5752 lines
272 KiB
Plaintext
5752 lines
272 KiB
Plaintext
Serial HOWTO
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David S.Lawyer dave@lafn.org original by Greg Hankins
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v2.27 February 2011
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This document is for the UART serial port. This port has mostly disap-
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peared from desktops and laptops is still used elsewhere such as for
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embedded systems. It covers information other than that which should
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be covered by Modem-HOWTO, PPP-HOWTO, Serial-Programming-HOWTO, or
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Text-Terminal-HOWTO. It lists info on multiport serial cards. It
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contains technical info about the serial port itself in more detail
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than found in the above HOWTOs and should be best for troubleshooting
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when the problem is the serial port itself. If you are dealing with a
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Modem, PPP (used for Internet access on a phone line), or a Text-Ter-
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minal, those HOWTOs should be consulted first.
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______________________________________________________________________
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Table of Contents
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1. Introduction
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1.1 Copyright, Disclaimer, & Credits
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1.1.1 Copyright
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1.1.2 Disclaimer
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1.1.3 Trademarks.
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1.1.4 Credits
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1.2 New Versions of this Serial-HOWTO
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1.3 New in Recent Versions
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1.4 Related HOWTO's, etc. about the Serial Port
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1.5 Feedback
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1.6 What is a Serial Port?
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2. Quick Help
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3. How the Hardware Transfers Bytes
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3.1 Transmitting
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3.2 Receiving
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3.3 The Large Serial Buffers
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4. Serial Port Basics
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4.1 What is a Serial Port ?
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4.1.1 Intro to Serial
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4.1.2 Pins and Wires
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4.1.3 RS-232 (EIA-232, etc.)
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4.2 IO Address & IRQ
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4.3 Names: ttyS0, ttyS1, etc.
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4.4 Interrupts
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4.5 Data Flow (Speeds)
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4.6 Flow Control
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4.6.1 Example of Flow Control
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4.6.2 Symptoms of No Flow Control
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4.6.3 Hardware vs. Software Flow Control
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4.7 Data Flow Path; Buffers
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4.8 Complex Flow Control Example
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4.9 Serial Driver Module
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4.10 The Serial Port is Now Obsolete on PCs
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4.11 RS-232 Cable Is Low Speed & Short Distance
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4.12 Inefficient PCI Interface to the Computer (in some cases)
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5. Multiport Serial Boards/Cards/Adapters
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5.1 Intro to Multiport Serial
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5.2 Dumb vs. Smart Cards
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5.3 Getting/Enabling a Driver
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5.3.1 Introduction
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5.3.2 Build (compile) support into the kernel?
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5.3.3 Using module support
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5.3.4 Getting info on multiport boards
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5.4 Multiport Devices in the /dev Directory,
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5.5 Making Legacy Multiport Devices in the /dev Directory
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5.6 Standard PC Serial Cards
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5.7 Dumb Multiport Serial Boards (with standard UART chips)
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5.8 Intelligent Multiport Serial Boards
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5.9 Unsupported Multiport Boards
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6. Servers for Serial Ports
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7. Configuring Overview
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8. Locating the Serial Port: IO address, IRQs
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8.1 What Bus is my Serial Port On?
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8.2 IO & IRQ Overview
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8.3 PCI Bus Support
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8.3.1 Introduction
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8.3.2 More info on PCI
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8.4 Common mistakes made re low-level configuring
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8.5 IRQ & IO Address Must be Correct
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8.6 What is the IO Address and IRQ per the driver ?
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8.6.1 Introduction
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8.6.2 I/O Address & IRQ: Boot-time messages
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8.6.3 The /proc directory and setserial
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8.7 What is the IO Address & IRQ of my Serial Port Hardware?
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8.7.1 Introduction
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8.7.2 PCI: What IOs and IRQs have been set?
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8.7.3 PCI: Enabling a disabled port
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8.7.4 ISA PnP ports
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8.7.5 Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
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8.7.6 Exploring via MS Windows (a last resort)
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8.8 Choosing Serial IRQs
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8.8.1 IRQ 0 is not an IRQ
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8.8.2 Interrupt sharing, Kernels 2.2+
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8.8.3 What IRQs to choose?
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8.9 Choosing Addresses --Video card conflict with ttyS3
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8.10 Set IO Address & IRQ in the hardware (mostly for PnP)
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8.10.1 Using a PnP BIOS to IO-IRQ Configure
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8.11 Giving the IRQ and IO Address to Setserial
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9. Configuring the Serial Driver (high-level) "stty"
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9.1 Overview
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9.2 Flow Control
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10. Serial Port Devices /dev/ttyS2, etc.
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10.1 Serial Port Names: ttyS4, etc
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10.2 The PCI Bus
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10.3 Serial Port Device Names & Numbers
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10.4 More on Serial Port Names
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10.5 USB (Universal Serial Bus) Serial Ports
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10.6 Link ttySN to /dev/modem
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10.7 Which Connector on the Back of my PC is ttyS1, etc?
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10.7.1 Inspect the connectors
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10.7.2 Send bytes to the port
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10.7.3 Connect a device to the connector
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10.7.4 Missing connectors
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10.8 Creating Devices In the /dev directory
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11. Interesting Programs You Should Know About
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11.1 Serial Monitoring/Diagnostics Programs
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11.2 Changing Interrupt Priority
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11.3 What is Setserial ?
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11.3.1 Setserial problems with linmodems, laptops
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11.3.2 Introduction
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11.3.3 Serial module unload
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11.3.4 Slow baud rates of 1200 or less
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11.3.5 Giving the setserial command
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11.3.6 Configuration file
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11.3.7 Probing
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11.3.8 Boot-time Configuration
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11.3.9 Edit a script (required prior to version 2.15)
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11.3.10 Configuration method using /etc/serial.conf, etc.
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11.3.11 IRQs
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11.3.12 Laptops: PCMCIA
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11.4 Stty
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11.4.1 Introduction
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11.4.2 Flow control options
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11.4.3 Using stty at a "foreign" terminal
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11.4.4 Two interfaces at a terminal
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11.4.5 Where to put the stty command ?
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11.4.6 Obsolete redirection method
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11.5 What is isapnp ?
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11.6 Connecting two PCs together via serial ports
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11.7 Connect the serial port to a fast network: ser2net
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12. Speed (Flow Rate)
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12.1 Very High Speeds
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12.1.1 Speeds over 115.2kbps
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12.1.2 How speed is set in hardware: the divisor and baud_base
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12.1.3 Setting the divisor, speed accounting
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12.1.4 Crystal frequency is higher than baud_base
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12.2 Higher Serial Throughput
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13. Locking Out Others
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13.1 Introduction
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13.2 Lock-Files
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13.3 Change Owners, Groups, and/or Permissions of Device Files
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14. Serial Communications Programs And Utilities
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14.1 List of Software
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14.2 kermit and zmodem
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15. Serial Tips And Miscellany
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15.1 Serial Modules
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15.2 Kernel Configuration
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15.3 Number of Serial Ports Supported
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15.4 Serial Console (console on the serial port)
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15.5 Line Drivers
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15.6 Stopping the Data Flow when Printing, etc.
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15.7 Known IO Address Conflicts
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15.7.1 Avoiding IO Address Conflicts with Certain Video Boards
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15.7.2 IO address conflict with ide2 hard drive
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15.8 Known Defective Hardware
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15.8.1 Problem with AMD Elan SC400 CPU (PC-on-a-chip)
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16. Troubleshooting
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16.1 Serial Electrical Test Equipment
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16.1.1 Breakout Gadgets, etc.
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16.1.2 Measuring voltages
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16.1.3 Taste voltage
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16.2 Serial Monitoring/Diagnostics
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16.3 (The following subsections are in both the Serial and Modem HOWTOs)
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16.4 Serial Port Can't be Found
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16.4.1 Scanning/probing legacy ports
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16.5 Linux Creates an Interrupt Conflict (your PC has an ISA slot)
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16.6 Extremely Slow: Text appears on the screen slowly after long delays
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16.7 Somewhat Slow: I expected it to be a few times faster
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16.8 The Startup Screen Shows Wrong IRQs for the Serial Ports
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16.9 "Cannot open /dev/ttyS?: Device or resource busy
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16.10 "Cannot open /dev/ttyS?: Permission denied"
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16.11 "Cannot open /dev/ttyS?"
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16.12 "Operation not supported by device" for ttyS?
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16.13 "Cannot create lockfile. Sorry"
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16.14 "Device /dev/ttyS? is locked."
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16.15 "/dev/tty? Device or resource busy"
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16.16 "Input/output error" from setserial, stty, pppd, etc.
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16.17 "LSR safety check engaged"
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16.18 Overrun errors on serial port
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16.19 Port gets characters only sporadically
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16.20 Troubleshooting Tools
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16.21 Almost all characters are wrong; Many missing or many extras
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17. Interrupt Problem Details
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17.1 Types of interrupt problems
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17.2 Symptoms of Mis-set or Conflicting Interrupts
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17.3 Mis-set Interrupts
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17.4 Interrupt Conflicts
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17.5 Resolving Interrupt Problems
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18. What Are UARTs? How Do They Affect Performance?
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18.1 Introduction to UARTS
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18.2 Two Types of UARTs
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18.3 FIFOs
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18.4 Why FIFO Buffers are Small
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18.5 UART Model Numbers
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19. Pinout and Signals
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19.1 Pinout of 9-pin and 25-pin serial connectors
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19.2 Signals May Have No Fixed Meaning
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19.3 Cabling Between Serial Ports
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19.4 RTS/CTS and DTR/DSR Flow Control
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19.4.1 The DTR and DSR Pins
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19.5 Preventing a Port From Opening
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20. Voltage Waveshapes
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20.1 Voltage for a Bit
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20.2 Voltage Sequence for a Byte
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20.3 Parity Explained
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20.4 Forming a Byte (Framing)
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20.5 How "Asynchronous" is Synchronized
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21. Other Serial Devices (not async RS-232)
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21.1 Successors to RS-232
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21.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
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21.3 EIA-485
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21.4 EIA-530
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21.5 EIA-612/613
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21.6 The Universal Serial Bus (USB)
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21.7 Firewire
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21.8 MIDI
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21.9 Synchronization & Synchronous
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21.9.1 Defining Asynchronous vs Synchronous
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21.9.2 Synchronous Communication
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22. Other Sources of Information
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22.1 Books
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22.2 Serial Software
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22.3 Related Linux Documents
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22.4 Serial Mailing List
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22.5 Internet
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23. Appendix A: Very Obsolete Hardware/Software
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23.1 Replacing pre 1990 UARTS
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23.2 Two Ports with the Same IO address
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23.3 Configuring by modifying source code
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23.4 Modems on Multiport Cards Obsolete for Sending at 56k
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23.5 Lock-Files if you used the depreciated devfs
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23.6 Devfs (The deprecated Device File System. History)
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______________________________________________________________________
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1. Introduction
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This HOWTO covers basic info on the serial port which is missing on
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newer personal computers but is being used in recent embedded systems
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as well as for routers, point-of-sale equipment, etc. It includes
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multiport serial cards. It was written when the serial port was a
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major port for connecting a PC to modems and printers, etc. and the
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style of this article reflects this. If you're having problems with
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the serial port, want to understand how it works, or need a detailed
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introduction to it before studying serial programming, this is one
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place to find out about it.
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This HOWTO is about the original serial port which uses a UART chip
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and is sometimes called a "UART serial port" to differentiate it from
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the newer types of serial devices: Universal Serial Bus or Firewire.
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It's ``slow'' compared to newer serial devices but can send text
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several times faster than you can read it. Information specific to
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devices which use serial ports: analog modems, text-terminals,
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infrared devices, and a few printers are found in Modem-HOWTO, Text-
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Terminal-HOWTO, Infrared-HOWTO, and Printing-HOWTO. Info on getty
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(the program that runs the login process or the like) has been also
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moved to other related HOWTOs since mgetty and uugetty are best for
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modems while agetty is best for text-terminals. If you are dealing
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with a modem, text terminal, infrared device, or printer, then you may
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not need to consult this HOWTO. But if you are using the serial port
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for some other device, using a multiport serial card, trouble-shooting
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the serial port itself, or want to understand more technical details
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of the serial port, then you may want to use this HOWTO as well as
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some of the other HOWTOs. (See ``Related HOWTO's'') This HOWTO lists
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info on various multiport serial cards. This HOWTO addresses Linux
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running on PCs (ISA and/or PCI buses), although it might be valid for
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other architectures.
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1.1. Copyright, Disclaimer, & Credits
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1.1.1. Copyright
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Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2005 by David S.
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Lawyer <mailto:dave@lafn.org>
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Please freely copy and distribute (sell or give away) this document in
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any format. Send any corrections and comments to the document
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maintainer. You may create a derivative work and distribute it
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provided that you:
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1. If it's not a translation: Email a copy of your derivative work (in
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a format LDP accepts) to the author(s) and maintainer (could be the
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same person). If you don't get a response then email the LDP
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(Linux Documentation Project): submit@en.tldp.org.
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2. License the derivative work in the spirit of this license or use
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GPL. Include a copyright notice and at least a pointer to the
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license used.
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3. Give due credit to previous authors and major contributors.
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If you're considering making a derived work other than a translation,
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it's requested that you discuss your plans with the current
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maintainer.
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1.1.2. Disclaimer
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While I haven't intentionally tried to mislead you, there are likely a
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number of errors in this document. Please let me know about them.
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Since this is free documentation, it should be obvious that I cannot
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be held legally responsible for any errors.
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1.1.3. Trademarks.
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Any brand names (starts with a capital letter such as MS Windows)
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should be assumed to be a trademark). Such trademarks belong to their
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respective owners.
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1.1.4. Credits
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Most of the original Serial-HOWTO was written by Greg Hankins.
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<mailto:gregh@twoguys.org> He also rewrote many contributions by
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others in order to maintain continuity in the writing style and flow.
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He wrote: ``Thanks to everyone who has contributed or commented, the
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list of people has gotten too long to list (somewhere over one
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hundred). Special thanks to Ted Ts'o for answering questions about
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the serial drivers.'' Approximately half of v2.00 was from Greg
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Hankins HOWTO and the other half were additions by David Lawyer. Ted
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Ts'o has continued to be helpful. In Jan. 2006 "Charles Brockman"
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reviewed it for typos which resulted in many typos being fixed.
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1.2. New Versions of this Serial-HOWTO
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New versions will be issued perhaps every year or two. They will be
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available to browse and/or download at LDP mirror sites see:
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<http://www.tldp.org/mirrors.html>. Various formats are available.
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If you only want to quickly check the date of the latest version look
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at <http://www.tldp.org/HOWTO/Serial-HOWTO.html> and compare it to
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this version: v2.27 February 2011 .
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1.3. New in Recent Versions
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For a full revision history going back to the time I started
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maintaining this HOWTO, see the source file (in linuxdoc format):
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(cvs) Serial-HOWTO.sgml
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<http://cvs.tldp.org/go.to/LDP/LDP/howto/linuxdoc/Serial-
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HOWTO.sgml?view=markup>
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· TO do: explain use of udev for serial port for setting names and
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permissions. Fix dead links.
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· 2.27 Feb. 2011: The serial port is still widely used in embedded
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systems, etc., and is not really obsolete. Better definition of
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input, output. Deleted devfs names such as tts.
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· 2.26 Nov. 2010 Changed EIA-232 to RS-232. PCI-e bus is serial. No
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serial port on new PCs. I incorrectly wrote in this update that
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the serial port was obsolete and that this HOWTO is now mainly of
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historical interest.
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· 2.25 Jan. 2007 picocom. devfs is obsolete. ser2net. Revised parts
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on drivers as modules vs. built into kernel. Serial Programming
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wikibook.
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1.4. Related HOWTO's, etc. about the Serial Port
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Modems, Text-Terminals, some printers, and other peripherals often
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used the serial port. Get these HOWTOs from the nearest mirror site
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as explained above.
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· Modem-HOWTO is about installing and configuring modems
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· Printing-HOWTO has info for serial printers using old lpr command
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· LPRng-HOWTO (not a LDP HOWTO, may come with software) has info for
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serial printing for "Next Generation" lpr
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· Serial Programming
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<http://en.wikibooks.org/wiki/Programming:Serial_Data_Communications>
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is a wiki-book on the Internet
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· Serial-Programming-HOWTO helps you write C programs that read and
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write to the serial port and/or check/set its state. A version
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written by Vern Hoxie but not submitted to LDP is at ``Internet
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section''.
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· Text-Terminal-HOWTO is about how they work, how to install
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configure, and repair them. It includes a section on "Make a
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Terminal the Console" which is useful for using a remote terminal
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to control a server (via the serial port).
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· Remote-Serial-Console-HOWTO is about making a text-terminal be the
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console so it can display boot-time messages, etc.
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1.5. Feedback
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Please send me any suggestions, correction or additional material.
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Tell me what you don't understand, or what could be clearer. You can
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reach me via email at <mailto:dave@lafn.org>.
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1.6. What is a Serial Port?
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The conventional serial port (not the newer USB port, or Firewire
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port) is a very old I/O (Input/Output) port. Until around 2006, most
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new desktop PC's had one, and old PC's from the 1990's sometimes had 2
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of them. Most laptops gave up them before the desktops did. Macs
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(Apple Computer) after mid-1998 only had the USB port. However, it's
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possible, to put a conventional serial port device on the USB bus
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which is on all modern PCs.
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Each serial port has a "file" associated with it in the /dev
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directory. It isn't really a file but it seems like one. For
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example, /dev/ttyS0. Other serial ports are /dev/ttyS1, /dev/ttyS2,
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etc. But ports on the USB bus, multiport cards, etc. have different
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names.
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The common specification for the conventional serial port is RS-232
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(or RS-232). So it's often called a "RS-232 serial port". The
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connector(s) for the serial port are often seen as one or two 9-pin
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connectors (in some cases 25-pin) on the back of a PC. But the serial
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port is more than just connectors. It includes the associated
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electronics which must produce signals conforming to the RS-232
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specification. See ``Voltage Waveshapes''. One pin is used to send
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out data bytes and another to receive data bytes. Another pin is a
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common signal ground. The other "useful" pins are used mainly for
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signalling purposes with a steady negative voltage meaning "off" and a
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steady positive voltage meaning "on".
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The UART (Universal Asynchronous Receiver-Transmitter) chip does most
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of the work. Today, the functionality of this chip is usually built
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into another chip. See ``What Are UARTs?'' These have improved over
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time and old models (prior to say 1994) are usually very obsolete.
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The serial port was originally designed for connecting external modems
|
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to a PC but it's used to connect many other devices also such as mice,
|
|
text-terminals, some printers, etc. to a computer. You just plug
|
|
these devices into the serial port using the correct cable. Many
|
|
internal modem cards have a built-in serial port so when you install
|
|
one inside your PC it's as if you just installed another serial port
|
|
in your PC.
|
|
|
|
|
|
2. Quick Help
|
|
|
|
This repeats more detailed information found elsewhere. If your
|
|
computer can't seem to find your serial port and you already know
|
|
something about hardware resources (addresses like 3F8 and IRQs like
|
|
5) then try this: First, get into the BIOS (often called "setup")
|
|
when the computer is powered on by pressing certain keys. To find out
|
|
what keys to press, watch the screen as your PC starts up. If the
|
|
words that flash by on the screen too fast to read, freeze them by
|
|
holding down the "pause" and "shift" keys at the same time. Then when
|
|
read, hit any key to resume (cease pausing) and hold down the key(s)
|
|
required to enter the BIOS setup. You may have to try this again
|
|
since there may be more than one screen which you can freeze with the
|
|
"pause" key. Also, look for messages about the serial ports on these
|
|
frozen screens.
|
|
|
|
Once in the BIOS menus, try to find menus dealing with the serial
|
|
port. They could be shown in a menu dealing with Resources, Plug-and-
|
|
Play, Peripherals, Ports, etc. Some old BIOSs setups (before 1995 ?)
|
|
didn't deal with the serial ports. Make sure the ports you need are
|
|
not disabled and note how they are configured (like 3F8 IRQ 4). You
|
|
may need to change the configuration to prevent conflicts. There
|
|
could be a shortage of IRQs if the BIOS has reserved some IRQs that it
|
|
didn't need to reserve.
|
|
|
|
For serial ports to be found, either the kernel must have been
|
|
compiled with serial support, or serial support must be provided by a
|
|
module. To check this look in the file /boot/config-2.6... and search
|
|
for SERIAL. =m means it's a module and you may check to see the
|
|
modules that are being used by typing: lsmod.
|
|
|
|
|
|
3. How the Hardware Transfers Bytes
|
|
|
|
Below is an introduction to the topic, but for a more advanced
|
|
treatment of it see ``FIFOs''.
|
|
|
|
|
|
3.1. Transmitting
|
|
|
|
Transmitting is sending bytes out of the serial port away from the
|
|
computer (output). Once you understand transmitting, receiving
|
|
(input) is easy to understand since it's similar. The first
|
|
explanation given here will be grossly oversimplified. Then more
|
|
detail will be added in later explanations. When the computer wants
|
|
to send a byte out the serial port (to the external cable) the CPU
|
|
sends the byte on the bus inside the computer to the I/O (Input
|
|
Output) address of the serial port. I/O is often written as just IO.
|
|
The serial port takes the byte, and sends it out one bit at a time (a
|
|
serial bit-stream) on the transmit pin of the serial cable connector.
|
|
For what a bit (and byte) look like electrically see ``Voltage
|
|
Waveshapes''.
|
|
|
|
Here's a replay of the above in a little more detail (but still very
|
|
incomplete). Most of the work at the serial port is done by the UART
|
|
chip (or the like). To transmit a byte, the serial device driver
|
|
program (running on the CPU) sends a byte to the serial port"s I/O
|
|
address. This byte gets into a 1-byte "transmit shift register" in
|
|
the serial port. From this shift register bits are taken from the
|
|
byte one-by-one and sent out bit-by-bit on the serial line. Then when
|
|
the last bit has been sent and the shift register needs another byte
|
|
to send, it could just ask the CPU to send it another byte. Thus
|
|
would be simple but it would likely introduce delays since the CPU
|
|
might not be able to get the byte immediately. After all, the CPU is
|
|
usually doing other things besides just handling the serial port.
|
|
|
|
A way to eliminate such delays is to arrange things so that the CPU
|
|
gets the byte before the shift register needs it and stores it in a
|
|
serial port buffer (in hardware). Then when the shift register has
|
|
sent out its byte and needs a new byte immediately, the serial port
|
|
hardware just transfers the next byte from its own buffer to the shift
|
|
register. No need to call the CPU to fetch a new byte.
|
|
|
|
|
|
The size of this serial port buffer was originally only one byte, but
|
|
today it is usually 16 bytes (more in higher priced serial ports).
|
|
Now there is still the problem of keeping this buffer sufficiently
|
|
supplied with bytes so that when the shift register needs a byte to
|
|
transmit it will always find one there (unless there are no more bytes
|
|
to send). This is done by contacting the CPU using an interrupt.
|
|
|
|
First we'll explain the case of the old fashioned one-byte buffer,
|
|
since 16-byte buffers work similarly (but are more complex). When the
|
|
shift register grabs the byte out of the buffer and the buffer needs
|
|
another byte, it sends an interrupt to the CPU by putting a voltage on
|
|
a dedicated wire on the computer bus. Unless the CPU is doing
|
|
something very important, the interrupt forces it to stop what it was
|
|
doing and start running a program which will supply another byte to
|
|
the port's buffer. The purpose of this buffer is to keep an extra
|
|
byte (waiting to be sent) queued in hardware so that there will be no
|
|
gaps in the transmission of bytes out the serial port cable.
|
|
|
|
Once the CPU gets the interrupt, it will know who sent the interrupt
|
|
since there is a dedicated interrupt wire for each serial port (unless
|
|
interrupts are shared). Then the CPU will start running the serial
|
|
device driver which checks registers at I/0 addresses to find out what
|
|
has happened. It finds out that the serial's transmit buffer is empty
|
|
and waiting for another byte. So if there are more bytes to send, it
|
|
sends the next byte to the serial port's I/0 address. This next byte
|
|
should arrive when the previous byte is still in the transmit shift
|
|
register and is still being transmitted bit-by-bit.
|
|
|
|
In review, when a byte has been fully transmitted out the transmit
|
|
wire of the serial port and the shift register is now empty the
|
|
following 3 things happen in rapid succession:
|
|
|
|
|
|
1. The next byte is moved from the transmit buffer into the transmit
|
|
shift register
|
|
|
|
2. The transmission of this new byte (bit-by-bit) begins
|
|
|
|
3. Another interrupt is issued to tell the device driver to send yet
|
|
another byte to the now empty transmit buffer
|
|
|
|
Thus we say that the serial port is interrupt driven. Each time the
|
|
serial port issues an interrupt, the CPU sends it another byte. Once
|
|
a byte has been sent to the transmit buffer by the CPU, then the CPU
|
|
is free to pursue some other activities until it gets the next
|
|
interrupt. The serial port transmits bits at a fixed rate which is
|
|
selected by the user (or an application program). It's sometimes
|
|
called the baud rate. The serial port also adds extra bits to each
|
|
byte (start, stop and perhaps parity bits) so there are often 10 bits
|
|
sent per byte. At a rate (also called speed) of 19,200 bits per
|
|
second (bps), there are thus 1,920 bytes/sec (and also 1,920
|
|
interrupts/sec).
|
|
|
|
Doing all this is a lot of work for the CPU. This is true for many
|
|
reasons. First, just sending one 8-bit byte at a time over a 32-bit
|
|
data bus (or even 64-bit) is not a very efficient use of bus width.
|
|
Also, there is a lot of overhead in handing each interrupt. When the
|
|
interrupt is received, the device driver only knows that something
|
|
caused an interrupt at the serial port but doesn't know that it's
|
|
because a character has been sent. The device driver has to make
|
|
various checks to find out what happened. The same interrupt could
|
|
mean that a character was received, one of the control lines changed
|
|
state, etc.
|
|
|
|
A major improvement has been the enlargement of the buffer size of the
|
|
serial port from 1-byte to 16-bytes. This means that when the CPU
|
|
gets an interrupt it gives the serial port up to 16 new bytes to
|
|
transmit. This is fewer interrupts to service but data must still be
|
|
transferred one byte at a time over a wide bus. The 16-byte buffer is
|
|
actually a FIFO (First In First Out) queue and is often called a FIFO.
|
|
See ``FIFOs'' for details about the FIFO along with a repeat of some
|
|
of the above info.
|
|
|
|
|
|
3.2. Receiving
|
|
|
|
Receiving bytes by a serial port is similar to sending them only it's
|
|
in the opposite direction. It's also interrupt driven. For the
|
|
obsolete type of serial port with 1-byte buffers, when a byte is fully
|
|
received from the external cable it goes into the 1-byte receive
|
|
buffer. Then the port gives the CPU an interrupt to tell it to pick
|
|
up that byte so that the serial port will have room for storing the
|
|
next byte which is currently being received. For newer serial ports
|
|
with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
|
|
after 14 bytes are in the receive buffer. The CPU then stops what it
|
|
was doing, runs the interrupt service routine, and picks up 14 to 16
|
|
bytes from the port. For an interrupt sent when the 14th byte has
|
|
been received, there could be 16 bytes to get if 2 more bytes have
|
|
arrived since the interrupt. But if 3 more bytes should arrive
|
|
(instead of 2), then the 16-byte buffer will overrun. It also may
|
|
pick up less than 14 bytes by setting it up that way or due to
|
|
timeouts. See ``FIFOs'' for more details.
|
|
|
|
|
|
3.3. The Large Serial Buffers
|
|
|
|
We've talked about small 16-byte serial port hardware buffers but
|
|
there are also much larger buffers in main memory. When the CPU takes
|
|
some bytes out of the receive buffer of the hardware, it puts them
|
|
into a much larger (say 8k-byte) receive buffer in main memory. Then
|
|
a program that is getting bytes from the serial port takes the bytes
|
|
it's receiving out of that large buffer (using a "read" statement in
|
|
the program).
|
|
|
|
A similar situation exists for bytes that are to be transmitted. When
|
|
the CPU needs to fetch some bytes to be transmitted it takes them out
|
|
of a large (8k-byte) transmit buffer in main memory and puts them into
|
|
the small 16-byte transmit buffer in the hardware. And when a program
|
|
wants to send bytes out the serial port, it writes them to this large
|
|
transmit buffer.
|
|
|
|
Both of these buffers are managed by the serial driver. But the
|
|
driver does more than just dealing with these buffers. It also does
|
|
limited filtering (minor modifications) of data passing thru these
|
|
buffers and listens for control certain characters. All this is
|
|
configurable using the ``Stty'' program.
|
|
|
|
|
|
4. Serial Port Basics
|
|
|
|
You don't have to understand the basics to use the serial port But
|
|
understanding it may help to determine what is wrong if you run into
|
|
problems. This section not only presents new topics but also repeats
|
|
some of what was said in the previous section ``How the Hardware
|
|
Transfers Bytes'' but in greater detail.
|
|
|
|
|
|
4.1. What is a Serial Port ?
|
|
|
|
|
|
|
|
4.1.1. Intro to Serial
|
|
|
|
The UART serial port (or just "serial port for short" is an I/O
|
|
(Input/Output) device.
|
|
|
|
An I/O device is just a way to get data into and out of a computer.
|
|
There are many types of I/O devices such as the older serial ports and
|
|
parallel ports, network cards, universal serial buses (USB), and
|
|
firewire etc. Most pre-2007 PC's have a serial port or two (on older
|
|
PC's). Each has a 9-pin connector (sometimes 25-pin) on the back of
|
|
the computer. Computer programs can send data (bytes) to the transmit
|
|
pin (output) and receive bytes from the receive pin (input). The
|
|
other pins are for control purposes and ground.
|
|
|
|
The serial port is much more than just a connector. It converts the
|
|
data from parallel to serial and changes the electrical representation
|
|
of the data. Inside the computer, data bits flow in parallel (using
|
|
many wires at the same time). Serial flow is a stream of bits over a
|
|
single wire (such as on the transmit or receive pin of the serial
|
|
connector). For the serial port to create such a flow, it must
|
|
convert data from parallel (inside the computer) to serial on the
|
|
transmit pin (and conversely).
|
|
|
|
Most of the electronics of the serial port is found in a computer chip
|
|
(or a part of a chip) known as a UART. For more details on UARTs see
|
|
the section ``What Are UARTS?'' But you may want to finish this
|
|
section first so that you will hopefully understand how the UART fits
|
|
into the overall scheme of things.
|
|
|
|
|
|
4.1.2. Pins and Wires
|
|
|
|
Old PC's used 25 pin connectors but only about 9 pins were actually
|
|
used so later on most connectors were only 9-pin. Each of the 9 pins
|
|
usually connects to a wire. Besides the two wires used for
|
|
transmitting and receiving data, another pin (wire) is signal ground.
|
|
The voltage on any wire is measured with respect to this ground. Thus
|
|
the minimum number of wires to use for 2-way transmission of data is
|
|
3. Except that it has been known to work with no signal ground wire
|
|
but with degraded performance and sometimes with errors.
|
|
|
|
There are still more wires which are for control purposes (signalling)
|
|
only and not for sending bytes. All of these signals could have been
|
|
shared on a single wire, but instead, there is a separate dedicated
|
|
wire for every type of signal. Some (or all) of these control wires
|
|
are called "modem control lines". Modem control wires are either in
|
|
the asserted state (on) of +5 volts or in the negated state (off) of
|
|
-5 volts. One of these wires is to signal the computer to stop
|
|
sending bytes out the serial port cable. Conversely, another wire
|
|
signals the device attached to the serial port to stop sending bytes
|
|
to the computer. If the attached device is a modem, other wires may
|
|
tell the modem to hang up the telephone line or tell the computer that
|
|
a connection has been made or that the telephone line is ringing
|
|
(someone is attempting to call in). See section ``Pinout and
|
|
Signals'' for more details.
|
|
|
|
|
|
|
|
4.1.3. RS-232 (EIA-232, etc.)
|
|
|
|
The serial port (not the USB) is usually a RS-232-C, EIA-232-D, or
|
|
EIA-232-E. These three are almost the same thing. The original RS
|
|
(Recommended Standard) prefix officially became EIA (Electronics
|
|
Industries Association) and later EIA/TIA after EIA merged with TIA
|
|
(Telecommunications Industries Association). The EIA-232 spec
|
|
provides also for synchronous (sync) communication but the hardware to
|
|
support sync is almost always missing on PC's. The RS designation is
|
|
was intended to become obsolete but is still widely used and will be
|
|
used in this howto for RS-232. Other documents may use the full
|
|
EIA/TIA designation, or just EIA or TIA. For info on other (non-
|
|
RS-232) serial ports see the section ``Other Serial Devices (not async
|
|
RS-232)''
|
|
|
|
|
|
4.2. IO Address & IRQ
|
|
|
|
Since the computer needs to communicate with each serial port, the
|
|
operating system must know that each serial port exists and where it
|
|
is (its I/O address). It also needs to know which wire (IRQ number)
|
|
the serial port must use to request service from the computer's CPU.
|
|
It requests service by sending an interrupt voltage on this wire.
|
|
Thus every serial port device must store in its non-volatile memory
|
|
both its I/O address and its Interrupt ReQuest number: IRQ. See
|
|
``Interrupts''. The PCI bus has its own system of interrupts. But
|
|
since the PCI-aware BIOS sets up these PCI interrupts to map to IRQs,
|
|
it seemingly behaves just as described above. Except that sharing of
|
|
PCI interrupts is allowed (2 or more devices may use the same IRQ
|
|
number).
|
|
|
|
I/O addresses are not the same as memory addresses. When an I/O
|
|
addresses is put onto the computer's address bus, another wire is
|
|
energized. This both tells main memory to ignore the address and
|
|
tells all devices which have I/O addresses (such as the serial port)
|
|
to listen to the address sent on the bus to see if it matches the
|
|
device's. If the address matches, then the I/O device reads the data
|
|
on the data bus.
|
|
|
|
The I/O address of a certain device (such as ttyS2) will actually be a
|
|
range of addresses. The lower address in this range is the base
|
|
address. "address" usually means just the "base address".
|
|
|
|
|
|
4.3. Names: ttyS0, ttyS1, etc.
|
|
|
|
The serial ports are named ttyS0, ttyS1, etc. (and usually correspond
|
|
respectively to COM1, COM2, etc. in DOS/Windows). The /dev directory
|
|
has a special file for each port. Type "ls /dev/ttyS*" to see them.
|
|
Just because there may be (for example) a ttyS3 file, doesn't
|
|
necessarily mean that there exists a physical serial port there.
|
|
|
|
Which one of these names (ttyS0, ttyS1, etc.) refers to which physical
|
|
serial port is determined as follows. The serial driver (software)
|
|
maintains a table showing which I/O address corresponds to which ttyS.
|
|
This mapping of names (such as ttyS1) to I/O addresses (and IRQ's) may
|
|
be both set and viewed by the "setserial" command. See ``What is
|
|
Setserial''. This does not set the I/O address and IRQ in the
|
|
hardware itself (which is set by jumpers or by plug-and-play
|
|
software). Thus which physical port corresponds to say ttyS1 depends
|
|
both on what the serial driver thinks (per setserial) and what is set
|
|
in the hardware. If a mistake has been made, the physical port may
|
|
not correspond to any name (such as ttyS2) and thus it can't be used.
|
|
See ``Serial Port Devices /dev/ttyS2, etc.'' for more details.
|
|
|
|
|
|
4.4. Interrupts
|
|
|
|
|
|
When the serial port receives a number of bytes (may be set to 1, 4,
|
|
8, or 14) into its FIFO buffer, it signals the CPU to fetch them by
|
|
sending an electrical signal known as an interrupt on a certain wire
|
|
normally used only by that port. Thus the FIFO waits until it has
|
|
received a number of bytes and then issues an interrupt.
|
|
|
|
However, this interrupt will also be sent if there is an unexpected
|
|
delay while waiting for the next byte to arrive (known as a timeout).
|
|
Thus if the bytes are being received slowly (such as from someone
|
|
typing on a terminal keyboard) there may be an interrupt issued for
|
|
every byte received. For some UART chips the rule is like this: If 4
|
|
bytes in a row could have been received in an interval of time, but
|
|
none of these 4 show up, then the port gives up waiting for more bytes
|
|
and issues an interrupt to fetch the bytes currently in the FIFO. Of
|
|
course, if the FIFO is empty, no interrupt will be issued.
|
|
|
|
Each interrupt conductor (inside the computer) has a number (IRQ) and
|
|
the serial port must know which conductor to use to signal on. For
|
|
example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4). A
|
|
list of them and more will be found in "man setserial" (search for
|
|
"Configuring Serial Ports"). Interrupts are issued whenever the
|
|
serial port needs to get the CPU's attention. It's important to do
|
|
this in a timely manner since the buffer inside the serial port can
|
|
hold only 16 incoming bytes. If the CPU fails to remove such received
|
|
bytes promptly, then there will not be any space left for any more
|
|
incoming bytes and the small buffer may overflow (overrun) resulting
|
|
in a loss of data bytes.
|
|
|
|
There is no ``Flow Control'' to prevent this.
|
|
|
|
Interrupts are also issued when the serial port has just sent out all
|
|
of its bytes from its small transmit FIFO buffer out the external
|
|
cable. It then has space for 16 more outgoing bytes. The interrupt
|
|
is to notify the CPU of that fact so that it may put more bytes in the
|
|
small transmit buffer to be transmitted. Also, when a modem control
|
|
line changes state, an interrupt is issued.
|
|
|
|
The buffers mentioned above are all hardware buffers. The serial port
|
|
also has large buffers in main memory. This will be explained later
|
|
|
|
Interrupts convey a lot of information but only indirectly. The
|
|
interrupt itself just tells a chip called the interrupt controller
|
|
that a certain serial port needs attention. The interrupt controller
|
|
then signals the CPU. The CPU then runs a special program to service
|
|
the serial port. That program is called an interrupt service routine
|
|
(part of the serial driver software). It tries to find out what has
|
|
happened at the serial port and then deals with the problem such a
|
|
transferring bytes from (or to) the serial port's hardware buffer.
|
|
This program can easily find out what has happened since the serial
|
|
port has registers at IO addresses known to the serial driver
|
|
software. These registers contain status information about the serial
|
|
port. The software reads these registers and by inspecting the
|
|
contents, finds out what has happened and takes appropriate action.
|
|
|
|
|
|
|
|
4.5. Data Flow (Speeds)
|
|
|
|
Data (bytes representing letters, pictures, etc.) flows into and out
|
|
of your serial port. Flow rates (such as 56k (56000) bits/sec) are
|
|
(incorrectly) called "speed". But almost everyone says "speed"
|
|
instead of "flow rate".
|
|
|
|
It's important to understand that the average speed is often less than
|
|
the specified speed. Waits (or idle time) result in a lower average
|
|
speed. These waits may include long waits of perhaps a second due to
|
|
``Flow Control''. At the other extreme there may be very short waits
|
|
(idle time) of several micro-seconds between bytes. If the device on
|
|
the serial port (such as a modem) can't accept the full serial port
|
|
speed, then the average speed must be reduced.
|
|
|
|
|
|
4.6. Flow Control
|
|
|
|
Flow control means the ability to slow down the flow of bytes in a
|
|
wire. For serial ports this means the ability to stop and then
|
|
restart the flow without any loss of bytes. Flow control is needed
|
|
for modems and other hardware to allow a jump in instantaneous flow
|
|
rates.
|
|
|
|
|
|
4.6.1. Example of Flow Control
|
|
|
|
For example, consider the case where you connect a 33.6k external
|
|
modem via a short cable to your serial port. The modem sends and
|
|
receives bytes over the phone line at 33.6k bits per second (bps).
|
|
Assume it's not doing any data compression or error correction. You
|
|
have set the serial port speed to 115,200 bits/sec (bps), and you are
|
|
sending data from your computer to the phone line. Then the flow from
|
|
the your computer to your modem over the short cable is at 115.2k bps.
|
|
However the flow from your modem out the phone line is only 33.6k bps.
|
|
Since a faster flow (115.2k) is going into your modem than is coming
|
|
out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
|
|
bps) in one of its buffers. This buffer would soon overrun (run out
|
|
of free storage space) unless the high 115.2k flow is stopped.
|
|
|
|
But now flow control comes to the rescue. When the modem's buffer is
|
|
almost full, the modem sends a stop signal to the serial port. The
|
|
serial port passes on the stop signal on to the device driver and the
|
|
115.2k bps flow is halted. Then the modem continues to send out data
|
|
at 33.6k bps drawing on the data it previous accumulated in its
|
|
buffer. Since nothing is coming into this buffer, the number of bytes
|
|
in it starts to drop. When almost no bytes are left in the buffer,
|
|
the modem sends a start signal to the serial port and the 115.2k flow
|
|
from the computer to the modem resumes. In effect, flow control
|
|
creates an average flow rate in the short cable (in this case 33.6k)
|
|
which is significantly less than the "on" flow rate of 115.2k bps.
|
|
This is "start-stop" flow control.
|
|
|
|
In the above simple example it was assumed that the modem did no data
|
|
compression. This could happen when the modem is sending a file which
|
|
is already compressed and can't be compressed further. Now let's
|
|
consider the opposite extreme where the modem is compressing the data
|
|
with a high compression ratio. In such a case the modem might need an
|
|
input flow rate of say 115.2k bps to provide an output (to the phone
|
|
line) of 33.6k bps (compressed data). This compression ratio is 3.43
|
|
(115.2/33.6). In this case the modem is able to compress the 115.2
|
|
bps PC-to-modem flow and send the same data (in compressed form) out
|
|
the phone line at 33.6bps. There's no need for flow control here so
|
|
long as the compression ratio remains higher than 3.43. But the
|
|
compression ratio varies from second to second and if it should drop
|
|
below 3.43, flow control will be needed
|
|
|
|
In the above example, the modem was an external modem. But the same
|
|
situation exists (as of early 2003) for most internal modems. There
|
|
is still a speed limit on the PC-to-modem speed even though this flow
|
|
doesn't take place over an external cable. This makes the internal
|
|
modems compatible with the external modems.
|
|
|
|
In the above example of flow control, the flow was from the computer
|
|
to a modem. But there is also flow control which is used for the
|
|
opposite direction of flow: from a modem (or other device) to a
|
|
computer. Each direction of flow involves 3 buffers: 1. in the modem
|
|
2. in the UART chip (called FIFOs) and 3. in main memory managed by
|
|
the serial driver. Flow control protects all buffers (except the
|
|
FIFOs) from overflowing.
|
|
|
|
Under Linux, the small UART FIFO buffers are not protected by flow
|
|
control but instead rely on a fast response to the interrupts they
|
|
issue. Some UART chips can be set to do hardware flow control to
|
|
protect their FIFOs but Linux (as of early 2003) doesn't seem to
|
|
support it. FIFO stand for "First In, First Out" which is the way it
|
|
handles bytes in a queue. All the 3 buffers use the FIFO rule but
|
|
only the one in the UART is named "FIFO". This is the essence of
|
|
flow control but there are still some more details.
|
|
|
|
|
|
|
|
4.6.2. Symptoms of No Flow Control
|
|
|
|
Understanding flow-control theory can be of practical use. The
|
|
symptom of no flow control is that chunks of data missing from files
|
|
sent without the benefit of flow control. When overflow happens,
|
|
often hundreds or even thousands of bytes get lost, and all in
|
|
contiguous chunks.
|
|
|
|
|
|
4.6.3. Hardware vs. Software Flow Control
|
|
|
|
If feasible, it's best to use "hardware" flow control that uses two
|
|
dedicated "modem control" wires to send the "stop" and "start"
|
|
signals. Hardware flow control at the serial port works like this:
|
|
The two pins, RTS (Request to send) and CTS (Clear to send) are used.
|
|
When the computer is ready to receive data it asserts RTS by putting a
|
|
positive voltage on the RTS pin (meaning "Request To Send to me").
|
|
When the computer is not able to receive any more bytes, it negates
|
|
RTS by putting a negative voltage on the pin saying: "stop sending to
|
|
me". The RTS pin is connected by the serial cable to another pin on
|
|
the modem, printer, terminal, etc. This other pin's only function is
|
|
to receive this signal.
|
|
|
|
For the case of a modem, this "other" pin will be the modem's RTS pin.
|
|
But for a printer, another PC, or a non-modem device, it's usually a
|
|
CTS pin so a "crossover" or "null modem" cable is required. This
|
|
cable connects the CTS pin at one end with the RTS pin at the other
|
|
end (two wires since each end of the cable has a CTS pin). For a
|
|
modem, a straight-thru cable is used.
|
|
|
|
For the opposite direction of flow a similar scheme is used. For a
|
|
modem, the CTS pin is used to send the flow control signal to the CTS
|
|
pin on the PC. For a non-modem, the RTS pin sends the signal. Thus
|
|
modems and non-modems have the roles of their RTS and CTS pins
|
|
interchanged. Some non-modems such as dumb terminals may use other
|
|
pins for flow control such as the DTR pin instead of RTS.
|
|
|
|
Software flow control uses the main receive and transmit data wires to
|
|
send the start and stop signals. It uses the ASCII control characters
|
|
DC1 (start) and DC3 (stop) for this purpose. They are just inserted
|
|
into the regular stream of data. Software flow control is not only
|
|
slower in reacting but also does not allow the sending of binary data
|
|
unless special precautions are taken. Since binary data will likely
|
|
contain DC1 and DC3 characters, special means must be taken to
|
|
distinguish between a DC3 that means a flow control stop and a DC3
|
|
that is part of the binary code. Likewise for DC1.
|
|
|
|
|
|
|
|
4.7. Data Flow Path; Buffers
|
|
|
|
It's been mentioned that there are 3 buffers for each direction of
|
|
flow (3 pairs altogether): 16-byte FIFO buffers (in the UART), a pair
|
|
of larger buffers inside a device connected to the serial port (such
|
|
as a modem), and a pair of buffers (say 8k) in main memory. When an
|
|
application program sends bytes to the serial port they first get
|
|
stashed in the transmit serial port buffer in main memory. The other
|
|
member of this pair consists of a receive buffer for the opposite
|
|
direction of byte-flow. Here's an example diagram for the case of
|
|
browsing the Internet with a browser. Transmit data flow is left to
|
|
right while receive flow is right to left. There is a separate buffer
|
|
for each direction of flow.
|
|
|
|
|
|
application 8k-byte 16-byte 1k-byte tele-
|
|
BROWSER ------- MEMORY -------- FIFO --------- MODEM -------- phone
|
|
program buffer buffer buffer line
|
|
|
|
|
|
|
|
For the transmit case, the serial device driver takes out say 15 bytes
|
|
from this transmit buffer (in main memory), one byte at a time and
|
|
puts them into the 16-byte transmit buffer in the serial UART for
|
|
transmission. Once in that transmit buffer, there is no way to stop
|
|
them from being transmitted. They are then transmitted to the modem
|
|
or (other device connected to the serial port) which also has a fair
|
|
sized (say 1k) buffer. When the device driver (on orders from flow
|
|
control sent from the modem) stops the flow of outgoing bytes from the
|
|
computer, what it actually stops is the flow of outgoing bytes from
|
|
the large transmit buffer in main memory. Even after this has
|
|
happened and the flow to the modem has stopped, an application program
|
|
may keep sending bytes to the 8k transmit buffer until it becomes
|
|
full. At the same time, the bytes stored in the FIFO continue to be
|
|
sent out. Bytes stored in the modem will continue to be sent out the
|
|
phone line unless the modem has gotten a modem-to-modem flow control
|
|
stop from the modem at the other end of the phone line.
|
|
|
|
When the memory buffer gets full, the application program can't send
|
|
any more bytes to it (a "write" statement in a C-program blocks) and
|
|
the application program temporarily stops running and waits until some
|
|
buffer space becomes available. Thus a flow control "stop" is
|
|
ultimately able to stop the program that is sending the bytes. Even
|
|
though this program stops, the computer does not necessarily stop
|
|
computing since it may switch to running other processes while it's
|
|
waiting at a flow control stop.
|
|
|
|
The above was a little oversimplified in three ways. First, some
|
|
UARTs can do automatic hardware flow control which can stop the
|
|
transmission out of the FIFO buffers if needed (not yet supported by
|
|
Linux). Second, while an application process is waiting to write to
|
|
the transmit buffer, it could possibly perform other tasks. Third,
|
|
the serial driver (located between the memory buffer and the FIFO) has
|
|
it's own small buffer (in main memory) used to process characters.
|
|
|
|
|
|
|
|
4.8. Complex Flow Control Example
|
|
|
|
For many situations, there is a transmit path involving several links,
|
|
each with its own flow control. For example, I type at a text-
|
|
terminal connected to a PC and the PC (under my control) dials out to
|
|
another computer using a modem. Today, a "text-terminal" is likely to
|
|
be just another PC emulating a text-terminal. The main (server) PC,
|
|
in addition to serving my text-terminal, could also have someone else
|
|
sitting at it doing something else. Note that calling this PC a
|
|
"server" is not technically correct but it does serve the terminal.
|
|
|
|
The text-terminal uses a command-line interface with no graphical
|
|
display. Every letter I type at the text-terminal goes over the
|
|
serial cable to my main PC and then over the phone line to the
|
|
computer that I've dialed out to. To dial out, I've used the
|
|
communication software: "minicom" which runs on my PC.
|
|
|
|
This sounds like a simple data path. I hit a key and the byte that
|
|
key generates flows over just two cables (besides the keyboard cable):
|
|
1. the cable from my text-terminal to my PC and 2. the telephone line
|
|
cable to some other computer. Of course, the telephone cable is
|
|
actually a number of telephone system cables and includes switches and
|
|
electronics so that a single physical cable can transmit many phone
|
|
calls. But I can think of it like one cable (or one link).
|
|
|
|
Now, let's count the number and type of electronic devices each
|
|
keystroke-byte has to pass thru. The terminal-to-PC cable has a
|
|
serial port at each end. The telephone cable has both a serial port
|
|
and a modem at each end. This adds up to 4 serial ports and 2 modems.
|
|
Since each serial port has 2 buffers, and each modem one buffer, that
|
|
adds up to 10 buffers. And that's just for one direction of flow.
|
|
Each byte also must pass thru the minicom software as well.
|
|
|
|
While there's just 2 cables in the above scenario, if external modems
|
|
were used there would be an additional cable between each modem and
|
|
it's serial port. This makes 4 cables in all. Even with internal
|
|
modems it's like there is a "virtual cable" between the modem and its
|
|
serial port. On all these 4 links (or cables), flow control takes
|
|
place.
|
|
|
|
Now lets consider an example of the operation of flow control.
|
|
Consider the flow of bytes from the remote computer at the other end
|
|
of the phone line to the screen on the text-terminal that I'm sitting
|
|
at. A real text-terminal has a limit to the speed at which bytes can
|
|
be displayed on its screen and issues a flow control "stop" from time
|
|
to time to slow down the flow. This "stop" propagates in a direction
|
|
opposite to the flow of bytes it controls. What happens when such a
|
|
"stop" is issued? Let's consider a case where the "stop" waits long
|
|
enough before canceling it with a "start", so that it gets thru to the
|
|
remote computer at the other end of the phone line. When it gets
|
|
there it will stop the program at the remote computer which is sending
|
|
out the bytes.
|
|
|
|
Let's trace out the flow of this "stop" (which may be "hardware" on
|
|
some links and "software" on others). First, suppose I'm "capturing"
|
|
a long file from the remote computer which is being sent
|
|
simultaneously to both my text-terminal and to a file on my hard-disk.
|
|
The bytes are coming in faster than the terminal can handle them so it
|
|
sends a "stop" out its serial port to a serial port on my PC. The
|
|
device driver detects it and stops sending bytes from the 8k PC serial
|
|
buffer (in main memory) to the terminal. But minicom still keeps
|
|
sending out bytes for the terminal into this 8k buffer.
|
|
|
|
When this 8k transmit buffer (on the first serial port) is full,
|
|
minicom must stop writing to it. Minicom stops and waits. But this
|
|
also causes minicom to stop reading from the 8k receive buffer on the
|
|
2nd serial port connected to the modem. Flow from the modem continues
|
|
until this 8k buffer too fills up and sends a different "stop" to the
|
|
modem. Now the modem's buffer ceases to send to the serial port and
|
|
also fills up. The modem (assuming error correction is enabled) sends
|
|
a "stop signal" to the other modem at the remote computer. This modem
|
|
stops sending bytes out of its buffer and when its buffer gets full,
|
|
another stop signal is sent to the serial port of the remote computer.
|
|
At the remote computer, the 8-k (or whatever) buffer fills up and the
|
|
program at the remote computer can't write to it anymore and thus
|
|
temporarily halts.
|
|
|
|
Thus a stop signal from a text terminal has halted a program on a
|
|
remote computer computer. What a long sequence of events! Note that
|
|
the stop signal passed thru 4 serial ports, 2 modems, and one
|
|
application program (minicom). Each serial port has 2 buffers (in one
|
|
direction of flow): the 8k one and the hardware 16-byte one. The
|
|
application program may have a buffer in its C_code. This adds up to
|
|
11 different buffers the data is passing thru. Note that the small
|
|
serial hardware buffers do not participate directly in flow control.
|
|
Also note that the two buffers associated with the text-terminal's
|
|
serial port are going to be dumping their contents to the screen
|
|
during this flow control halt. This leaves 9 other buffers that may
|
|
be getting filled up during the flow control halt.
|
|
|
|
If the terminal speed limitation is the bottleneck in the flow from
|
|
the remote computer to the terminal, then its flow control "stop" is
|
|
actually stopping the program that is sending from the remote computer
|
|
as explained above. But you may ask: How can a "stop" last so long
|
|
that 9 buffers (some of them large) all get filled up? It seldom
|
|
happens, but it can actually happen this way if all the buffers were
|
|
near their upper limits when the terminal sent out the "stop".
|
|
|
|
But if you were to run a simulation on this you would discover that
|
|
it's usually more complicated than this. At an instant of time some
|
|
links are flowing and others are stopped (due to flow control). A
|
|
"stop" from the terminal seldom propagates back to the remote computer
|
|
neatly as described above. It may take a few "stops" from the
|
|
terminal to result in one "stop" at the remote computer, etc. To
|
|
understand what is going on you really need to observe a simulation
|
|
which can be done for a simple case with coins on a table. Use only a
|
|
few buffers and set the upper level for each buffer at only a few
|
|
coins.
|
|
|
|
Does one really need to understand all this? Well, understanding this
|
|
explained to me why capturing text from a remote computer was loosing
|
|
text. The situation was exactly the above example but modem-to-modem
|
|
flow control was disabled. Chunks of captured text that were supposed
|
|
to also get to my hard-disk never got there because of an overflow at
|
|
my modem buffer due to flow control "stops" from the terminal. Even
|
|
though the remote computer had a flow path to the hard-disk without
|
|
bottlenecks, the same flow also went to a terminal which issued flow
|
|
control "stops" with disastrous results for the branch of the flow
|
|
going to a file on the hard-disk. The flow to the hard-disk passed
|
|
thru my modem and since the overflow happened at the modem, bytes
|
|
intended for the hard-disk were lost.
|
|
|
|
|
|
|
|
4.9. Serial Driver Module
|
|
|
|
The device driver for the serial port is the software that operates
|
|
the serial port. It is now provided as a serial module. From kernel
|
|
2.2 on, this module will normally get loaded automatically if it's
|
|
needed. In earlier kernels, you had to have kerneld running in order
|
|
to do auto-load modules on demand. Otherwise the serial module needed
|
|
to be explicitly listed in /etc/modules. Before modules became
|
|
popular with Linux, the serial driver was usually built into the
|
|
kernel (and sometimes still is). If it's built-in don't let the
|
|
serial module load or else you will have two serial drivers running at
|
|
the same time. With 2 drivers there are all sorts of errors including
|
|
a possible "I/O error" when attempting to open a serial port. Use
|
|
"lsmod" to see if the module is loaded.
|
|
|
|
|
|
When the serial module is loaded it displays a message on the screen
|
|
about the existing serial ports (often showing a wrong IRQ). But once
|
|
the module is used by setserial to tell the device driver the
|
|
(hopefully) correct IRQ then you should see a second display similar
|
|
to the first but with the correct IRQ, etc. See ``Serial Module'' See
|
|
``What is Setserial'' for more info on setserial.
|
|
|
|
|
|
|
|
4.10. The Serial Port is Now Obsolete on PCs
|
|
|
|
The serial port is today (2011) sort of obsolete on PCs (and often
|
|
called a "legacy" device) but it is still in use on some older
|
|
computers and is used in imbedded systems, for communication with
|
|
routers and point-of-sale equipment, etc. The serial port is ``slow''
|
|
and after about 2005 most new PC's no longer had them. Most laptops
|
|
and Macs discontinued them even earlier. During the era when some new
|
|
PC's came with serial ports and others didn't, the PC's that didn't
|
|
have serial ports were euphemistically called "legacy-free". However,
|
|
while PC's today no longer have serial ports, some do have them built
|
|
into a "legacy" modem (which plugs into a telephone line). Such a
|
|
serial port only works with the modem and can't be used for any other
|
|
device. The reason they have such a "built in" serial port is that
|
|
analog modems are designed to only work thru a serial port.
|
|
|
|
The physical serial port on the back of a PC (including the chip its
|
|
connected to inside the PC), must pass data between the computer and
|
|
an external cable. Thus it has two interfaces: the serial-port-to
|
|
cable and the serial-port-to-computer-bus. Both of these interfaces
|
|
are slow. First we'll consider the interface via external cable to
|
|
the outside world.
|
|
|
|
|
|
4.11. RS-232 Cable Is Low Speed & Short Distance
|
|
|
|
The conventional RS-232 serial port is inherently low speed and is
|
|
severely limited in distance. Ads often read "high speed" but it can
|
|
only work at "high speed" over very short distances such as to a modem
|
|
located right next to the computer. Compared to a network card, even
|
|
this "high speed" is actually low speed. All of the RS-232 serial
|
|
cable wires use a common ground return wire so that twisted-pair
|
|
technology (needed for high speeds) can't be used without additional
|
|
hardware. More modern interfaces for serial ports exist but they are
|
|
not standard on PC's like the RS-232 is. See ``Successors to
|
|
RS-232''. Some multiport serial cards support them.
|
|
|
|
It is somewhat tragic that the RS-232 standard from 1969 did not use
|
|
twisted pair technology which could operate about a hundred times
|
|
faster. Twisted pairs have been used in telephone cables since the
|
|
late 1800's. In 1888 (over 120 years ago) the "Cable Conference"
|
|
reported its support of twisted-pair (for telephone systems) and
|
|
pointed out its advantages. But over 80 years after this approval by
|
|
the "Cable Conference", RS-232 failed to utilize it. Since RS-232
|
|
was originally designed for connecting a terminal to a low speed modem
|
|
located nearby, the need for high speed and longer distance
|
|
transmission was apparently not recognized. The result was that since
|
|
the serial port couldn't handle high speeds, new types of serial
|
|
interfaces were devised that could: Ethernet, USB, Firewire, etc. The
|
|
final outcome was the demise of the serial port which due to it's
|
|
ancient technology became obsolete for high speed uses.
|
|
|
|
|
|
|
|
4.12. Inefficient PCI Interface to the Computer (in some cases)
|
|
|
|
The serial port communicates with the computer via the PCI bus, the
|
|
LPC bus, X-bus, or ISA bus. The PCI bus is now 32 or 64 bits wide,
|
|
but the serial port only sends a byte at a time (8 bits wide) which is
|
|
a waste of PCI bus bandwidth. Not so for the LPC bus which has only a
|
|
4-bit wide bus and thus provides an efficient interface. The ISA bus
|
|
is usually 16-bits wide and the efficiency is intermediate as compared
|
|
to efficient LPC and inefficient PCI.
|
|
|
|
|
|
5. Multiport Serial Boards/Cards/Adapters
|
|
|
|
5.1. Intro to Multiport Serial
|
|
|
|
Multiport serial cards install in slots in a PC on the ISA or PCI bus.
|
|
They are also called "... adapters" or "... boards". Each such card
|
|
provides you with many serial ports. Today they are commonly used for
|
|
the control of external devices (including automation for both
|
|
industry and the home). They can connect to computer servers for the
|
|
purpose of monitoring/controlling the server from a remote location.
|
|
They were once mainly used for connecting up many dumb terminals
|
|
and/or modems to serial ports. Today, use of dumb terminals has
|
|
declined, and several modems (or digital modems) can now be built into
|
|
an internal card. So multiport serial cards are not as significant as
|
|
they once were.
|
|
|
|
Each multiport card has a number of external connecters (DB-25 or
|
|
RJ45) so that one may connect up a number of devices (modems,
|
|
terminals, etc.). Each such physical device would then be connected
|
|
to its own serial port. Since the space on the external-facing part
|
|
of the card is limited there is often not enough room for all the
|
|
serial port connectors. To solve this problem, the connectors may be
|
|
on the ends of cables which come out (externally) from the card
|
|
(octopus cable). Or they may be on an external box (possibly rack
|
|
mountable) which is connected by a cable to a multiport card.
|
|
|
|
|
|
|
|
5.2. Dumb vs. Smart Cards
|
|
|
|
Dumb multiport cards are not too much different than ordinary serial
|
|
ports. They are interrupt driven and the CPU of the computer does
|
|
most all the work servicing them. They usually have a system of
|
|
sharing a single interrupt for all the ports. This doesn't decrease
|
|
the load on the CPU since the single interrupt will be sent to the CPU
|
|
each time any one port needs servicing. Such devices usually require
|
|
special drivers that you must either compile into the kernel or use as
|
|
a module.
|
|
|
|
Smart boards may use ordinary UARTs but handle most interrupts from
|
|
the UARTs internally within the board. This frees the CPU from the
|
|
burden of handling all these interrupts. The board may save up bytes
|
|
in its large internal FIFOs and transfer perhaps 1k bytes at a time to
|
|
the serial buffer in main memory. It may use the full bus width of 32
|
|
bits for making data transfers to main memory (instead of transferring
|
|
only 8-bit bytes like dumb serial cards do). Not all "smart" boards
|
|
are equally efficient. Many boards today are Plug-and-Play.
|
|
|
|
|
|
5.3. Getting/Enabling a Driver
|
|
|
|
5.3.1. Introduction
|
|
|
|
For a multiport board to work, a special driver for it must be used.
|
|
This driver may either be built into the kernel source code or
|
|
supplied as a module. For the 2.6 kernels on, most drivers are
|
|
supplied both ways: as a module or it can be built into the kernel.
|
|
Take care not to both build support into the kernel and force the
|
|
module to load for a certain serial card. For older kernels, there
|
|
were often no modules for dumb serial multiport boards so support was
|
|
built into the kernel.
|
|
|
|
|
|
5.3.2. Build (compile) support into the kernel?
|
|
|
|
A pre-compiled kernel may not have a driver for your multiport card
|
|
built in. So then you must either compile the kernel yourself and
|
|
build in the right driver, or insure that the module is available and
|
|
loads. Of course if the driver doesn't come both ways (as a compile-
|
|
time option and as a module) you have no such choice.
|
|
|
|
If you want to see what has already been compiled into an existing
|
|
working kernel, go the the /boot directory (or wherever the compiled
|
|
kernel(s) reside) and look in the config... file.
|
|
|
|
In the 2.6 kernel there are many options to select from in the
|
|
configuration file for compiling. Adding support for certain
|
|
multiport cards is listed under the headings "Character devices" or
|
|
"Serial drivers". Old multiport cards had support as part of the
|
|
serial driver and are found under "Serial Drivers". More advanced
|
|
cards have their own driver found under "Character devices"
|
|
|
|
For compiling kernel 2.6 you should select
|
|
"CONFIG_SERIAL_8250_EXTENDED". (or just "CONFIG_SERIAL_EXTENDED" for
|
|
2.4). Then you will be asked more questions about your serial ports
|
|
with more options to select. If the resulting configuration is not
|
|
quite right, then you may need to edit the kernel configuration file
|
|
manually.
|
|
|
|
|
|
5.3.3. Using module support
|
|
|
|
A pre-compiled kernel may come with a pre-compiled module for the
|
|
board so that you don't need to recompile the kernel. This module
|
|
must be loaded in order to use it and if there is installation
|
|
software for the driver, it should also set up Linux to load the
|
|
module (probably at boottime). Some of the modules to load at
|
|
boottime are listed in /etc/modules or /etc/modules.conf Also certain
|
|
parameters may need to be passed to the driver via entries in these
|
|
files or via lilo's "append" command or via grub's "kernel" command.
|
|
For kernel 2.6 (and 2.4) the (unloaded) modules should be found in
|
|
/lib/modules/.../kernel/drivers/char.
|
|
|
|
|
|
5.3.4. Getting info on multiport boards
|
|
|
|
The board's manufacturer should have info on their website.
|
|
Unfortunately, info for old boards is sometimes not there but might be
|
|
found somewhere else on the Internet (including discussion groups).
|
|
You might also want to look at the kernel documentation in
|
|
/usr/share/doc/linux-doc... (formerly kernel-doc in pre 2.6 kernels).
|
|
For configuring the kernel or modules prior to compiling see:
|
|
Configure.help and search for "serial", etc. There are also kernel
|
|
documentation files for certain boards including computone, hayes-esp,
|
|
moxa-smartio, riscom8, specialix, stallion, and sx (specialix).
|
|
|
|
|
|
5.4. Multiport Devices in the /dev Directory,
|
|
|
|
The serial ports your multiport board uses depends on what kind of
|
|
board you have. Some have their own device names like /dev/ttyE27
|
|
(Stallion) or /dev/ttyD2 (Digiboard), etc. For various other brands,
|
|
see see devices.txt in the kernel documentation. Some use the
|
|
standard names like /dev/ttyS14 and may be found in configuration
|
|
files that used as arguments to setserial. Such files may be included
|
|
in a setserial or serial package.
|
|
|
|
|
|
5.5. Making Legacy Multiport Devices in the /dev Directory
|
|
|
|
An installation script may do this for you. But if not, here's some
|
|
examples of how to create a device name in the /dev directory. If you
|
|
use udev, MAKEDEV will not create devices in the device directory
|
|
since this directory is only in memory and will be lost when you turn
|
|
off the computer. Instead it will create the device indev/.static/dev
|
|
directory.
|
|
|
|
For the names and numbers of other types of serial ports other than
|
|
ttyS.. See devices.txt in the kernel documentation. Either use the
|
|
mknod command, or the MAKEDEV script. Typing "man makedev" may show
|
|
instructions on using it.
|
|
|
|
Using the MAKEDEV script, you would first become the superuser (root)
|
|
and type (for example) either:
|
|
|
|
|
|
|
|
linux# MAKEDEV ttyS17
|
|
|
|
|
|
|
|
Or if the above doesn't work cd to /dev before giving the above
|
|
command>. Substitute whatever your port is for ttyS17.
|
|
|
|
Using mknod is a more complicated option since you need to know the
|
|
major and minor device numbers. These numbers are in the "devices"
|
|
file in the kernel documentation. For ttyS serial ports the minor
|
|
number is: 64 + port number (=81 for the example below). Note the
|
|
"major" number is always 4 for ttyS devices (and 5 for the obsolete
|
|
cua devices). So, if you wanted to create a device for ttyS17 using
|
|
mknod, you would type:
|
|
|
|
|
|
|
|
linux# mknod -m 666 /dev/ttyS17 c 4 81
|
|
|
|
|
|
|
|
5.6. Standard PC Serial Cards
|
|
|
|
In olden days, PCs came with a serial card installed. Later on, the
|
|
serial function was put on the hard-drive interface card. In the
|
|
1990s and early 2000s one or two serial ports were usually built into
|
|
the motherboard (on-board). Most of them (as of 2002) use a 16550 but
|
|
some use 16650 (32-byte FIFOs). But one may still buy the individual
|
|
PC serial cards if they need more serial ports. They can be used to
|
|
connect external serial devices (modems, serial mice, etc...). Only a
|
|
tiny percentage of retail computer stores carry such cards. But one
|
|
can purchase them on the Internet. Before getting one for the PCI
|
|
bus, make sure Linux supports it.
|
|
|
|
Here's a list of a few popular brands:
|
|
|
|
|
|
· Byte Runner (may order directly, shows prices)
|
|
<http://www.byterunner.com>
|
|
|
|
· SIIG <http://www.siig.com/products/io/>
|
|
|
|
· Dolphin <http://www.dolphinfast.com/sersol.html>
|
|
|
|
Note: due to address conflicts, you may not be able to use /dev/ttyS3
|
|
with a IBM8514 video card (and some others) simultaneously. See
|
|
``Avoiding IO Address Conflicts with Certain Video Boards''
|
|
|
|
|
|
5.7. Dumb Multiport Serial Boards (with standard UART chips)
|
|
|
|
They are also called "serial adapters". Each port has its own
|
|
address. They often have a special method of sharing interrupts which
|
|
requires that you compile support for them into the kernel.
|
|
|
|
* => The file that ran setserial in Debian shows some details of
|
|
configuring
|
|
# => See note below for this board
|
|
|
|
· AST FourPort and clones (4 ports) * #
|
|
|
|
· Accent Async-4 (4 ports) *
|
|
|
|
· Arnet Multiport-8 (8 ports)
|
|
|
|
· Bell Technologies HUB6 (6 ports)
|
|
|
|
· Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports; See
|
|
the Boca mini-howto revised in 2001) * #
|
|
|
|
· Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
|
|
sharing ?? Uses odd-ball 10-cond RJ45-like connectors)
|
|
|
|
· Boca 2by4 (4 serial ports, 2 parallel ports)
|
|
|
|
· Byte Runner <http://www.byterunner.com>
|
|
|
|
· Computone ValuePort V4-ISA (AST FourPort compatible) *
|
|
|
|
· Digi PC/8 (8 ports) #
|
|
|
|
· Dolphin <http://www.dolphinfast.com/sersol/>
|
|
|
|
· Globetek <http://www.globetek.com/>
|
|
|
|
· GTEK BBS-550 (8 ports; See the mini-howto)
|
|
|
|
· Hayes ESP (after kernel 2.1.15)
|
|
|
|
· HUB-6 See Bell Technologies.
|
|
|
|
· Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible) *
|
|
|
|
· Moxa C104, Moxa C104+ (AST FourPort compatible) *
|
|
|
|
· NI-SERIAL
|
|
<http://digital.natinst.com/manuals.nsf/web%2Fbyproductcurrent?OpenView&Start=1&Count=500&Expand=15.1#15.1>
|
|
by National Instruments
|
|
|
|
· NetBus (2 ports) <http://www.netbus.com> using patch from
|
|
<http://lists.insecure.org/linux-kernel/2001/Feb/2809.html>
|
|
Discontinued.
|
|
|
|
· PC-COMM (4 ports)
|
|
|
|
· Sealevel Systems <http://www.sealevel.com> COMM-2 (2 ports), COMM-4
|
|
(4 ports) and COMM-8 (8 ports)
|
|
|
|
· SIIG I/O Expander 2S IO1812 (4 ports) #
|
|
|
|
· STB-4COM (4 ports)
|
|
|
|
· Twincom ACI/550
|
|
|
|
· Usenet Serial Board II (4 ports) *
|
|
|
|
· VScom (uses same driver as ByteRunner)
|
|
|
|
In general, Linux will support any serial board which uses a 8250,
|
|
16450, 16550, 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and
|
|
16954. UART. See the latest man page for "setserial" for a more
|
|
complete list.
|
|
|
|
Notes:
|
|
|
|
AST Fourport: You might need to specify skip_test in rc.serial.
|
|
|
|
BB-1004 and BB-1008 do not support DCD and RI lines, and thus are not
|
|
usable for dialin modems. They will work fine for all other purposes.
|
|
|
|
Digi PC/8 Interrupt Status Register is at 0x140.
|
|
|
|
SIIG IO1812 manual for the listing for COM5-COM8 is wrong. They
|
|
should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.
|
|
|
|
|
|
5.8. Intelligent Multiport Serial Boards
|
|
|
|
Make sure that a Linux-compatible driver is available and read the
|
|
information that comes with it. These boards use special devices (in
|
|
the /dev directory), and not the standard ttyS ones. This information
|
|
varies depending on your hardware. If you have updated info which
|
|
should be shown here please email it to me.
|
|
|
|
Names of Linux driver modules are *.ko (*.o prior to kernel 2.6) but
|
|
these may not work for all models shown. See ``Modules (mostly for
|
|
smart boards)'' The needed module may have been supplied with your
|
|
Linux distribution. Also, parameters (such as the io and irq often
|
|
need to be given to the module so you need to find instructions on
|
|
this (possibly in the source code tree).
|
|
|
|
There are many different brands, each of which often offers many
|
|
different cards. No attempt is currently being made to list all the
|
|
cards here (and many listed are obsolete and have bad internet links
|
|
to them which need to be fixed). But all major brands and websites
|
|
should be shown here so it something is missing let me know. Go to
|
|
the webpage shown for more information. These websites often also
|
|
have info (ads) on related hardware such as modem pools, remote access
|
|
servers (RASs), and terminal servers. Where there is no webpage, the
|
|
cards are likely obsolete. If you would like to put together a better
|
|
list, let me know.
|
|
|
|
|
|
· Chase Research, now Perle Systems Ltd (UK based, ISA/PCI cards)
|
|
webpage: <http://www.perle.com>
|
|
driver status: included in kernel 2.4+ for PCI only; otherwise
|
|
supported by Perle
|
|
driver and manual location:
|
|
<http://www.perle.com/downloads/multi_port.shtml>
|
|
· Comtrol RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)
|
|
webpage: http://www.comtrol.com
|
|
driver status: supported by Comtrol. rocket.o
|
|
driver location: ftp://tsx-11.mit.edu/pub/linux/packages/comtrol
|
|
|
|
· Computone IntelliPort II (ISA, PCI and EISA busses up to 64 ports)
|
|
webpage: <http://www.computone.com>
|
|
driver location: old patch at
|
|
<http://www.wittsend.com/computone/linux-2.2.10-ctone.patch.gz>
|
|
mailing list: <mailto:majordomo@lazuli.wittsend.com> with
|
|
"subscribe linux-computone" in body
|
|
note: Old ATvantage and Intelliport cards are not supported by
|
|
Computone
|
|
|
|
· Connecttech
|
|
website: <http://www.connecttech.com/>
|
|
driver location: <ftp://ftp.connecttech.com/pub/linux/>
|
|
|
|
· Cyclades
|
|
Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
|
|
Cyclom-Z (MIPS R3000; 8 - 64 ports)
|
|
website: <http://www.cyclades.com/products/svrbas/zseries.php>
|
|
driver status: supported by Cyclades
|
|
driver location: ftp://ftp.cyclades.com/pub/cyclades and included
|
|
in Linux kernel since version 1.1.75: cyclades.o
|
|
|
|
· Decision PCCOM (2-8 ports; ISA and PCI; aka PC COM)
|
|
ISA:
|
|
contact: <mailto:info@cendio.se>
|
|
driver location: (dead link) ftp://ftp.cendio.se/pub/pccom8
|
|
PCI:
|
|
drivers: <http://www.decision.com.tw>
|
|
driver status: Support in serial driver 5.03. For an earlier
|
|
driver, there exists a patch for kernel 2.2.16 at
|
|
<http://www.qualica.com/serial/> and for kernels 2.2.14-2.2.17 at
|
|
<http://www.pccompci.com/mains/installing_pci_linux1.html>
|
|
|
|
· Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),
|
|
PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),
|
|
PC/Xr (16MHz IDT3041; 4 or 8 ports),
|
|
PC/Xem (20MHz IDT3051; 8 - 64 ports)
|
|
website: <http://www.dgii.com>
|
|
driver status: supported by Digi
|
|
driver location: ftp://ftp.dgii.com/drivers/linux and included in
|
|
Linux kernel since version 2.0. epca.o
|
|
|
|
· Digi COM/Xi (10MHz 80188; 4 or 8 ports)
|
|
contact: Simon Park, si@wimpol.demon.co.uk
|
|
driver status: ?
|
|
note: Simon is often away from email for months at a time due to
|
|
his job. Mark Hatle, <mailto:fray@krypton.mankato.msus.edu> has
|
|
graciously volunteered to make the driver available if you need it.
|
|
Mark is not maintaining or supporting the driver.
|
|
|
|
· Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)
|
|
website: http://www.equinox.com
|
|
driver status: supported by Equinox
|
|
driver location: ftp://ftp.equinox.com/library/sst
|
|
|
|
· Globetek
|
|
website: <http://www.globetek.com/products.shtml>
|
|
driver location:
|
|
<http://www.globetek.com/media/files/linux.tar.gz>
|
|
|
|
· GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),
|
|
SmartCard (24MHz Dallas DS80C320; 8 ports),
|
|
BlackBoard-8A (16C654 UARTs; 8 ports),
|
|
PCSS (15/24MHz 8032; 8 ports)
|
|
website: http://www.gtek.com
|
|
driver status: supported by GTEK
|
|
driver location: ftp://ftp.gtek.com/pub
|
|
|
|
· Hayes ESP (COM-bic; 1 - 8 ports)
|
|
website: http://www.nyx.net/~arobinso
|
|
driver status: Supported by Linux kernel (1998) since v. 2.1.15.
|
|
esp.o. Setserial 2.15+ supports. Also supported by author
|
|
driver location: http://www.nyx.net/~arobinso
|
|
|
|
· Intelligent Serial Interface by Multi-Tech Systems
|
|
PCI: 4 or 8 port. ISA 8 port. DTE speed 460.8k. Discontinued
|
|
webpage: <http://www.multitech.com/en_US/products/>
|
|
|
|
· Maxpeed SS (Toshiba; 4, 8 and 16 ports)
|
|
website: http://www.maxpeed.com
|
|
driver status: supported by Maxpeed
|
|
driver location: ftp://maxpeed.com/pub/ss
|
|
|
|
· Microgate SyncLink ISA and PCI high speed multiprotocol serial.
|
|
Intended for synchronous HDLC.
|
|
website: <http://ww/microgate.com/products/sllinux/hdlcapi.htm>
|
|
driver status: supported by Microgate: synclink.o
|
|
|
|
· Moxa C218 (12MHz 80286; 8 ports),
|
|
Moxa C320 (40MHz TMS320; 8 - 32 ports)
|
|
website: http://www.moxa.com
|
|
driver status: supported by Moxa
|
|
driver locations:
|
|
<http://www.moxa.com/support/download/download.php3>>
|
|
<ftp://ftp.moxa.com/drivers/linux> (also from Taiwan at
|
|
www.moxa.com.tw/...) where ... is the same as above)
|
|
|
|
· SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
|
|
website: http://www.sdlcomm.com
|
|
driver status: supported by SDL
|
|
driver location: ftp://ftp.sdlcomm.com/pub/drivers
|
|
|
|
· Specialix SX (25MHz T225; 8? - 32 ports),
|
|
SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)
|
|
webpage: Old link is broken. Out of business?
|
|
driver status: Was supported by Specialix
|
|
driver location: <http://www.BitWizard.nl/specialix/>
|
|
old driver location:
|
|
<ftp://metalab.unc.edu/pub/Linux/kernel/patches/serial>
|
|
|
|
· Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and
|
|
EasyConnection (8 - 32 ports) - each with Cirrus Logic CD1400
|
|
UARTs,
|
|
Stallion (8MHz 80186 CPU; 8 or 16 ports),
|
|
Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),
|
|
ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),
|
|
EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)
|
|
contact: sales@stallion.com or http://www.stallion.com
|
|
driver status: supported by Stallion
|
|
driver location: ftp://ftp.stallion.com/drivers/ata5/Linux and
|
|
included in linux kernel since 1.3.27. Moved: it's now at ?.
|
|
|
|
· System Base website: <http://www.sysbas.com/>
|
|
|
|
|
|
|
|
A review of Comtrol, Cyclades, Digi, and Stallion products was printed
|
|
in the June 1995 issue of the Linux Journal. The article is available
|
|
at Review: Intelligent Multiport Serial Boards
|
|
<http://m.linuxjournal.com/article/1097> Besides the listing of
|
|
various brands of multiports found above in this HOWTO there is Gary's
|
|
Encyclopedia - Serial Cards <http://eupedia.org/serialcards.html>.
|
|
It's not as complete, but may have some different links.
|
|
|
|
|
|
5.9. Unsupported Multiport Boards
|
|
|
|
The following brands that formerly made boards for with Linux support
|
|
don't mention any Linux support as of 1 Jan. 2000. Let me know if
|
|
this changes.
|
|
|
|
· Aurora (PCI only) <http://sie-cs.com/en/browse/product/serial-io>
|
|
|
|
|
|
|
|
6. Servers for Serial Ports
|
|
|
|
A computer that has many serial ports (with many serial cables
|
|
connected to it) is often called a server. Of course, most servers
|
|
serve other functions besides just serving serial ports, and many do
|
|
not serve serial ports at all (although they likely have a serial port
|
|
on them). For example, a "serial server" may have serial cables, each
|
|
of which runs to a different (non-serial) server. The serial server
|
|
(perhaps called a "console server") controls, via a console, all the
|
|
other servers. The console may be physically located remote from the
|
|
serial server, communicating with the server over a network.
|
|
|
|
There are two basic types of serial servers. One type is just an
|
|
ordinary computer (perhaps rack mounted) that uses multiport cards on
|
|
a PCI bus (or the like). The other type is a proprietary server that
|
|
is a dedicated computer that serves a special purpose. Servers of
|
|
both types may be called: serial servers, console servers, print
|
|
servers, or terminal servers. They are not the same.
|
|
|
|
The terminal server was originally designed to provide many serial
|
|
ports, each connected to a dumb text-terminal. Today, a terminal
|
|
server often connects to graphic terminals over a fast network and
|
|
doesn't use serial ports since they are too slow. One network cable
|
|
takes the place of many serial cables and each graphic terminal uses
|
|
far more bandwidth than the text-terminals did. However, graphic
|
|
terminals may be run in text mode to reduce the bandwidth required. A
|
|
more detailed discussion of terminal servers (serial port) is in Text-
|
|
Terminal-HOWTO. For networked terminal servers (not serial port) see
|
|
Linux Terminal Server Project (LTSP) <http://www.ltsp.org/index.php>
|
|
|
|
(To-do: Discuss other types of serial servers, but the author knows
|
|
little about them.)
|
|
|
|
|
|
7. Configuring Overview
|
|
|
|
In most cases configuring is automatically taken care of and you have
|
|
nothing to do. Linux should detect your serial ports, and load driver
|
|
modules if needed. Then the driver should insure that IRQ and address
|
|
space resources have been allocated. Finally, the application program
|
|
which uses the serial port should set the port speed, etc.
|
|
|
|
For any of this to work, serial support must either be compiled into
|
|
the kernel (by you or by whoever compiled your kernel) or provided by
|
|
modules that are loaded into the kernel when you start to use the
|
|
serial port. In most cases, if it hasn't been compiled into the
|
|
kernel, a module(s) will do the job and Linux should hopefully
|
|
automatically find and load the correct modules.
|
|
|
|
But if you have more than 4 (or perhaps 2) serial ports, then the
|
|
kernel must be told this as it doesn't seem to do it automatically.
|
|
See ``Number of Serial Ports Supported'', ``Kernel Configuration''
|
|
and. ``Serial Modules''.
|
|
|
|
Once the proper support is in your kernel and modules, then The rest
|
|
of the configuring of the serial port should happen automatically.
|
|
This is done by the serial driver software often with help from your
|
|
application software. But sometimes it doesn't get configured right
|
|
and then you need to do it yourself. Or perhaps you need to configure
|
|
it in a special way, etc. This HOWTO only covers configuration of the
|
|
serial port itself and not the configuring of any devices attached to
|
|
the port (such as a modem or printer).
|
|
|
|
Resource allocation (locating the hardware or low-level configuring)
|
|
is assigning each port an IO address, IRQ, and name (such as ttyS2).
|
|
This IO-IRQ pair must be set in both the hardware and become known to
|
|
the serial driver. We might just call this "io-irq" configuring for
|
|
short. The "setserial" program is sometimes used to tell the driver
|
|
io-irq info that an administrator has put in a configuration file or
|
|
given as parameters to the setserial command. PnP methods, jumpers,
|
|
etc, are used to set the IO and IRQ in the hardware. Details will be
|
|
supplied later. If you need to configure but don't understand certain
|
|
details it's easy to get into trouble. See ``Locating the Serial
|
|
Port: IO address IRQs'' ``What is Setserial''
|
|
|
|
The second part (high-level configuring) is assigning it a speed (such
|
|
as 115.2k bits/sec), selecting flow control, etc. This is often
|
|
initiated by communication programs such as wvdial, PPP, minicom,
|
|
picocom or by getty (which you may run on the port so that others may
|
|
log into your computer from an old-fashioned dumb terminal connected
|
|
to the port). However you will need to tell these programs what speed
|
|
you want, etc. by using a menu or a configuration file. This high-
|
|
level configuring may also be done manually with the stty program.
|
|
stty is also useful to view the current status if you're having
|
|
problems. See the section ``Stty''
|
|
|
|
|
|
8. Locating the Serial Port: IO address, IRQs
|
|
|
|
8.1. What Bus is my Serial Port On?
|
|
|
|
If you need to find a serial port it often helps if you know what bus
|
|
it's on. If the serial port is on a card, you may know what bus the
|
|
card inserts into such as a PCI slot. But if the serial port is built
|
|
into the motherboard it may not be clear what bus it's on. For old
|
|
motherboards that have ISA bus slots, it's likely on the ISA bus and
|
|
may not even be Plug-and-Play. But even if all your slots are PCI,
|
|
the serial port is likely to be on either an ISA bus or LPC bus (also
|
|
called a "LPC interface"). LPC is common on laptop computers. Type
|
|
"lspci" to see if it shows "LPC". Unfortunately, the LPC bus has no
|
|
standard Plug-and-Play method for low-level configuring devices on it.
|
|
But according to the specs, the BIOS is supposed to do such
|
|
configuring (using ACPI ??). To see if you have a LPC bus, type
|
|
"lspci" and look for a LPC bridge or the like.
|
|
|
|
|
|
8.2. IO & IRQ Overview
|
|
|
|
For a serial port to work properly it first must be given both an IO
|
|
address and an IRQ. For old hardware (of mid 1990s), jumpers on a
|
|
card or a saved BIOS setting does it. For newer hardware the BIOS or
|
|
Linux must set them at boot-time, and the new hardware doesn't
|
|
remember how it was set once it's powered down. Enabling the hardware
|
|
(by using software) gives it both an IRQ and an IO address. Without
|
|
an IO address, it can't be used. Without an IRQ it will need to use
|
|
inefficient polling methods for which one must set the IRQ to 0 in the
|
|
serial driver. Using digital signals sent to the hardware by the BIOS
|
|
or Linux, it all should get configured automatically (provided the
|
|
BIOS has not been previously set up to disable it). So you only need
|
|
to read this if you're having problems or if you want to understand
|
|
how it works.
|
|
|
|
The driver must of course know both the IO address and IRQ so that it
|
|
can talk to the serial port chip. Modern serial port drivers (kernel
|
|
2.4) try to determine this by PnP methods so one doesn't normally need
|
|
to tell the driver (by using "setserial"). A driver may also set an
|
|
IO address or IRQ in the hardware. But unfortunately, there is some
|
|
PCI serial port hardware that the driver doesn't recognize so you
|
|
might need to enable the port yourself. See ``PCI: Enabling a
|
|
disabled port''
|
|
|
|
For the old ISA bus, the driver also probes likely serial port
|
|
addresses to see if there are any serial ports there. This works for
|
|
the case of jumpers and sometimes works for a ISA PnP port when the
|
|
driver doesn't do ISA PnP (prior to kernel 2.4).
|
|
|
|
Locating the serial port by giving it an IRQ and IO address is low-
|
|
level configuring. It's often automatically done by the serial driver
|
|
but sometimes you have to do it yourself. What follows repeats what
|
|
was said above but in more detail.
|
|
|
|
The low-level configuring consists of assigning an IO address, IRQ,
|
|
and names (such as ttyS2) . This IO-IRQ pair must be set in both the
|
|
hardware and told to the serial driver. And the driver needs to call
|
|
this pair a name (such as ttyS2). We could call this "io-irq"
|
|
configuring for short. The modern way to do this is for the driver to
|
|
use PnP methods to detect/set the IO/IRQ and then remember what it
|
|
did. An easy way for a driver to do this is for the driver to ask the
|
|
kernel to enable the device and then the kernel tells the driver what
|
|
IO/IRQ it has used. The old way is using the "setserial" program to
|
|
tell the driver. For jumpers, there is no PnP but the driver might
|
|
detect the port if the jumpers are set to the usual IO/IRQ. If you
|
|
need to configure but don't understand certain details it's easy to
|
|
get into trouble.
|
|
|
|
When Linux starts, an effort is made to detect and configure (low-
|
|
level) the serial ports. Exactly what happens depends on your BIOS,
|
|
hardware, Linux distribution, kernel version, etc. If the serial
|
|
ports work OK, there may be no need for you to do any more low-level
|
|
configuring.
|
|
|
|
If you're having problems with the serial ports, then you may need to
|
|
do low-level configuring. If you have kernel 2.2 or lower, then you
|
|
need to do it if you:
|
|
|
|
|
|
· Plan to use more than 2 ISA serial ports
|
|
|
|
· Are installing a new serial port (such as an internal modem)
|
|
|
|
· One or more of your serial ports have non-standard IRQs or IO
|
|
addresses
|
|
|
|
Starting with kernel 2.2 you may be able to use more than 2 serial
|
|
ports without doing any low-level configuring by sharing interrupts.
|
|
All PCI ports should support this but for ISA it only works for some
|
|
hardware. It may be just as easy to give each port a unique interrupt
|
|
if they are available. See ``Interrupt sharing and Kernels 2.2+''
|
|
|
|
The low-level configuring (setting the IRQ and IO address) seems to
|
|
cause people more trouble than the high-level stuff, although for many
|
|
it's fully automatic and there is no configuring to be done. Until
|
|
the port is enabled and the serial driver knows the correct IRQ and IO
|
|
address, the port will not usually not work at all.
|
|
|
|
A port may be disabled, either by the BIOS or by failure of Linux to
|
|
find and enable the port. For modern ports (provided the BIOS hasn't
|
|
disabled them) manual PnP tools such as lspci should be able to find
|
|
them. Applications, and utilities such as "setserial" and "scanport"
|
|
(Debian only ??) only probe IO addresses, don't use PnP tools, and
|
|
thus can't detect disabled ports.
|
|
|
|
Even if an ISA port can be found by the probing done by the serial
|
|
driver it may work extremely slow if the IRQ is wrong. See
|
|
``Extremely Slow: Text appears on the screen slowly after long
|
|
delays''. PCI ports are less likely to get the IRQ wrong.
|
|
|
|
IO address, IRQs, etc. are called "resources" and we are thus
|
|
configuring certain resources. But there are many other types of
|
|
"resources" so the term has many other meanings. In summary, the low-
|
|
level configuring consists of enabling the device, giving it a name
|
|
(ttyS2 for example) and putting two values (an IRQ number and IO
|
|
address) into two places:
|
|
|
|
|
|
1. The device driver (done by PnP or "setserial")
|
|
|
|
2. Configuration registers of the serial port hardware itself, done by
|
|
PnP software (or jumpers on legacy hardware).
|
|
|
|
You may watch the start-up (= boot-time) messages. They are usually
|
|
correct. But if you're having problems, your serial port may not show
|
|
up at all or if you do see a message from "setserial" it may not show
|
|
the true configuration of the hardware (and it is not necessarily
|
|
supposed to). See ``I/O Address & IRQ: Boot-time messages''.
|
|
|
|
|
|
8.3. PCI Bus Support
|
|
|
|
8.3.1. Introduction
|
|
|
|
|
|
|
|
If you have kernel 2.4 or better, then there should be support for PnP
|
|
for both the PCI and ISA buses (either built-in or by modules). Some
|
|
PCI serial ports can be automatically detected and low-level
|
|
configured by the serial driver. Others may not be.
|
|
|
|
While kernel 2.2 supported PCI in general, it had no support for PCI
|
|
serial ports (although some people got them working anyway). Starting
|
|
with kernel 2.4, the serial driver will read the id number digitally
|
|
stored in the serial hardware to determine how to support it (if it
|
|
knows how). It should assign an I/O address to it, determine its IRQ,
|
|
etc. So you don't need to use "setserial" for it.
|
|
|
|
There is a possible problem if you don't use the device filesystem.
|
|
The driver may assign the port to say tt/ttyS4 per a boot-time message
|
|
(use dmesg to see it). But if you don't have a "file" /dev/ttyS4 then
|
|
the port will not work. So you will then need to create it, using
|
|
cd /dev; ./MAKEDEV ttyS4
|
|
|
|
|
|
8.3.2. More info on PCI
|
|
|
|
PCI ports are not well standardized. Some use main memory for
|
|
communication with the PC. Some require special enabling of the IRQ.
|
|
The output of "lspci -vv" can help determine if one can be supported.
|
|
If you see a 4-digit IO port, the port might work by just telling
|
|
"setserial" the IO port and the IRQ. For example, if lspci shows IRQ
|
|
10, I/O at 0xecb8 and you decide to name it ttyS2 then the command is:
|
|
|
|
setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig
|
|
|
|
Note that the boot-time message "Probing PCI hardware" means reading
|
|
the PnP configuration registers in the PCI devices which detects info
|
|
about all PCI cards and on-board PCI devices. This is different than
|
|
the probing of IO addresses by the serial driver which means reading
|
|
certain IO addresses to see if what's read looks like there's a serial
|
|
port at that address.
|
|
|
|
|
|
8.4. Common mistakes made re low-level configuring
|
|
|
|
Here are some common mistakes people make:
|
|
|
|
· setserial command: They run it (without the "autoconfig" and
|
|
auto_irq options) and think it has checked the hardware to see if
|
|
what it shows is correct (it hasn't).
|
|
|
|
· setserial messages: They see them displayed on the screen at boot-
|
|
time (or by giving the setserial command) and erroneously think
|
|
that the result always shows how their hardware is actually
|
|
configured.
|
|
|
|
· /proc/interrupts: When their serial device isn't in use they don't
|
|
see its interrupt there, and erroneously conclude that their serial
|
|
port can't be found (or doesn't have an interrupt set).
|
|
|
|
· /proc/ioports and /proc/tty/driver/serial: People think this shows
|
|
the actual hardware configuration when it only shows about the same
|
|
info (possibly erroneous) as setserial.
|
|
|
|
|
|
8.5. IRQ & IO Address Must be Correct
|
|
|
|
There are really two answers to the question "What is my IO and IRQ?"
|
|
1. What the device driver thinks has been set (This is what setserial
|
|
usually sets and shows.). 2. What is actually set in the hardware.
|
|
Both 1. and 2. above should be the same. If they're not it spells
|
|
trouble since the driver has incorrect info on the physical serial
|
|
port. In some cases the hardware is disabled so it has no IO address
|
|
or IRQ.
|
|
|
|
If the driver has the wrong IO address it will try to send data to a
|
|
non-existing serial port --or even worse, to some other device. If it
|
|
has the wrong IRQ the driver will not get interrupt service requests
|
|
from the serial port, resulting in a very slow or no response. See
|
|
``Extremely Slow: Text appears on the screen slowly after long
|
|
delays''. If it has the wrong model of UART there is also apt to be
|
|
trouble. To determine if both IO-IRQ pairs are identical you must
|
|
find out how they are set in both the driver and the hardware.
|
|
|
|
|
|
8.6. What is the IO Address and IRQ per the driver ?
|
|
|
|
8.6.1. Introduction
|
|
|
|
What the driver thinks is not necessarily how the hardware is actually
|
|
set. If everything works OK then what the driver thinks is likely
|
|
correct (set in the hardware) and you don't need to investigate
|
|
(unless you're curious or want to become a guru). Ways to determine
|
|
what the driver thinks include: boot-time messages ``I/O Address &
|
|
IRQ: Boot-time messages'', the /proc directory "files" ``The /proc
|
|
directory and setserial'', and the "setserial" command.
|
|
|
|
|
|
|
|
8.6.2. I/O Address & IRQ: Boot-time messages
|
|
|
|
In many cases your ports will automatically get low-level configured
|
|
at boot-time (but not always correctly). To see what is happening,
|
|
look at the start-up messages on the screen. Don't neglect to check
|
|
the messages from the BIOS before Linux is loaded (no examples shown
|
|
here). These BIOS messages may be frozen by pressing the Pause key
|
|
(while holding down shift). It's often tricky to freeze them and you
|
|
may need to hit Ctrl-Alt-Del while Linux is booting to start rebooting
|
|
and try again. What these messages display may change as booting
|
|
progresses and it's often tricky to freeze it at exactly the right
|
|
words.
|
|
|
|
Use Shift-PageUp to scroll back to the messages after they have
|
|
flashed by. Shift-PageDown will scroll in the opposite direction.
|
|
The dmesg command (or looking at logs in /var/log) will show only the
|
|
first of these two messages. Here's an example of the start-up
|
|
messages (as of 2004, almost the same as for 1999). Note that in
|
|
older versions ttyS1 was displayed in these messages as ttyS01, etc.
|
|
|
|
At first you see what was detected (but the irq is only a wild guess):
|
|
|
|
|
|
|
|
Serial driver version 4.27 with no serial options enabled
|
|
ttyS0 at 0x03f8 (irq = 4) is a 16550A
|
|
ttyS1 at 0x02f8 (irq = 3) is a 16550A
|
|
ttyS2 at 0x03e8 (irq = 4) is a 16550A
|
|
ttyS4 at port 0xeff0 (irq = 10) is a 16550A
|
|
|
|
|
|
|
|
Note that ttyS0-ttyS2 were detected by probing the standard addresses
|
|
while ttyS4 is a PCI port detected by probing the PCI configuration.
|
|
Later setserial shows you what was saved in a configuration file
|
|
(which you may edit), but it's not necessarily correct either:
|
|
|
|
|
|
|
|
Loading the saved-state of the serial devices...
|
|
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
|
|
/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A
|
|
|
|
|
|
|
|
Note that the configuration file only had ttyS1-2 in it so that ttyS0
|
|
and ttyS4 were not affected by it. There is also a slight
|
|
discrepancy: The first message shows ttyS2 at irq=4 while the second
|
|
shows it at irq=5. In most cases the second message is the correct
|
|
one. But if you're having trouble, it may be misleading. Before
|
|
reading the explanation of all of this complexity in the rest of this
|
|
section, you might just try using your serial port and see if it works
|
|
OK. If so it may not be essential to read further.
|
|
|
|
The second message is from the setserial program being run at boot-
|
|
time from a script in the /etc directory tree. It shows what the
|
|
device driver thinks is the correct configuration. But this too could
|
|
be wrong. For example, the irq could actually be set to irq=8 in the
|
|
hardware (both messages wrong). The irq=5 could be there because the
|
|
configuration file is incorrect.
|
|
With old jumper-set serial ports Linux sometimes gets IRQs wrong
|
|
because it doesn't by default probe for IRQs. It just assumes the
|
|
"standard" ones (first message) or accepts what is in a configuration
|
|
file (second message). Neither of these is necessarily correct. If
|
|
the serial driver has the wrong IRQ, the serial port is very slow or
|
|
doesn't seem to work at all.
|
|
|
|
The first message is a result of Linux probing the ISA serial port
|
|
addresses but it doesn't probe for IRQs. If a port shows up here it
|
|
exists but the IRQ may be wrong. Linux doesn't check IRQs because
|
|
doing so is not foolproof. It just assumes the IRQs are as shown
|
|
because they are the "standard" values. You may check them manually
|
|
with setserial using the autoconfig and auto_irq options but this
|
|
isn't guaranteed to be correct either.
|
|
|
|
The data shown by the BIOS messages (which you see at first before
|
|
Linux is booted) are what is initially set in the hardware. If your
|
|
serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or
|
|
"setpci" will run and change these settings. Look for messages about
|
|
this after Linux starts. The last serial port message shown in the
|
|
example above should agree with the BIOS messages (as possibly
|
|
modified by isapnp or setpci). If they don't agree then you either
|
|
need to change the setting in the port hardware or use setserial to
|
|
tell the driver what is actually set in the hardware.
|
|
|
|
Also, if you have Plug-and-Play (PnP) serial ports, they can only be
|
|
found by PnP software unless the IRQ and IO has been set inside the
|
|
hardware by Plug-and-Play software. Prior to kernel 2.4 this was a
|
|
common reason why the start-up messages did not show a serial port
|
|
that physically exists. A PnP BIOS may automatically low-level
|
|
configure them. PnP configuring will be explained later.
|
|
|
|
|
|
8.6.3. The /proc directory and setserial
|
|
|
|
Type "setserial -g /dev/ttyS*". There are some other ways to find
|
|
this info by looking at "files" in the /proc directory. Be warned
|
|
that there is no guarantee that the same is set in the hardware.
|
|
|
|
/proc/ioports will show the IO addresses that the drivers are using.
|
|
/proc/interrupts shows the IRQs that are used by drivers of currently
|
|
running processes (that have devices open). It shows how many
|
|
interrupts have actually be issued. /proc/tty/driver/serial shows
|
|
much of the above, plus the number of bytes that have been received
|
|
and sent (even if the device is not now open).
|
|
|
|
Note that for the IO addresses and IRQ assignments, you are only
|
|
seeing what the driver thinks and not necessarily what is actually set
|
|
in the hardware. The data on the actual number of interrupts issued
|
|
and bytes processed is real however. If you see a large number of
|
|
interrupts and/or bytes then it probably means that the device is (or
|
|
was) working. But the interrupts might be from another device. If
|
|
there are no bytes received (rx:0) but bytes were transmitted (tx:3749
|
|
for example), then only one direction of flow is working (or being
|
|
utilized).
|
|
|
|
Sometimes a showing of just a few interrupts doesn't mean that the
|
|
interrupt is actually being physically generated by any serial port.
|
|
Thus if you see almost no interrupts for a port that you're trying to
|
|
use, that interrupt might not be set in the hardware. To view
|
|
/proc/interrupts to check on a program that you're currently running
|
|
(such as "minicom") you need to keep the program running while you
|
|
view it.
|
|
|
|
|
|
|
|
8.7. What is the IO Address & IRQ of my Serial Port Hardware?
|
|
|
|
8.7.1. Introduction
|
|
|
|
If it's PCI or ISA PnP then what's set in the hardware has been done
|
|
by PnP methods. Even if nothing has been set or the port disabled,
|
|
PnP ports may still be found by using "lspci -v" or "isapnp
|
|
--dumpregs". Ports disabled by jumpers (or hardware failures) are
|
|
completely lost. See ``ISA PnP ports'', ``PCI: What IOs and IRQs have
|
|
been set?'', ``PCI: Enabling a disabled port''
|
|
|
|
PnP ports don't store their configuration in the hardware when the
|
|
power is turned off. This is in contrast to Jumpers (non-PnP) which
|
|
remain the same with the power off. That's why a PnP port is more
|
|
likely to be found in a disabled state than an old non-PnP one.
|
|
|
|
|
|
8.7.2. PCI: What IOs and IRQs have been set?
|
|
|
|
For PCI, the BIOS almost always sets the IRQ and may set the IO
|
|
address as well. To see how it's set use "lspci -vv" (best) or look
|
|
in /proc/bus/pci (or for kernels <2.2 /proc/pci). The modem's serial
|
|
port is often called a "Communication controller". Look for this. If
|
|
lspci shows "I/O ports at ... [disabled]" then the serial port is
|
|
disabled and the hardware has no IO address so it's lost and can't be
|
|
used. See ``PCI: Enabling a disabled port'' for how to enable it.
|
|
|
|
If more than one IO address is shown, the first one is more likely to
|
|
be it. You can't change the IRQ (at least not with "setpci") This
|
|
is because if one writes an IRQ it it's hardware register no action is
|
|
taken on it. It's the BIOS that should actually set up the IRQs and
|
|
then write the correct value to this register for lspci to view. If
|
|
you must, change the IO address with "setpci" by changing the
|
|
BASE_ADDRESS_0 or the like.
|
|
|
|
|
|
8.7.3. PCI: Enabling a disabled port
|
|
|
|
If the port communicates via an IO address then "lspci -vv" should
|
|
show "Control: I/O+ ..." with + meaning that the IO address is
|
|
enabled. If it shows "I/O-" (and "I/O ports at ... [disabled]") then
|
|
you may need to use the setpci command to enable it. For example
|
|
"setpci -d 151f:000 command=101". 151f is the vendor id, and 000 is
|
|
the device id both obtained from "lspci -n -v" or from /proc/bus/pci
|
|
or from "scanpci -v". The "command=101" means that 101 is put into
|
|
the command register which is the same as the "Control" register
|
|
displayed by "lspci". The 101h sets two bits: the 1 sets I/O to + and
|
|
the 100 part keeps SERR# set to +. In this case only the SERR# bit
|
|
of the Control register was initially observed to be + when the lspci
|
|
command was run. So we kept it enabled to + by setting bit 8 (where
|
|
bit 0 is I/O) to 1 by the first 1 in 101. Some serial cards don't use
|
|
SERR# so if you see SERR#- then there's no need to enable it so then
|
|
use: command=1. Then you'll need to set up "setserial" to tell the
|
|
driver the IO and IRQ.
|
|
|
|
Bit 8 is actually the 9th bit since we started counting bits from 0.
|
|
Don't be alarmed that lspci shows a lot of - signs showing that the
|
|
card doesn't have many features available (or enabled). Serial ports
|
|
are relatively slow and don't need these features.
|
|
|
|
Another way to enable it is to let the BIOS do it by telling the BIOS
|
|
that you don't have a plug-and-play operating system. Then the BIOS
|
|
should enable it when you start your PC. If you have MS Windows9x on
|
|
the same PC then doing this might cause problems with Windows (see
|
|
Plug-and-Play-HOWTO).
|
|
|
|
8.7.4. ISA PnP ports
|
|
|
|
For an ISA Plug-and-Play (PnP) port one may try the pnpdump program
|
|
(part of isapnptools). If you use the --dumpregs option then it
|
|
should tell you the actual IO address and IRQ set in the port. It
|
|
should also find an ISA PnP port that is disabled. The address it
|
|
"trys" is not the device's IO address, but a special address used for
|
|
communicating with PnP cards.
|
|
|
|
|
|
8.7.5. Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
|
|
|
|
Perhaps the BIOS messages will tell you some info before Linux starts
|
|
booting. Use the shift-PageUp key to step back thru the boot-time
|
|
messages and look at the very first ones which are from the BIOS.
|
|
This is how it was before Linux started. Setserial can't change it
|
|
but isapnp or setpci can. Starting with kernel 2.4, the serial driver
|
|
can make such changes for many (but not all) serial ports.
|
|
|
|
Using "scanport" (Debian only ??) will probe all I/O ports and will
|
|
indicate what it thinks may be serial port. After this you could try
|
|
probing with setserial using the "autoconfig" option. You'll need to
|
|
guess the addresses to probe at (using clues from "scanport"). See
|
|
``What is Setserial''.
|
|
|
|
For a port set with jumpers, the IO ports and IRQs are set per the
|
|
jumpers. If the port is not Plug-and-Play (PnP) but has been setup by
|
|
using a DOS program, then it's set at whatever the person who ran that
|
|
program set it to.
|
|
|
|
|
|
8.7.6. Exploring via MS Windows (a last resort)
|
|
|
|
For PnP ports, checking on how it's configured under DOS/Windows may
|
|
(or may not) imply how it's under Linux. MS Windows stores its
|
|
configuration info in its Registry which is not used by Linux so they
|
|
are not necessarily configured the same. If you let a PnP BIOS
|
|
automatically do the configuring when you start Linux (and have told
|
|
the BIOS that you don't have a PnP operating system when starting
|
|
Linux) then Linux should use whatever configuration is in the BIOS's
|
|
non-volatile memory. Windows also makes use of the same non-volatile
|
|
memory but doesn't necessarily configure it that way.
|
|
|
|
|
|
8.8. Choosing Serial IRQs
|
|
|
|
If you have Plug-and-Play ports then either a PnP BIOS or a serial
|
|
driver may configure all your devices for you so then you may not need
|
|
to choose any IRQs. PnP software determines what it thinks is best
|
|
and assigns them (but it's not always best). But if you directly use
|
|
isapnp (ISA bus) or jumpers then you have to choose. If you already
|
|
know what IRQ you want to use you could skip this section except that
|
|
you may want to know that IRQ 0 has a special use (see the following
|
|
paragraph).
|
|
|
|
|
|
8.8.1. IRQ 0 is not an IRQ
|
|
|
|
While IRQ 0 is actually the timer (in hardware) it has a special
|
|
meaning for setting a serial port with setserial. It tells the driver
|
|
that there is no interrupt for the port and the driver then will use
|
|
polling methods. Such polling puts more load on the CPU but can be
|
|
tried if there is an interrupt conflict or mis-set interrupt. The
|
|
advantage of assigning IRQ 0 is that you don't need to know what
|
|
interrupt is set in the hardware. It should be used only as a
|
|
temporary expedient until you are able to find a real interrupt to
|
|
use.
|
|
|
|
|
|
8.8.2. Interrupt sharing, Kernels 2.2+
|
|
|
|
Sharing of IRQs is where two devices use the same IRQ. As a general
|
|
rule, this wasn't allowed for the ISA bus. The PCI bus may share
|
|
IRQs but one can't share the same IRQ between the ISA and the PCI bus.
|
|
Most multi-port boards may share IRQs. Sharing is not as efficient
|
|
since every time a shared interrupt is given a check must be made to
|
|
determine where it came from. Thus if it's feasible, it's nicer to
|
|
allocate every device its own interrupt.
|
|
|
|
Prior to kernel 2.2, serial IRQs could not be shared with each other
|
|
except for most multiport boards. Starting with kernel 2.2 serial
|
|
IRQs may be sometimes shared between serial ports. In order for
|
|
sharing to work in 2.2 the kernel must have been compiled with
|
|
CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
|
|
sharing (so that if two serial cards put different voltages on the
|
|
same interrupt wire, only the voltage that means "this is an
|
|
interrupt" will prevail). Since the PCI bus specs permit sharing, any
|
|
PCI card should allow sharing.
|
|
|
|
|
|
8.8.3. What IRQs to choose?
|
|
|
|
The serial hardware often has only a limited number of IRQs. Also you
|
|
don't want IRQ conflicts. So there may not be much of a choice. Your
|
|
PC may normally come with ttyS0 and ttyS2 at IRQ 4, and ttyS1 and
|
|
ttyS3 at IRQ 3. Looking at /proc/interrupts will show which IRQs are
|
|
being used by programs currently running. You likely don't want to
|
|
use one of these. Before IRQ 5 was used for sound cards, it was often
|
|
used for a serial port.
|
|
|
|
Here is how Greg (original author of Serial-HOWTO) set his up in
|
|
/etc/rc.d/rc.serial. rc.serial is a file (shell script) which runs at
|
|
start-up (it may have a different name or location). For versions of
|
|
"setserial" after 2.15 it's not always done this way anymore but this
|
|
example does show the choice of IRQs.
|
|
|
|
|
|
|
|
/sbin/setserial /dev/ttyS0 irq 3 # my serial mouse
|
|
/sbin/setserial /dev/ttyS1 irq 4 # my Wyse dumb terminal
|
|
/sbin/setserial /dev/ttyS2 irq 5 # my Zoom modem
|
|
/sbin/setserial /dev/ttyS3 irq 9 # my USR modem
|
|
|
|
|
|
|
|
Standard IRQ assignments:
|
|
|
|
|
|
|
|
IRQ 0 Timer channel 0 (May mean "no interrupt". See below.)
|
|
IRQ 1 Keyboard
|
|
IRQ 2 Cascade for controller 2
|
|
IRQ 3 Serial port 2
|
|
IRQ 4 Serial port 1
|
|
IRQ 5 Parallel port 2, Sound card
|
|
IRQ 6 Floppy diskette
|
|
IRQ 7 Parallel port 1
|
|
IRQ 8 Real-time clock
|
|
IRQ 9 Redirected to IRQ2
|
|
IRQ 10 not assigned
|
|
IRQ 11 not assigned
|
|
IRQ 12 not assigned
|
|
IRQ 13 Math co-processor
|
|
IRQ 14 Hard disk controller 1
|
|
IRQ 15 Hard disk controller 2
|
|
|
|
|
|
|
|
There is really no Right Thing to do when choosing interrupts. Try to
|
|
find one that isn't being used by the motherboard, or any other
|
|
boards. 2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices. Note
|
|
that IRQ 2 is the same as IRQ 9. You can call it either 2 or 9, the
|
|
serial driver is very understanding. If you have a very old serial
|
|
board it may not be able to use IRQs 8 and above.
|
|
|
|
Make sure you don't use IRQs 1, 6, 8, 13 or 14! These are used by
|
|
your motherboard. You will make her very unhappy by taking her IRQs.
|
|
When you are done you might want to double-check /proc/interrupts when
|
|
programs that use interrupts are being run and make sure there are no
|
|
conflicts.
|
|
|
|
|
|
8.9. Choosing Addresses --Video card conflict with ttyS3
|
|
|
|
Here's a problem with some old serial cards. The IO address of the
|
|
IBM 8514 video board (and others like it) is allegedly 0x?2e8 where ?
|
|
is 2, 4, 8, or 9. This may conflict with the IO address of ttyS3 at
|
|
0x02e8. Your may think that this shouldn't happen since the addresses
|
|
are different in the high order digit (the leading 0 in 02e8). You're
|
|
right, but a poorly designed serial port may ignore the high order
|
|
digit and respond to any address that ends in 2e8. That is bad news
|
|
if you try to use ttyS3 (ISA bus) at this IO address.
|
|
|
|
For the ISA bus you should try to use the default addresses shown
|
|
below. PCI cards use different addresses so as not to conflict with
|
|
ISA addresses. The addresses shown below represent the first address
|
|
of an 8-byte range. For example 3f8 is really the range 3f8-3ff.
|
|
Each serial device (as well as other types of devices that use IO
|
|
addresses) needs its own unique address range. There should be no
|
|
overlaps (conflicts). Here are the default addresses for commonly
|
|
used serial ports on the ISA bus:
|
|
|
|
|
|
|
|
ttyS0 address 0x3f8
|
|
ttyS1 address 0x2f8
|
|
ttyS2 address 0x3e8
|
|
ttyS3 address 0x2e8
|
|
|
|
|
|
|
|
Suppose there is an address conflict (as reported by setserial -g
|
|
/dev/ttyS*) between a real serial port and another port which does not
|
|
physically exist (and shows UART: unknown). Such a conflict shouldn't
|
|
cause problems but it sometimes does in older kernels. To avoid this
|
|
problem don't permit such address conflicts or delete /dev/ttySx if it
|
|
doesn't physically exist.
|
|
|
|
|
|
8.10. Set IO Address & IRQ in the hardware (mostly for PnP)
|
|
|
|
After it's set in the hardware don't forget to insure that it also
|
|
gets set in the driver by using setserial. For non-PnP serial ports
|
|
they are either set in hardware by jumpers or by running a DOS program
|
|
("jumperless") to set them (it may disable PnP). The rest of this
|
|
subsection is only for PnP serial ports. Here's a list of the
|
|
possible methods of configuring PnP serial ports:
|
|
|
|
|
|
· Using a PnP BIOS CMOS setup menu (usually only for external devices
|
|
on ttyS0 (Com1) and ttyS1 (Com2))
|
|
|
|
· Letting a PnP BIOS automatically configure a PnP serial port See
|
|
``Using a PnP BIOS to IO-IRQ Configure''
|
|
|
|
· Doing nothing if the serial driver recognized your card OK
|
|
|
|
· Using isapnp for a PnP serial port (non-PCI)
|
|
|
|
· Using setpci (pciutils or pcitools) for the PCI bus
|
|
|
|
The IO address and IRQ must be set (by PnP) in their registers each
|
|
time the system is powered on since PnP hardware doesn't remember how
|
|
it was set when the power is shut off. A simple way to do this is to
|
|
let a PnP BIOS know that you don't have a PnP OS and the BIOS will
|
|
automatically do this each time you start. This might cause problems
|
|
in Windows (which is a PnP OS) if you start Windows with the BIOS
|
|
thinking that Windows is not a PnP OS. See Plug-and-Play-HOWTO.
|
|
|
|
Plug-and-Play (PnP) was designed to automate this io-irq configuring,
|
|
but for Linux it initially made life much more complicated. In modern
|
|
Linux (2.4 kernels --partially in 2.2 kernels), each device driver has
|
|
to do its own PnP (using supplied software which it may utilize).
|
|
There is unfortunately no centralized planning for assigning IO
|
|
addresses and IRQs as there is in MS Windows. But it usually works
|
|
out OK in Linux anyway.
|
|
|
|
|
|
8.10.1. Using a PnP BIOS to IO-IRQ Configure
|
|
|
|
While the explanation of how to use setpci or isapnp for io-irq
|
|
configuring should come with such software, this is not the case if
|
|
you want to let a PnP BIOS do such configuring. Not all PnP BIOS can
|
|
do this. The BIOS usually has a CMOS menu for setting up the first
|
|
two serial ports. This menu may be hard to find. For an "Award" BIOS
|
|
it was found under "chipset features setup" There is often little to
|
|
choose from. For ISA serial ports, the first two ports normally get
|
|
set at the standard IO addresses and IRQs. See ``More on Serial Port
|
|
Names''
|
|
|
|
Whether you like it or not, when you start up a PC, a PnP BIOS starts
|
|
to do PnP (io-irq) configuring of hardware devices. It may do the job
|
|
partially and turn the rest over to a PnP OS (which Linux is in some
|
|
sense) or if thinks you don't have a PnP OS it may fully configure all
|
|
the PnP devices but not configure the device drivers.
|
|
|
|
If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
|
|
should do the configuring of all PnP serial ports --not just the first
|
|
two. An indirect way to control what the BIOS does (if you have
|
|
Windows 9x on the same PC) is to "force" a configuration under
|
|
Windows. See Plug-and-Play-HOWTO and search for "forced". It's
|
|
easier to use the CMOS BIOS menu which may override what you "forced"
|
|
under Windows. There could be a BIOS option that can set or disable
|
|
this "override" capability.
|
|
|
|
If you add a new PnP device, the BIOS should PnP configure it. It
|
|
could even change the io-irq of existing devices if required to avoid
|
|
any conflicts. For this purpose, it keeps a list of non-PnP devices
|
|
provided that you have told the BIOS how these non-PnP devices are io-
|
|
irq configured. One way to tell the BIOS this is by running a program
|
|
called ICU under DOS/Windows.
|
|
|
|
But how do you find out what the BIOS has done so that you set up the
|
|
device drivers with this info? The BIOS itself may provide some info,
|
|
either in its setup menus of via messages on the screen when you turn
|
|
on your computer. See ``What is set in my serial port hardware?''.
|
|
Other ways of finding out is to use lspci for the PCI bus or isapnp
|
|
--dumpregs for the ISA bus. The cryptic results it shows you may not
|
|
be clear to a novice.
|
|
|
|
|
|
8.11. Giving the IRQ and IO Address to Setserial
|
|
|
|
Once you've set the IRQ and IO address in the hardware (or arranged
|
|
for it to be done by PnP) you also need to insure that the "setserial"
|
|
command is run each time you start Linux. See the subsection ``Boot-
|
|
time Configuration''
|
|
|
|
|
|
|
|
9. Configuring the Serial Driver (high-level) "stty"
|
|
|
|
9.1. Overview
|
|
|
|
See the section ``Stty''. The "stty" command sets many things such as
|
|
flow control, speed, and parity. The only one discussed in this
|
|
section is flow control.
|
|
|
|
|
|
9.2. Flow Control
|
|
|
|
Configuring Flow Control: Hardware Flow Control is Usually Best See
|
|
``Flow Control'' for an explanation of it. It's usually better to use
|
|
hardware flow control rather than software flow control using
|
|
Xon/Xoff. To use full hardware flow control you must normally have
|
|
two dedicated wires for it in the cable between the serial port and
|
|
the device. If the device is on a card or the motherboard, then it
|
|
should always be possible to use hardware flow control.
|
|
|
|
Many applications (and the getty program) give you an option regarding
|
|
flow control and will set it as you specify or it might enable
|
|
hardware flow control by default if you don't set it. It must be set
|
|
both in the serial driver and in the hardware connected to the serial
|
|
port. How it's set into this hardware is hardware dependent.
|
|
Sometimes there is a certain "init string" you send to the hardware
|
|
device via the serial port from your PC. For a modem, the
|
|
communication program should set it in both places.
|
|
|
|
If a program you use doesn't set flow control in the serial driver,
|
|
then you may do it yourself using the stty command. Since the driver
|
|
doesn't remember the setting after you stop Linux, you could put the
|
|
stty command in a file that runs at start-up or when you login (such
|
|
as /etc/profile for the bash shell). Here's what you would add for
|
|
hardware flow control for port ttyS2:
|
|
|
|
stty -F /dev/ttyS2 crtscts
|
|
|
|
|
|
|
|
or for old stty versions < 1.17:
|
|
|
|
|
|
stty crtscts < /dev/ttyS2
|
|
|
|
|
|
|
|
crtscts stands for a Control setting to use the RTS and CTS pins of
|
|
the serial port for hardware flow control. Note that RTS+CTS almost
|
|
spells: crtscts and the initial "c" means "control".
|
|
|
|
|
|
10. Serial Port Devices /dev/ttyS2, etc.
|
|
|
|
10.1. Serial Port Names: ttyS4, etc
|
|
|
|
Common serial port names are /dev/ttyS0, /dev/ttyS1, etc. Then around
|
|
the year 2000 came the USB bus with names like /dev/ttyUSB0 and
|
|
/dev/ttyACM1 (for the ACM modem on the USB bus). Multiport serial
|
|
card used somewhat differnt names (depending on the brand) such as
|
|
/dev/ttyE5.
|
|
|
|
|
|
10.2. The PCI Bus
|
|
|
|
Since DOS provided for 4 serial ports on the old ISA bus: COM1-COM4,
|
|
or ttyS0-ttyS3 in Linux, most serial ports on the newer PCI bus used
|
|
higher numbers such as ttyS4 or ttyS14 (prior to kernel 2.6.13). But
|
|
since most PCs only came with one or two serial ports, ttyS0 and
|
|
possibly ttyS1 (for the second port) the PCI bus can now use ttyS2
|
|
(kernel 2.6.15 on). All this permits one to have both ISA serial
|
|
ports and PCI serial ports on the same PC with no name conflicts. 0-1
|
|
(or 0-3) are reserved for the old ISA bus (or the newer LPC bus) and
|
|
2-upward (or 4-upward or 14-upward) are used for PCI, where older
|
|
schemes are shown in parentheses . It's not required to be this way
|
|
but it often is.
|
|
|
|
If you're using udev (which puts only the device you have on your
|
|
computer into the /dev directory at boottime) then there's an easy way
|
|
to change the device names by editing files in /etc/udev/. For
|
|
example, to change the name of what the kernel detects as ttyS3 to
|
|
what you want to name it: ttyS14, add a line similar to this to
|
|
/etc/udev/udev.rules
|
|
NAME="ttyS14"
|
|
|
|
On-board serial ports on motherboards which have both PCI and ISA
|
|
slots are likely to still be ISA ports. Even for all-PCI-slot
|
|
motherboards, the serial ports are often not PCI. Instead, they are
|
|
either ISA, on an internal ISA bus or on a LPC bus which is intended
|
|
for slow legacy I/O devices: serial/parallel ports and floppy drives.
|
|
|
|
|
|
|
|
10.3. Serial Port Device Names & Numbers
|
|
|
|
Devices in Linux have major and minor numbers. The serial port ttySx
|
|
(x=0,1,2, etc.) is major number 4. You can see this (and the minor
|
|
numbers too) by typing: "ls -l ttyS*" in the /dev directory. To find
|
|
the device names for various devices, see the "devices" file in the
|
|
kernel documentation.
|
|
There formerly was a "cua" name for each serial port and it behaved
|
|
just a little differently. For example, ttyS2 would correspond to
|
|
cua2. It was mainly used for modems. The cua major number was 5 and
|
|
minor numbers started at 64. You may still have the cua devices in
|
|
your /dev directory but they are now deprecated. For details see
|
|
Modem-HOWTO, section: cua Device Obsolete.
|
|
|
|
For creating the old devices in the device directory see:
|
|
|
|
``Creating Devices In the /dev directory''
|
|
|
|
|
|
10.4. More on Serial Port Names
|
|
|
|
Dos/Windows use the COM name while the messages from the serial driver
|
|
use ttyS00, ttyS01, etc. Older serial drivers (2001 ?) used just
|
|
tty00, tty01, etc.
|
|
|
|
|
|
The tables below shows some examples of serial device names. The IO
|
|
addresses are the default addresses for the old ISA bus (not for the
|
|
newer PCI and USB buses).
|
|
|
|
|
|
|
|
dos common IO USB-BUS ( ACM => acm modem )
|
|
name name major minor address || common name common name
|
|
COM1 /dev/ttyS0 4, 64; 3F8 || /dev/ttyUSB0 | /dev/ttyACM0
|
|
COM2 /dev/ttyS1 4, 65; 2F8 || /dev/ttyUSB1 | /dev/ttyACM1
|
|
COM3 /dev/ttyS2 4, 66; 3E8 || /dev/ttyUSB2 | /dev/ttyACM2
|
|
COM4 /dev/ttyS3 4, 67; 2E8 || /dev/ttyUSB3 | /dev/ttyACM3
|
|
- /dev/ttyS4 4, 68; various
|
|
|
|
|
|
|
|
10.5. USB (Universal Serial Bus) Serial Ports
|
|
|
|
For more info see the usb subdirectory in the kernel documentation
|
|
directory for files: usb-serial, acm, etc.
|
|
|
|
|
|
10.6. Link ttySN to /dev/modem
|
|
|
|
On some installations, two extra devices will be created, /dev/modem
|
|
for your modem and /dev/mouse for a mouse. Both of these are symbolic
|
|
links to the appropriate device in /dev.
|
|
|
|
Historical note: Formerly (in the 1990s) the use of /dev/modem (as a
|
|
link to the modem's serial port) was discouraged since lock files
|
|
might not realize that it was really say /dev/ttyS2. The newer lock
|
|
file system doesn't fall into this trap so it's now OK to use such
|
|
links.
|
|
|
|
|
|
|
|
10.7. Which Connector on the Back of my PC is ttyS1, etc?
|
|
|
|
10.7.1. Inspect the connectors
|
|
|
|
Inspecting the connectors may give some clues but is often not
|
|
definitive. The serial connectors on the back side of a PC are
|
|
usually DB connectors with male pins. 9-pin is the most common but
|
|
some are 25-pin (especially older PCs like 486s). There may be one
|
|
9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??). For two
|
|
9-pin ones the top one might be ttyS0.
|
|
|
|
If you only have one serial port connector on the back of your PC,
|
|
this may be easy. If you also have an internal modem, a program like
|
|
wvdial may be able to tell you what port it's on (unless it's a PnP
|
|
that hasn't been enabled yet). A report from setserial (at boot-time
|
|
or run by you from the command line) should help you identify the non-
|
|
modem ports.
|
|
|
|
If you have two serial ports it may be more difficult. You could have
|
|
only one serial connector but actually have 2 ports, one of which
|
|
isn't used (but it's still there electronically). First check manuals
|
|
(if any) for your computer. Look at the connectors for meaningful
|
|
labels. You might even want to take off the PC's cover and see if
|
|
there are any meaningful labels on the card where the internal ribbon
|
|
serial cables plug in. Labels (if any) are likely to say something
|
|
like "serial 1", "serial 2" or A, B. Which com port it actually is
|
|
will depend on jumper or PnP settings (sometimes shown in a BIOS setup
|
|
menu). But 1 or A are more likely to be ttyS0 with 2 or B ttyS1.
|
|
|
|
|
|
10.7.2. Send bytes to the port
|
|
|
|
Labels are not apt to be definitive so here's another method. If the
|
|
serial ports have been configured correctly per setserial, then you
|
|
may send some bytes out a port and try to detect which connector (if
|
|
any) they are coming out of. One way to send such a signal is to copy
|
|
a long text file to the port using a command like: cp my_file_name
|
|
/dev/ttyS1. A voltmeter connected to the DTR pin (see Serial-HOWTO
|
|
for Pinout) will display a positive voltage as soon as you give the
|
|
copy command.
|
|
|
|
The transmit pin should go from several volts negative to a voltage
|
|
fluctuating around zero after you start sending the bytes. If it
|
|
doesn't (but the DTR went positive) then you've got the right port but
|
|
it's blocked from sending. This may be due to a wrong IRQ, -clocal
|
|
being set, etc. The command "stty -F /dev/ttyS1 -a" should show
|
|
clocal (and not -clocal). If not, change it to clocal.
|
|
|
|
Another test is to jumper the transmit and receive pins (pins 2 and 3
|
|
of either the 25-pin or 9-pin connector) of a test serial port. Then
|
|
send something to each port (from the PCs keyboard) and see if it gets
|
|
sent back. If it does it's likely the port with the jumper on it.
|
|
Then remove the jumper and verify that nothing gets sent back. Note
|
|
that if "echo" is set (per stty) then a jumper creates an infinite
|
|
loop. Bytes that pass thru the jumper go into the port and come right
|
|
back out of the other pin back to the jumper. Then they go back in
|
|
and out again and again. Whatever you send to the port repeats itself
|
|
forever (until you interrupt it by removing the jumper, etc.). This
|
|
may be a good way to test it as the repeating test messages halt when
|
|
the jumper is removed.
|
|
|
|
As a jumper you could use a mini (or micro) jumper cable (sold in some
|
|
electronic parts stores) with mini alligator clips. A small scrap of
|
|
paper may be used to prevent the mini clips from making electrical
|
|
contact where it shouldn't. Metal paper clips can sometimes be bent
|
|
to use as jumpers. Whatever you use as a jumper take care not to bend
|
|
or excessively scratch the pins. To receive something from a port,
|
|
you can go to a virtual terminal (for example Alt-F2 and login) and
|
|
type something like "cp /dev/ttyS2 /dev/tty". Then at another virtual
|
|
terminal you may send something to ttyS2 (or whatever) by "echo
|
|
test_message > /dev/ttyS2". Then go back to the receive virtual
|
|
terminal and look for the test_message. See ``Serial Electrical Test
|
|
Equipment'' for more info.
|
|
|
|
10.7.3. Connect a device to the connector
|
|
|
|
Another way to try to identify a serial port is to connect some
|
|
physical serial device to it and see if it works. But a problem here
|
|
is that it might not work because it's not configured right. A serial
|
|
mouse might get detected at boot-time if connected.
|
|
|
|
You may put a device, such as a serial mouse (use 1200 baud), on a
|
|
port and then use minicom or picocom to communicate with that port.
|
|
Then by clicking on the mouse, or otherwise sending characters with
|
|
the device, see if they get displayed. It not you may have told
|
|
picocom the wrong port (such as ttyS0 instead of ttyS1) so try again.
|
|
|
|
|
|
10.7.4. Missing connectors
|
|
|
|
If the software shows that you have more serial ports than you have
|
|
connectors for (including an internal modem which counts as a serial
|
|
port) then you may have a serial port that has no connector. Some
|
|
motherboards come with a serial port with no cable or external serial
|
|
DB connector. Someone may build a PC from this and decide not to use
|
|
this serial port. There may be a "serial" connector and label on the
|
|
motherboard but no ribbon cable connects to its pins. To use this
|
|
port you must get a ribbon cable and connector. I've seen different
|
|
wiring arrangements for such ribbon cables so beware.
|
|
|
|
|
|
10.8. Creating Devices In the /dev directory
|
|
|
|
If you don't use devfs (which automatically creates such devices) and
|
|
don't have a device "file" that you need, you will have to create it.
|
|
Use the mknod command or with the MAKEDEV shell script. Example,
|
|
suppose you needed to create ttyS0:
|
|
|
|
|
|
|
|
linux# mknod -m 666 /dev/ttyS0 c 4 64
|
|
|
|
|
|
|
|
The MAKEDEV script is easier to use. See the man page for it. For
|
|
example, if you needed to make the device for ttyS0 you would just
|
|
type:
|
|
|
|
|
|
|
|
linux# MAKEDEV ttyS0
|
|
|
|
|
|
|
|
If the above command doesn't work (and you are the root user), look
|
|
for the MAKEDEV script in the /dev directory and run it.
|
|
|
|
This handles the devices creation and should set the correct
|
|
permissions. For making multiport devices see ``Making multiport
|
|
devices in the /dev directory''.
|
|
|
|
|
|
11. Interesting Programs You Should Know About
|
|
|
|
Most info on getty has been moved to Modem-HOWTO with a little info on
|
|
the use of getty with directly connected terminals now found in Text-
|
|
Terminal-HOWTO.
|
|
|
|
11.1. Serial Monitoring/Diagnostics Programs
|
|
|
|
A few Linux programs (and one "file") will monitor various modem
|
|
control lines and indicate if they are positive (1 or green) or
|
|
negative (0 or red).
|
|
|
|
· serlook <http://sourceforge.net/projects/serlook/files/> can snoop
|
|
on serial line traffic (via a wiretap) and also send/receive on a
|
|
serial line.
|
|
|
|
· The "file": /proc/tty/driver/serial lists those that are asserted
|
|
(positive voltage)
|
|
|
|
· modemstat (Only works correctly on Linux PC consoles.) Status
|
|
monitored in a tiny window. Color-coded and compact. Must kill it
|
|
(a process) to quit.
|
|
|
|
· statserial (Info displayed on entire screen)
|
|
|
|
· serialmon (Doesn't monitor RTS, CTS, DSR but logs other functions)
|
|
|
|
As of June 1998, I know of no diagnostic program in Linux for the
|
|
serial port.
|
|
|
|
|
|
11.2. Changing Interrupt Priority
|
|
|
|
|
|
· irqtune will give serial port interrupts higher priority to improve
|
|
performance.
|
|
|
|
· hdparm for hard-disk tuning may help some more.
|
|
|
|
|
|
11.3. What is Setserial ?
|
|
|
|
This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There are
|
|
some minor differences, depending on which HOWTO it appears in.
|
|
|
|
|
|
11.3.1. Setserial problems with linmodems, laptops
|
|
|
|
|
|
If you have a Laptop (PCMCIA) don't use setserial until you read
|
|
``Laptops: PCMCIA''.
|
|
|
|
|
|
11.3.2. Introduction
|
|
|
|
setserial is a program used for the user to communicate with the
|
|
serial device driver. You normally never need to use it, provided
|
|
that you only use the one or two serial ports that come as standard
|
|
equipment with a PC. Even in other cases, most extra serial ports
|
|
should be auto-detected by modern kernels. Except you'll need to use
|
|
setserial if you have an old ISA serial port set by jumpers on the
|
|
physical hardware or if your kernel (such as 2.2 or older) doesn't
|
|
both detect and set your add-on PCI serial ports.
|
|
|
|
setserial allows you (or a shell script) to talk to the serial
|
|
software. But there's also another program, tt/stty/, that also deals
|
|
with the serial port and is used for setting the port speed, etc.
|
|
|
|
setserial deals with the lower-level configuring of the serial port,
|
|
such as dealing with IRQs (such as 5), port addresses (such as 3f8),
|
|
and the like. A major problem with it is that it can't set or
|
|
configure the serial port hardware: It can't set the IRQ or port
|
|
addresses into the hardware. Furthermore, when it seemingly reports
|
|
the configuration of the hardware, it's sometimes wrong since it
|
|
doesn't actually probe the hardware unless you specifically tell it
|
|
to. Even then, it doesn't do the modern type of bus probing and some
|
|
hardware may never be found by it. Still, what it shows is right most
|
|
all the time but if you're having trouble getting a serial port to
|
|
work, then there's a fair chance it's wrong.
|
|
|
|
In olden days, when the IRQ and port address was set by jumpers on the
|
|
serial card, one would use setserial to tell the driver how these
|
|
jumpers were set. Today, when plug-and-play methods detect how the
|
|
jumperless serial port is set, setserial is not really needed anymore
|
|
unless you're having problems or using old hardware. Furthermore, if
|
|
the configuration file used by setserial is wrong, then there's
|
|
trouble. In this case, if you use setserial to try to find out how
|
|
the port is configured, it may just repeat the incorrect information
|
|
in the configuration file.
|
|
|
|
setserial can sometimes be of help to find a serial port. But it's
|
|
only of use if you know the port address and use the right options.
|
|
For modern ports, there's usually better ways to look for them by
|
|
plug-and-play methods.
|
|
|
|
Thus the name setserial is somewhat of a misnomer since it doesn't set
|
|
the I/O address nor IRQ in the hardware, it just "sets" them in the
|
|
driver software. And the driver naively believes that what setserial
|
|
tells it, even if it conflicts with what the driver has found by using
|
|
plug-and-play methods. Too bad that it fails to at least issue a
|
|
warning message for such a conflict. Since the device driver is
|
|
considered to be part of the kernel, the word "kernel" is often used
|
|
in other documentation with no mention made of any "serial driver".
|
|
|
|
Some distributions (and versions) set things up so that setserial is
|
|
run at boot-time by an initialization shell script (in the /etc
|
|
directory tree). But the configuration file which this script uses
|
|
may be either in the /etc tree or the /var tree. In some cases, if
|
|
you want setserial to run at boot-time, you may have to take some
|
|
action. setserialwill not work without either serial support built
|
|
into the kernel or loaded as a module. The module may get loaded
|
|
automatically if you (or a script) attempt to use setserial.
|
|
|
|
While setserial can be made to probe the hardware IO port addresses to
|
|
try to determine the UART type and IRQ, this has severe limitations.
|
|
See ``Probing''. It can't set the IRQ or the port address in the
|
|
hardware of PnP or PCI serial ports (but the plug-and-play features of
|
|
the serial driver may do this). It also can't directly read the PnP
|
|
data stored in configuration registers in the hardware. But since the
|
|
device driver can read these registers and setserial tells you what
|
|
the device driver thinks, it might be correct. Or it could be telling
|
|
you what setserial had previously (and perhaps erroneously) told the
|
|
driver. There's no way to know for sure without doing some other
|
|
checks.
|
|
|
|
The serial driver (for Linux Kernel 2.4+) looks for a few "standard"
|
|
legacy serial ports, for PnP ports on the ISA bus, and for all
|
|
supported port hardware on the PCI bus. If it finds your ports
|
|
correctly, then there's no need to use setserial. The driver doesn't
|
|
probe for the IRQs of old ISA serial ports set with jumpers on the
|
|
card and may get these wrong.
|
|
|
|
Besides the man page for setserial, check out info in
|
|
/usr/doc/setserial.../ or /usr/share/doc/setserial. This should tell
|
|
you how setserial is handled for your distribution of Linux. While
|
|
setserial behaves the same in all distributions, the scripts for
|
|
running it, how to configure such scripts (including automatic
|
|
configuration), and the names and locations of the script files, etc.,
|
|
are all distribution-dependent.
|
|
|
|
|
|
11.3.3. Serial module unload
|
|
|
|
If a serial module gets unloaded, the changes previously made by
|
|
setserial will be forgotten by the driver. But while the driver
|
|
forgets it, a script provided by the distribution may save it in a
|
|
file somewhere so that it can the restored if the module is reloaded.
|
|
|
|
|
|
11.3.4. Slow baud rates of 1200 or less
|
|
|
|
There once was a problem with slow serial printers (especially the old
|
|
ones of the 1980s). The printing program would close the serial port
|
|
at the "end" of printing well before all the characters from the large
|
|
serial buffer (in main memory) were sent to the printer. The result
|
|
was a truncated print job that didn't print the last paragraph or last
|
|
page, etc.
|
|
|
|
But the newer lprng print program (and possibly other printing
|
|
programs) keeps the port open until printing is finished so "problem
|
|
solved", even if you're using an antique printer. Setserial can
|
|
modify the time that the port will keep operating after it's closed
|
|
(in order to output any characters still in its buffer in main RAM).
|
|
This is done by the "closing_wait" option per the setserial man page.
|
|
For "bad" software that closes the port too soon, it might also be
|
|
needed at speeds above 1200 if there are a lot of "flow control"
|
|
waits.
|
|
|
|
|
|
11.3.5. Giving the setserial command
|
|
|
|
Remember, that setserial can't set any I/O addresses or IRQs in the
|
|
hardware. That's done either by plug-and-play software (run by the
|
|
driver) or by jumpers for legacy serial ports. Even if you give an
|
|
I/O address or IRQ to the driver via setserial it will not set such
|
|
values and assumes that they have already been set. If you give it
|
|
wrong values, the serial port will not work right (if at all).
|
|
|
|
For legacy ports, if you know the I/O address but don't know the IRQ
|
|
you may command setserial to attempt to determine the IRQ.
|
|
|
|
You can see a list of possible commands by just typing setserial with
|
|
no arguments. This fails to show you the one-letter options such as
|
|
-v for verbose which you should normally use when troubleshooting.
|
|
Note that setserial calls an IO address a "port". If you type:
|
|
|
|
|
|
setserial -g /dev/ttyS*
|
|
|
|
|
|
|
|
You'll see some info about how the device driver is configured for
|
|
your ports. In many cases you'll see some ports displayed with what
|
|
appears at first glance to be erroneous IRQs and addresses. But if
|
|
you also see: "UART: unknown" just ignore the entire line since no
|
|
serial port exists at that address.
|
|
|
|
If you add -a to the option -g you will see more info although few
|
|
people need to deal with (or understand) this additional info since
|
|
the default settings you see usually work fine. In normal cases the
|
|
hardware is set up the same way as "setserial" reports. But if you
|
|
are having problems there is a good chance that setserial has it
|
|
wrong. In fact, you can run "setserial" and assign a purely
|
|
fictitious I/O port address, any IRQ, and whatever uart type you would
|
|
like to have. Then the next time you type "setserial ..." it will
|
|
display these bogus values you've supplied to the driver. They will
|
|
also be officially registered with the kernel as displayed (at the top
|
|
of the screen) by the "scanport" command (Debian). Of course the
|
|
serial port driver will not work correctly (if at all) if you attempt
|
|
to use such a port. Thus, when giving parameters to setserial,
|
|
"anything goes". Well almost. If you assign one port a base address
|
|
that is already assigned (such as 3e8) it may not accept it. But if
|
|
you use 3e9 it will accept it. Unfortunately 3e9 is actually assigned
|
|
since it is within the range starting at base address 3e8. Thus the
|
|
moral of the story is to make sure your data is correct before
|
|
assigning resources with setserial.
|
|
|
|
|
|
11.3.6. Configuration file
|
|
|
|
While assignments made by setserial are lost when the PC is powered
|
|
off, a configuration file may restore them when the PC is started up
|
|
again. In newer versions, what you change by setserial might get
|
|
automatically saved to a configuration file. When setserial runs it
|
|
uses the info from the configuration file.
|
|
|
|
Where this configuration file resides depends on your distribution.
|
|
Look at the start-up scripts somewhere in the /etc/ tree (such as
|
|
/etc/init.d/ or /etc/rc.d/) and read the startup script for "serial"
|
|
or "setserial" or the like. It should show where the configuration
|
|
file(s) reside. In Debian there are 4 options for use of this
|
|
configuration file:
|
|
|
|
|
|
1. Don't use this file at all. At each boot, the serial driver alone
|
|
detects the ports and setserial doesn't ever run. ("kernel"
|
|
option)
|
|
|
|
2. Save what setserial reports when the system is first shutdown and
|
|
put it in the configuration file. After that, don't ever make any
|
|
changes to the configuration file, even if someone has made changes
|
|
by running the setserial command on the command line and then shuts
|
|
down the system. ("autosave-once" option)
|
|
|
|
3. At every shutdown, save whatever setserial detects to the
|
|
configuration file. ("autosave" option)
|
|
|
|
4. Manually edit the configuration file to set the configuration.
|
|
Don't ever do any automatic saves to it. ("manual" option)
|
|
|
|
In olden days (perhaps before 2000), there wasn't any configuration
|
|
file and the configuration was manually set (hard coded) inside the
|
|
shell script that ran setserial. See ``Edit a script (prior to
|
|
version 2.15)''.
|
|
|
|
|
|
11.3.7. Probing
|
|
|
|
You probe for a port with setserial only when you suspect that it has
|
|
been enabled (by PnP methods, the BIOS, jumpers, etc.). Otherwise
|
|
setserial probing will never find it since its address doesn't exist.
|
|
A problem is where the software looks for a port at specified I/O
|
|
addresses. Prior to probing with "setserial", one may run the
|
|
"scanport" (Debian) command to check all possible ports in one scan.
|
|
It makes crude guesses as to what is on some ports but doesn't
|
|
determine the IRQ. It's a fast first start. It may hang your PC but
|
|
so far it's worked fine for me. Note that non-Debian distributions
|
|
don't seem to supply "scanport". Is there another scan program?
|
|
|
|
With appropriate options, setserial can probe (at a given I/O address)
|
|
for a serial port but you must guess the I/O address. If you ask it
|
|
to probe for /dev/ttyS2 for example, it will only probe at the address
|
|
it thinks ttyS2 is at (2F8). If you tell setserial that ttyS2 is at a
|
|
different address, then it will probe at that address, etc. See
|
|
``Probing''
|
|
|
|
The purpose of such probing is to see if there is a uart there, and if
|
|
so, what its IRQ is. Use setserial mainly as a last resort as there
|
|
are faster ways to attempt it such as wvdialconf to detect modems,
|
|
looking at very early boot-time messages, or using pnpdump --dumpregs,
|
|
or lspci -vv. But if you want to detect hardware with setserial use
|
|
for example :
|
|
/dev/ttyS2 -v autoconfig
|
|
If the resulting message shows a uart type such as 16550A, then you're
|
|
OK. If instead it shows "unknown" for the uart type, then there is
|
|
supposedly no serial port at all at that I/O address. Some cheap
|
|
serial ports don't identify themselves correctly so if you see
|
|
"unknown" you still might have a serial port there.
|
|
|
|
Besides auto-probing for a uart type, setserial can auto-probe for
|
|
IRQ's but this doesn't always work right either. In one case it first
|
|
gave the wrong irq but when the command was repeated it found the
|
|
correct irq. In versions of setserial >= 2.15, the results of your
|
|
last probe test could be automatically saved and put into a
|
|
distribution-specific configuration file such as /etc/serial.conf or
|
|
/etc/sysconfig/serial or /var/lib/setserial/autoserial.conf for
|
|
Debian. This will be used next time you start Linux.
|
|
|
|
It may be that two serial ports both have the same IO address set in
|
|
the hardware. Of course this is not normally permitted for the ISA
|
|
bus but it sometimes happens anyway. Probing detects one serial port
|
|
when actually there are two. However if they have different IRQs,
|
|
then the probe for IRQs may show IRQ = 0. For me, it only did this if
|
|
I first used setserial to give the IRQ a fictitious value.
|
|
|
|
|
|
11.3.8. Boot-time Configuration
|
|
|
|
While setserial may run via an initialization script, something akin
|
|
to setserial also runs earlier when the serial module is loaded (or
|
|
when the kernel starts the built-in serial driver if it was compiled
|
|
into the kernel). Thus when you watch the start-up messages on the
|
|
screen it may look like it ran twice, and in fact it has.
|
|
|
|
If the first message is for a legacy port, the IRQs shown may be wrong
|
|
since it didn't probe for IRQs. If there is a second report of serial
|
|
ports, it may the result of a script such as /etc/init.d/setserial.
|
|
It usually does no probing and thus could be wrong about how the
|
|
hardware is actually set. It only shows configuration data that got
|
|
saved in a configuration files. The old method, prior to setserial
|
|
2.15, was to manually write such data directly into the script.
|
|
|
|
When the kernel loads the serial module (or if the "module equivalent"
|
|
is built into the kernel) then all supported PnP ports are detected.
|
|
For legacy (non-PnP) ports, only ttyS{0-3} are auto-detected and the
|
|
driver is set to use only IRQs 4 and 3 (regardless of what IRQs are
|
|
actually set in the hardware). No probing is done for IRQs but it's
|
|
possible to do this manually. You see this as a boot-time message
|
|
just as if setserial had been run.
|
|
|
|
To correct possible errors in IRQs (or for other reasons) there may be
|
|
a script file somewhere that runs setserial. Unfortunately, if this
|
|
file has some IRQs wrong, the kernel will still have incorrect info
|
|
about the IRQs. This file is usually part of the initialization done
|
|
at boot-time. Whether it runs or not depends on how you (and/or your
|
|
distribution) have set things up. It may also depends on the
|
|
runlevel.
|
|
|
|
Before modifying a configuration file, you can test out a "proposed"
|
|
setserial command by just typing it on the command line. In some
|
|
cases the results of this use of setserial will automatically get
|
|
saved somewhere such as /etc/serial.conf (or autoserial.conf or
|
|
serial) when you shutdown. So if it worked OK (and solved your
|
|
problem) then there's no need to modify any configuration file. See
|
|
``Configuration method using /etc/serial.conf, etc.''.
|
|
|
|
|
|
11.3.9. Edit a script (required prior to version 2.15)
|
|
|
|
This is how it was done prior to setserial 2.15 (1999) The objective
|
|
was to modify (or create) a script file in the /etc tree that runs
|
|
setserial at boot-time. Most distributions provided such a file (but
|
|
it may not have initially resided in the /etc tree).
|
|
|
|
So prior to version 2.15 (1999) it was simpler. All you did was edit
|
|
a script. There was no /etc/serial.conf file (or the like) to
|
|
configure setserial. Thus you needed to find the file that runs
|
|
"setserial" at boot time and edit it. If it didn't exist, you needed
|
|
to create one (or place the commands in a file that ran early at boot-
|
|
time). If such a file was currently being used it's likely was
|
|
somewhere in the /etc directory-tree. But Redhat <6.0 has supplied it
|
|
in /usr/doc/setserial/ but you need to move it to the /etc tree before
|
|
using it.
|
|
|
|
The script /etc/rc.d/rc.serial was commonly used in the past. The
|
|
Debian distribution used /etc/rc.boot/0setserial. Another file once
|
|
used was /etc/rc.d/rc.local but it's may not have run early enough.
|
|
It was reported that other processes may try to open the serial port
|
|
before rc.local ran resulting in serial communication failure. Later
|
|
on it most likely was found in /etc/init.d/ but wasn't normally
|
|
intended to be edited.
|
|
|
|
If such a file was supplied, it likely contained a number of
|
|
commented-out examples. By uncommenting some of these and/or
|
|
modifying them, you could set things up correctly. It was important
|
|
use a valid path for setserial, and a valid device name. You could do
|
|
a test by executing this file manually (just type its name as the
|
|
super-user) to see if it works right. Testing like this was a lot
|
|
faster than doing repeated reboots to get it right.
|
|
|
|
For versions >= 2.15 (provided your distribution implemented the
|
|
change, Redhat didn't at first) it may be more tricky to do since the
|
|
file that runs setserial on startup, /etc/init.d/setserial or the like
|
|
was not intended to be edited by the user. See ``Configuration method
|
|
using /etc/serial.conf, etc.''.
|
|
|
|
An example line in such a script was:
|
|
|
|
|
|
/sbin/setserial /dev/ttyS3 irq 5 uart 16550A skip_test
|
|
|
|
|
|
|
|
or, if you wanted setserial to automatically determine the uart and
|
|
the IRQ for ttyS3 you would have used something like this:
|
|
|
|
|
|
|
|
/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
|
|
|
|
This was done for every serial port you wanted to auto configure,
|
|
using a device name that really does exist on your machine. In some
|
|
cases it didn't work right due to the hardware.
|
|
|
|
|
|
11.3.10. Configuration method using /etc/serial.conf, etc.
|
|
|
|
Prior to setserial version 2.15 (1999), the way to configure setserial
|
|
was to manually edit the shell-script that ran setserial at boot-time.
|
|
See ``Edit a script (before version 2.15)''. This was simple, but the
|
|
simple and clear method has been changed to something that is
|
|
unnecessarily complex. Today the script and configuration file are
|
|
two different files instead of one. This shell-script is not edited
|
|
but gets its data from a configuration file such as /etc/serial.conf
|
|
(or /var/lib/setserial/autoserial.conf).
|
|
|
|
Furthermore you may not even need to edit serial.conf (or the like)
|
|
because using the "setserial" command on the command line may
|
|
automatically cause serial.conf to be edited appropriately. This was
|
|
done so that you may not need to edit any file in order to set up (or
|
|
change) what setserial does each time that Linux is booted.
|
|
|
|
What often happens is this: When you shut down your PC the script
|
|
that ran "setserial" at boot-time is run again, but this time it only
|
|
does what the part for the "stop" case says to do: It uses
|
|
"setserial" to find out what the current state of "setserial" is, and
|
|
it puts that info into the serial configuration file such as
|
|
serial.conf. Thus when you run "setserial" to change the serial.conf
|
|
file, it doesn't get changed immediately but only when and if you shut
|
|
down normally.
|
|
|
|
Now you can perhaps guess what problems might occur. Suppose you
|
|
don't shut down normally (someone turns the power off, etc.) and the
|
|
changes don't get saved. Suppose you experiment with "setserial" and
|
|
forget to run it a final time to restore the original state (or make a
|
|
mistake in restoring the original state). Then your "experimental"
|
|
settings are saved. And worst of all, unless you know which options
|
|
were set in the configuration file, you don't know what will happen.
|
|
One option in Debian (and likely other distributions) is known as
|
|
"AUTOSAVE-ONCE" which saves changes only for the first time you make
|
|
them with the setserial command.
|
|
|
|
If the option "###AUTOSAVE###" is set and you manually edit
|
|
serial.conf, then your editing is destroyed when you shut down because
|
|
it gets changed back to the state of setserial at shutdown. There is
|
|
a way to disable the changing of serial.conf at shutdown and that is
|
|
to remove "###AUTOSAVE###" or the like from first line of serial.conf.
|
|
In the Debian distribution, the removal of "###AUTOSAVE###" from the
|
|
first line was once automatically done after the first time you
|
|
shutdown just after installation. To retain this effect the
|
|
"AUTOSAVE-ONCE" option was created which only does a save when time
|
|
the system is shut down for the first time (just after you install or
|
|
update the setserial program).
|
|
|
|
The file most commonly used to run setserial at boot-time (in
|
|
conformance with the configuration file) is now /etc/init.d/setserial
|
|
(Debian) or /etc/init.d/serial (Redhat), or etc., but it should not
|
|
normally be edited. For 2.15, Redhat 6.0 just had a file
|
|
/usr/doc/setserial-2.15/rc.serial which you have to move to
|
|
/etc/init.d/ if you want setserial to run at boot-time.
|
|
|
|
To disable a port, use setserial to set it to "uart none". This will
|
|
not be saved. The format of /etc/serial.conf appears to be just like
|
|
that of the parameters placed after "setserial" on the command line
|
|
with one line for each port. If you don't use autosave, you may edit
|
|
/etc/serial.conf manually.
|
|
In order to force the current settings set by setserial to be saved to
|
|
the configuration file (serial.conf) without shutting down, do what
|
|
normally happens when you shutdown: Run the shell-script
|
|
/etc/init.d/{set}serial stop. The "stop" command will save the
|
|
current configuration but the serial ports still keep working OK.
|
|
|
|
In some cases you may wind up with both the old and new configuration
|
|
methods installed but hopefully only one of them runs at boot-time.
|
|
Debian labeled obsolete files with "...pre-2.15".
|
|
|
|
|
|
11.3.11. IRQs
|
|
|
|
By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
|
|
ttyS3 share IRQ 3. But while sharing serial interrupts (using them in
|
|
running programs) is OK for the PCI bus, it's not permitted for the
|
|
ISA bus unless you: 1. have kernel 2.2 or better, and 2. you've
|
|
compiled in support for this, and 3. your serial hardware supports it.
|
|
See
|
|
|
|
``Interrupt sharing and Kernels 2.2+''
|
|
|
|
If you only have two serial ports, ttyS0 and ttyS1, you're still OK
|
|
since IRQ sharing conflicts don't exist for non-existent devices.
|
|
|
|
If you add a legacy internal modem (without plug-and-play) and retain
|
|
ttyS0 and ttyS1, then you should attempt to find an unused IRQ and set
|
|
it in your serial port (or modem card) and then use setserial to
|
|
assign it to your device driver. If IRQ 5 is not being used for a
|
|
sound card, this could be used for a modem.
|
|
|
|
|
|
11.3.12. Laptops: PCMCIA
|
|
|
|
If you have a Laptop, read PCMCIA-HOWTO for info on the serial
|
|
configuration. For serial ports on the motherboard, setserial is used
|
|
just like it is for a desktop. But for PCMCIA cards (such as a modem)
|
|
it's a different story. The configuring of the PCMCIA system should
|
|
automatically run setserial so you shouldn't need to run it. If you
|
|
do run it (by a script file or by /etc/serial.conf) it might be
|
|
different and cause trouble. The autosave feature for serial.conf
|
|
shouldn't save anything for PCMCIA cards (but Debian did until
|
|
2.15-7). Of course, it's always OK to use setserial to find out how
|
|
the driver is configured for PCMCIA cards.
|
|
|
|
|
|
|
|
11.4. Stty
|
|
|
|
11.4.1. Introduction
|
|
|
|
stty does much of the configuration of the serial port but since
|
|
application programs (and the getty program) often handle this, you
|
|
may not need to use it much. It's handy if you're having problems or
|
|
want to see how the port is set up. Try typing ``stty -a'' at your
|
|
terminal/console to see how it's now set. Also try typing it without
|
|
the -a (all) for a short listing which shows how it's set different
|
|
than "normal" which is how it's set using the command "stty sane".
|
|
Don't try to learn all the setting unless you want to become a serial
|
|
historian since many of the settings are only for slow antique dumb
|
|
terminals of the 1970's. Most of the defaults should work OK.
|
|
|
|
stty is documented in the man pages with a more detailed account in
|
|
the info pages. Type "man stty" or "info stty".
|
|
|
|
Many of the stty options start with an "o" (output) or an "i" (input).
|
|
For example: onlcr. Output is the flow of bytes out of the computer
|
|
while input is the flow of bytes into the computer. The "point of
|
|
view" is the computer, not the serial port or the device connected to
|
|
the serial port. For example, received input data comes in on a cable
|
|
and goes to the serial port chip. This chip, after converting the
|
|
bits from the serial to parallel representation, then sends it (via a
|
|
program read) to the large serial port buffer in main computer memory.
|
|
Thus the chip has both input and output but since it's input data to
|
|
the computer, its output is considered to be input. The situation is
|
|
similar for output flowing thru this chip. The "input" and "output"
|
|
refer to the direction of flow with respect to the computer and not
|
|
the serial port hardware (the chip).
|
|
|
|
Whereas setserial only deals with actual serial ports, stty is used
|
|
both for serial ports and for virtual terminals such as the standard
|
|
Linux text interface on a PC monitor. For the PC monitor, many of the
|
|
stty settings are meaningless. Changing the baud rate, etc. doesn't
|
|
appear to actually do anything.
|
|
|
|
Here are some of the items stty configures: speed (bits/sec), parity,
|
|
bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
|
|
control, break signal, end-of-line markers, change case, padding, beep
|
|
if buffer overrun?, echo what you type to the screen, allow background
|
|
tasks to write to terminal?, define special (control) characters (such
|
|
as what key to press for interrupt). See the stty man or info page
|
|
for more details. Also see the man page: termios which covers the
|
|
same options set by stty but (as of mid 1999) covers features which
|
|
the stty man page fails to mention.
|
|
|
|
|
|
With some implementations of getty (getty_ps package), the commands
|
|
that one would normally give to stty are typed into a getty
|
|
configuration file: /etc/gettydefs. Even without this configuration
|
|
file, the getty command line may be sufficient to set things up so
|
|
that you don't need stty.
|
|
|
|
One may write C programs which change the stty configuration, etc.
|
|
Looking at some of the documentation for this may help one better
|
|
understand the use of the stty command (and its many possible
|
|
arguments). Serial-Programming-HOWTO may be useful but it's outdated.
|
|
The manual page: termios contains a description of the C-language
|
|
structure (of type termios) which stores the stty configuration in
|
|
computer memory. Many of the flag names in this C-structure are
|
|
almost the same (and do the same thing) as the arguments to the stty
|
|
command.
|
|
|
|
|
|
11.4.2. Flow control options
|
|
|
|
To set hardware flow control use "crtscts". For software flow control
|
|
there are 3 settings: ixon, ixoff, and ixany.
|
|
|
|
ixany: Mainly for terminals. Hitting any key will restart the flow
|
|
after a flow-control stop. If you stop scrolling with the "stop
|
|
scroll" key (or the like) then hitting any key will resume scrolling.
|
|
It's seldom needed since hitting the "scroll lock" key again will do
|
|
the same thing.
|
|
|
|
ixon: Enables the port to listen for Xoff and to stop transmitting
|
|
when it gets an Xoff. Likewise, it will resume transmitting if it
|
|
gets an Xon.
|
|
|
|
ixoff: enables the port to send the Xoff signal out the transmit line
|
|
when its buffers in main memory are nearly full. It protects the
|
|
device where the port is located from being overrun.
|
|
For a slow dumb terminal (or other slow device) connected to a fast
|
|
PC, it's unlikely that the PC's port will be overrun. So you seldom
|
|
actually need to enable ixoff. But it's often enabled "just in case".
|
|
|
|
|
|
11.4.3. Using stty at a "foreign" terminal
|
|
|
|
How do you use stty to view or set a terminal other than the terminal
|
|
you are currently using? It's usually impossible to do it if the
|
|
foreign terminal is in use and has a shell running on it. In other
|
|
cases for dealing with say ttyS2 while typing at another terminal
|
|
(such as tty1) use stty -F /dev/ttyS2 ... (or --file instead of F).
|
|
If ... is -a it displays all the stty settings (-a means all).
|
|
|
|
But if the foreign terminal (ttyS2 in this example) has a shell
|
|
running on it, then what you see will likely be deceptive and trying
|
|
to set it will not work. This problem exists for virtual terminals
|
|
also such as dealing with tty3 from tty1, etc. See ``Two interfaces
|
|
at a terminal'' to understand it.
|
|
|
|
|
|
11.4.4. Two interfaces at a terminal
|
|
|
|
When using a shell (such as bash) with command-line-editing enabled
|
|
there are two different terminal interfaces (what you see when you
|
|
type stty -a). When you type in modern shells at the command line you
|
|
have a temporary "raw" interface (or raw mode) where each character is
|
|
read by the command-line-editor as you type it. Once you hit the
|
|
<return> key, the command-line-editor is exited and the terminal
|
|
interface is changed to the nominal "cooked" interface (cooked mode)
|
|
for the terminal. This cooked mode lasts until the next prompt is
|
|
sent to the terminal (which is only a small fraction of a second).
|
|
Note that one never gets to type any command in this cooked mode but
|
|
what was typed in raw mode on the command line gets read by the shell
|
|
while in cooked mode.
|
|
|
|
When a prompt is sent to the terminal, the terminal goes from "cooked"
|
|
to "raw" mode (just like it does when you start an editor such as vim.
|
|
The prompt signals starting the command-line editor. The settings for
|
|
the "raw" mode are based only on the basic stty settings taken from
|
|
the "cooked" mode. Raw mode keeps these setting but changes several
|
|
other settings in order to change the mode to "raw". It is not at all
|
|
based on the settings used in the previous "raw" mode. Thus if one
|
|
uses stty to change settings for the raw mode, such settings will be
|
|
permanently lost as soon as one hits the <return> key at the terminal
|
|
that has supposedly been "set".
|
|
|
|
Now when one types stty to look at the terminal interface, one may
|
|
either get a view of the cooked mode or the raw mode. You need to
|
|
figure out which one you're looking at. It you use stty from a
|
|
foreign terminal (other than the terminal you are currently typing at)
|
|
then you will see the raw mode settings. Any changes made will only
|
|
be made to the raw mode and will be lost when someone presses <return>
|
|
at the foreign terminal you tried to "set". But if you type a stty
|
|
command to view/change the configuration of the terminal you are
|
|
using, and then hit <return> it's a different story. The <return>
|
|
puts the terminal in cooked mode. Your changes are saved and will
|
|
still be there when the terminal goes back into raw mode (unless of
|
|
course it's a setting not allowed in raw mode).
|
|
|
|
This situation can create problems. For example, suppose you corrupt
|
|
your terminal interface. To restore it you go to another terminal and
|
|
"stty -F dev/ttyS1 sane" (or the like). It will not work! Of course
|
|
you can try to type "stty sane ..." at the terminal that is corrupted
|
|
but you can't see what you typed. All the above not only applies to
|
|
dumb terminals but to virtual terminals used on a PC Monitor as well
|
|
as to the terminal windows in X. In other words, it applies to almost
|
|
everyone who uses Linux.
|
|
|
|
Luckily, when you start up Linux, any file that runs stty at boot-time
|
|
will likely deal with a terminal (or serial port with no terminal)
|
|
that has no shell running on it so there's no problem for this special
|
|
case.
|
|
|
|
|
|
11.4.5. Where to put the stty command ?
|
|
|
|
Should you need to have stty set up the serial interface each time the
|
|
computer starts up then you need to put the stty command in a file
|
|
that will be executed each time the computer is started up (Linux
|
|
boots). It should be run before the serial port is used (including
|
|
running getty on the port). There are many possible places to put it.
|
|
If it gets put in more than one place and you only know about (or
|
|
remember) one of those places, then a conflict is likely. So make
|
|
sure to document what you do.
|
|
|
|
One place to put it would be in the same file that runs setserial when
|
|
the system is booted. The location is distribution and version
|
|
dependent. It would seem best to put it after the setserial command
|
|
so that the low level stuff is done first. If you have directories in
|
|
the /etc tree where every file in them is executed at boot-time
|
|
(System V Init) then you could create a file named "stty" for this
|
|
purpose.
|
|
|
|
|
|
11.4.6. Obsolete redirection method
|
|
|
|
Prior to about 2000 you needed to use the redirection operator "<" if
|
|
you wanted to use stty on a foreign terminal. For example to use stty
|
|
on ttyS2 sitting at tty1 you would type: stty .... < /dev/ttyS2.
|
|
After 2000 (provided your version of setserial is >= 1.17 and stty >=
|
|
2.0) a better method was created using the -F option: stty -F
|
|
/dev/ttyS2. This will work when the old redirection method fails.
|
|
|
|
The old redirection example above makes ttyS2 the standard input to
|
|
stty. This gives the stty program a link to the "file" ttyS2 so that
|
|
it may "read" it. But instead of reading the bytes sent to ttyS2 as
|
|
one might expect, it uses the link to find the configuration settings
|
|
of the port so that it may read or change them. In olden days, some
|
|
people tried to use ``stty ... > /dev/ttyS2'' to set the terminal.
|
|
This didn't work. Instead, it takes the message normal displayed by
|
|
the stty command for the terminal you are on (say tty1) and sends this
|
|
message to ttyS2. But it doesn't change any settings for ttyS2.
|
|
|
|
Here's a problem with the old redirection operator (which doesn't
|
|
happen if you use the newer -F option instead). Sometimes when trying
|
|
to use stty, the command hangs and nothing happens (you don't get a
|
|
prompt for a next command even after hitting <return>). This is
|
|
likely due to the port being stuck because it's waiting for one of the
|
|
modem control lines to be asserted. For example, unless you've set
|
|
"clocal" to ignore modem control lines, then if no CD signal is
|
|
asserted the port will not open and stty will not work for it (unless
|
|
you use the newer -F option). A similar situation seems to exist for
|
|
hardware flow control. If the cable for the port doesn't even have a
|
|
conductor for the pin that needs to be asserted then there is no easy
|
|
way to stop the hang.
|
|
|
|
One way to try to get out of the above hang is to use the newer -F
|
|
option and set "clocal" and/or "crtscts" as needed. If you don't have
|
|
the -F option then you may try to run some program (such as minicom)
|
|
on the port that will force it to operate even if the control lines
|
|
say not to. Then hopefully this program might set the port so it
|
|
doesn't need the control signal in the future in order to open: clocal
|
|
or -crtscts. To use "minicom" to do this you likely will have to
|
|
reconfigure minicom and then exit it and restart it. Instead of all
|
|
this bother, it may be simpler to just reboot the PC or via using a
|
|
virtual terminal kill the process using "top" (or "ps" to get the
|
|
process number and then "kill" to kill that process.
|
|
|
|
The obsolete redirection method (which still works in later versions)
|
|
is to type ``stty ... < /dev/ttyS2''. If the new method using -F
|
|
works but the obsolete one hangs, it implies that the port is hung due
|
|
to a modem control line not being asserted. Thus the obsolete
|
|
redirection method might still useful for troubleshooting.
|
|
|
|
|
|
|
|
11.5. What is isapnp ?
|
|
|
|
isapnp is a program to configure Plug-and-Play (PnP) devices on the
|
|
ISA bus including internal modems. It comes in a package called
|
|
"isapnptools" and includes another program, "pnpdump" which finds all
|
|
your ISA PnP devices and shows you options for configuring them in a
|
|
format which may be added to the PnP configuration file:
|
|
/etc/isapnp.conf. The isapnp command may be put into a startup file
|
|
so that it runs each time you start the computer and thus will
|
|
configure ISA PnP devices. It is able to do this even if your BIOS
|
|
doesn't support PnP. See Plug-and-Play-HOWTO.
|
|
|
|
|
|
11.6. Connecting two PCs together via serial ports
|
|
|
|
This is where you run a serial cable (crossover type = null-modem
|
|
type) between the serial ports of two PCs. Then how do you use this
|
|
line? One way is for one PC to run login on the serial line and for
|
|
the other PC to run say minicom or picocom to emulate a terminal. See
|
|
Text-Terminal-HOWTO. There is no network protocol used in this case
|
|
and no error detection.
|
|
|
|
The other method is to run a network protocol on the line. For
|
|
example, to use PPP in combination with TCP/IP see Serial Laplink
|
|
HOWTO. Although this HOWTO doesn't mention the old program "slattach"
|
|
(serial line attach) it can put the serial line into a networking mode
|
|
using the protocol you select. Protocols for slattach include PPP or
|
|
SLIP (an older protocol widely used prior to PPP).
|
|
|
|
The Debian package, net-tools, contains slattach. SLIP is provided as
|
|
a kernel module or can be built into the kernel (2.2, 2.4, or 2.6).
|
|
|
|
|
|
11.7. Connect the serial port to a fast network: ser2net
|
|
|
|
ser2net is a Linux program which will connect a network to the serial
|
|
port. For example, someone connects to your PC via an ethernet port
|
|
or fast modem using say telnet. Then (without ser2net) they could
|
|
remotely login to your PC and then run programs on your PC that
|
|
utilize a serial port on the PC. However, it might be better if they
|
|
didn't need to login and use your software, but instead could
|
|
immediately connect to the serial port. ser2net running on your PC
|
|
can make this happen.
|
|
|
|
It could be like a bridge between the ethernet cable and the serial
|
|
cable. The ethernet cable would have TCP/IP protocol on it but the
|
|
serial port would just have the raw data taken out of the TCP/IP
|
|
packets. Optionally, you could use TCP/IP packets on the serial line
|
|
too. Since an ethernet port has high bandwidth, it could communicate
|
|
with several serial ports at the same time and also have data flowing
|
|
elsewhere as well.
|
|
To set up ser2net, you must specify which network ports (on the
|
|
ethernet) will connect to which serial ports. Then when network
|
|
packets arrive at your PC over ethernet which are addressed to a
|
|
network port you've tied to a serial port, the data in those packets
|
|
flows to the serial port. And conversely. Of course the network
|
|
doesn't have to be ethernet. It could be a cable modem or DSL line,
|
|
etc.
|
|
|
|
|
|
12. Speed (Flow Rate)
|
|
|
|
By "speed" we really mean the "data flow rate" but almost everybody
|
|
incorrectly calls it speed. The speed is measured in bits/sec (or
|
|
baud). Speed is set using the "stty" command or by a program which
|
|
uses the serial port. See ``Stty''
|
|
|
|
|
|
12.1. Very High Speeds
|
|
|
|
12.1.1. Speeds over 115.2kbps
|
|
|
|
The top speed of 115.2k has been standard since the mid 1990's. But
|
|
by the year 2000, most new serial ports supported higher speeds of
|
|
230.4k and 460.8k. Some also support 921.6k. Unfortunately Linux
|
|
seldom uses these speeds due to lack of drivers. Thus such ports
|
|
behave just like 115.2k ports unless the higher speeds are enabled by
|
|
special software. To get these speeds you need to compile the kernel
|
|
with special patches or use modules until support is built into the
|
|
kernel's serial driver.
|
|
|
|
Unfortunately serial port manufacturers never got together on a
|
|
standard way to support high speeds, so the serial driver needs to
|
|
support a variety of hardware. Once high speed is enabled, a standard
|
|
way to choose it is to set baud_base to the highest speed with
|
|
setserial (unless the serial driver does this for you). The software
|
|
will then use a divisor of 1 to set the highest speed. All this will
|
|
hopefully be supported by the Linux kernel sometime in 2003.
|
|
|
|
A driver for the w83627hf chip (used on many motherboards such as the
|
|
Tyan S2460) is at <https://www.muru.com/linux/w83627hf/>
|
|
|
|
A non-standard way that some manufacturers have implemented high speed
|
|
is to use a very large number for the divisor to get the high speed.
|
|
This number isn't really a divisor at all since it doesn't divide
|
|
anything. It's just serves as a code number to tell the hardware what
|
|
speed to use. In such cases you need to compile the kernel with
|
|
special patches.
|
|
|
|
One patch to support this second type of high-speed hardware is called
|
|
shsmod (Super High Speed Mode). There are both Windows and Linux
|
|
versions of this patch. See <http://www.devdrv.com/shsmod/>. There
|
|
is also a module for the VIA VT82C686 chip
|
|
<http://www.kati.fi/viahss/>. Using it may result in buffer overflow.
|
|
|
|
For internal modems, only a minority of them advertise that they
|
|
support speeds of over 115.2k for their built-in serial ports. Does
|
|
shsmod support these ??
|
|
|
|
|
|
12.1.2. How speed is set in hardware: the divisor and baud_base
|
|
|
|
Speed is set by having the serial port's clock change frequency. But
|
|
this change happens not by actually changing the frequency of the
|
|
oscillator driving the clock but by "dividing" the clock's frequency.
|
|
For example, to divide by two, just ignore every other clock tick.
|
|
This cuts the speed in half. Dividing by 3 makes the clock run at 1/3
|
|
frequency, etc. So to slow the clock down (meaning set speed), we
|
|
just send the clock a divisor. It's sent by the serial driver to a
|
|
register in the port. Thus speed is set by a divisor.
|
|
|
|
If the clock runs at a top speed of 115,000 bps (common), then here
|
|
are the divisors for various speeds (assuming a maximum speed of
|
|
115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24
|
|
(4.8k), 48 (2.4k), 96 (1.2k), etc. The serial driver sets the speed
|
|
in the hardware by sending the hardware only a "divisor" (a positive
|
|
integer). This "divisor" divides the "maximum speed" of the hardware
|
|
resulting in a slower speed (except a divisor of 1 obviously tells the
|
|
hardware to run at maximum speed).
|
|
|
|
There are exceptions to the above since for certain serial port
|
|
hardware, speeds above 115.2k are set by using a very high divisor.
|
|
Keep that exception in mind as you read the rest of this section.
|
|
Normally, if you specify a speed of 115.2k (in your communication
|
|
program or by stty) then the serial driver sets the port hardware to
|
|
divisor 1 which sets the highest speed.
|
|
|
|
Besides using a very high divisor to set high speed, the conventional
|
|
way to do it is as follows: If you happen to have hardware with a
|
|
maximum speed of say 230.4k (and the 230.4k speed has been enabled in
|
|
the hardware), then specifying 115.2k will result in divisor 1. For
|
|
some hardware this will actually give you 230.4k. This is double the
|
|
speed that you set. In fact, for any speed you set, the actual speed
|
|
will be double. If you had hardware that could run at 460.8k then the
|
|
actual speed would be quadruple what you set. All the above assumes
|
|
that you don't use "setserial" to modify things.
|
|
|
|
|
|
12.1.3. Setting the divisor, speed accounting
|
|
|
|
To correct this accounting (but not always fix the problem) you may
|
|
use "setserial" to change the baud_base to the actual maximal speed of
|
|
your port such as 230.4k. Then if you set the speed (by your
|
|
application or by stty) to 230.4k, a divisor of 1 will be used and
|
|
you'll get the same speed as you set.
|
|
|
|
If you have very old software which will not allow you to tell it such
|
|
a high speed (but your hardware has it enabled) then you might want to
|
|
look into using the "spd_cust" parameter. This allows you to tell the
|
|
application that the speed is 38,400 but the actual speed for this
|
|
case is determined by the value of "divisor" which has also been set
|
|
in setserial. I think it best to try to avoid using this kludge.
|
|
|
|
There are some brands of UARTs that uses a very high divisor to set
|
|
high speeds. There isn't any satisfactory way to use "setserial" (say
|
|
set "divisor 32770") to get such a speed since then setserial would
|
|
then think that the speed is very low and disable the FIFO in the
|
|
UART.
|
|
|
|
|
|
12.1.4. Crystal frequency is higher than baud_base
|
|
|
|
Note that the baud_base setting is usually much lower than the
|
|
frequency of the crystal oscillator since the crystal frequency of say
|
|
1.8432 MHz is divided by 16 in the hardware to get the actual top
|
|
speed of 115.2k. The reason the crystal frequency needs to be higher
|
|
is so that this high crystal speed can generate clock ticks to take a
|
|
number of samples of each bit to determine if it's a 1 or a 0.
|
|
|
|
Actually, the 1.8432 MHz "crystal frequency" may be obtained from a
|
|
18.432 MHz crystal oscillator by dividing by 10 before being fed to
|
|
the UART. Other schemes are also possible as long as the UART
|
|
performs properly.
|
|
12.2. Higher Serial Throughput
|
|
|
|
If you are seeing slow throughput and serial port overruns on a system
|
|
with (E)IDE disk drives, you can get hdparm. This is a utility that
|
|
can modify (E)IDE parameters, including unmasking other IRQs during a
|
|
disk IRQ. This will improve responsiveness and will help eliminate
|
|
overruns. Be sure to read the man page very carefully, since some
|
|
drive/controller combinations don't like this and may corrupt the
|
|
filesystem.
|
|
|
|
Also have a look at a utility called irqtune that will change the IRQ
|
|
priority of a device, for example the serial port that your modem is
|
|
on. This may improve the serial throughput on your system. The
|
|
irqtune FAQ is at http://www.best.com/~cae/irqtune
|
|
<http://www.best.com/~cae/irqtune>
|
|
|
|
|
|
13. Locking Out Others
|
|
|
|
13.1. Introduction
|
|
|
|
When you are using a serial port, you may want to prevent others from
|
|
using it at the same time. However there may be cases where you do
|
|
want others to use it, such as sending you an important message if you
|
|
are using a text-terminal.
|
|
|
|
There are various ways of preventing others (or other processes) from
|
|
using your serial port when you are using it (locking). This should
|
|
all happen automatically but it's important to know about this if it
|
|
gives you trouble. If a program is abnormally exited or the PC is
|
|
abruptly turned off (by pulling the plug, etc.) your serial port might
|
|
wind up locked. Even if the lock remains, it's usually automatically
|
|
removed when you want to use the serial port again. But in rare cases
|
|
it isn't. That's when you need to understand what happened.
|
|
|
|
One way to implement locking is to design the kernel to handle it but
|
|
Linux thus far has shunned this solution (with an exception involving
|
|
the cua device which is now obsolete). Two solutions used by Linux is
|
|
to:
|
|
|
|
1. create lock-files
|
|
|
|
2. modify the permissions and/or owners of devices such as /dev/ttyS2
|
|
|
|
|
|
13.2. Lock-Files
|
|
|
|
If you use the new device-filesystem (devfs) then see the next
|
|
section. A lock-file is simply a file created to mean that a
|
|
particular device is in use. They are kept in /var/lock. Formerly
|
|
they were in /usr/spool/uucp. Linux lock-files are usually named
|
|
LCK..name, where name may be a device name, a process id number, a
|
|
device's major and minor numbers, or a UUCP site name. Most processes
|
|
(an exception is getty) create these locks so that they can have
|
|
exclusive access to devices. For instance if you dial out on your
|
|
modem, some lockfiles will appear to tell other processes that someone
|
|
else is using the modem. In older versions (in the 1990s) there was
|
|
usually only one lockfile per process. Lock files contain the PID of
|
|
the process that has locked the device. Note that if a process
|
|
insists on using a device that is locked, it may ignore the lockfile
|
|
and use the device anyway. This is useful in sending a message to a
|
|
text-terminal, etc.
|
|
|
|
When a program wants to use a serial port but finds it locked with
|
|
lock-files it should check to see if the lock-file's PID is still in
|
|
use. If it's not it means that the lock is stale and it's OK to go
|
|
ahead and use the port anyway (after removing the stale lock-files).
|
|
Unfortunately, there may be some programs that don't do this and give
|
|
up by telling you that a device is already in use when it really
|
|
isn't.
|
|
|
|
If the lockfile only uses device names, the following problem could
|
|
arise: If the same device has two different names then two different
|
|
processes could each use a different name for the same device. This
|
|
results in lockfiles with different names that actually are the same
|
|
device. Formerly each physical serial port was known by two different
|
|
device names: ttyS0 and cua0. To solve this lockfile alias problem, 3
|
|
methods have been used. It may be overkill since any one of these
|
|
methods would have fixed the problem.
|
|
|
|
|
|
1. The lock checking software was made aware of ttyS vs. cua.
|
|
|
|
2. The device cua was deprecated
|
|
|
|
3. Additional locks were created which use unique device numbers
|
|
instead of names.
|
|
|
|
Using alternate names such as /dev/modem for /dev/ttyS2 may cause
|
|
problems if one program opens /dev/ttyS2 while another program opens
|
|
/dev/modem. This problem was supposedly fixed around 2000, but is
|
|
still present in 2005. For dumb terminals, lockfiles are not used
|
|
since this would not permit someone else to send a message to your
|
|
terminal using the write or talk program.
|
|
|
|
|
|
|
|
13.3. Change Owners, Groups, and/or Permissions of Device Files
|
|
|
|
In order to use a device, you (or the program you run if you have "set
|
|
user id") needs to have permission to read and write the device "file"
|
|
in the /dev directory. So a logical way to prevent others from using
|
|
a device is to make yourself the temporary owner of the device and set
|
|
permissions so that no one else can use it. A program may do this for
|
|
you. A similar method can be used with the group of the device file.
|
|
|
|
While lock files prevent other process from using the device, changing
|
|
device file owners/permissions restricts other users (or the group)
|
|
from using it. One case is where the group is permitted to write to
|
|
the port, but not to read from it. Writing to the port might just
|
|
mean a message sent to a text-terminal while reading means destructive
|
|
reading. The original process that needs to read the data may find
|
|
data missing if another process has already read that data. Thus a
|
|
read can do more harm that a write since a read causes loss of data
|
|
while a write only adds extra data. That's a reason to allow writes
|
|
but not reads. This is exactly the opposite of the case for ordinary
|
|
files where you allow others to read the file but not write (modify)
|
|
it. Use of a port normally requires both read and write permissions.
|
|
|
|
A program that changes the device file attributes should undo these
|
|
changes when it exits. But if the exit is abnormal, then a device
|
|
file may be left in such a condition that it gives the error
|
|
"permission denied" when one attempts to use it again.
|
|
|
|
|
|
14. Serial Communications Programs And Utilities
|
|
|
|
14.1. List of Software
|
|
|
|
Here is a list of some communication software you can choose from,
|
|
available via FTP, if they didn't come with your Linux distribution.
|
|
|
|
· ecu - a communications program
|
|
|
|
· C-Kermit <http://www.columbia.edu/kermit/> - portable, scriptable,
|
|
serial and TCP/IP communications including file transfer,
|
|
character-set translation, and zmodem support
|
|
|
|
· gkermit Tiny GPLed kermit run only from the command line. Can't
|
|
connect to another computer
|
|
|
|
· gtkterm - a simple gtk terminal, X-based
|
|
|
|
· minicom - telix-like communications program
|
|
|
|
· picocom - like a small minicom but no automatic phone dialing
|
|
|
|
· pppd - establishes a ppp connection on the serial line
|
|
|
|
· seyon - X based communication program
|
|
|
|
· xc - xcomm communication package
|
|
|
|
· term and SLiRP offer TCP/IP functionality using a shell account.
|
|
|
|
· screen is another multi-session program. This one behaves like the
|
|
virtual consoles.
|
|
|
|
· callback is where you dial out to a remote modem and then that
|
|
modem hangs up and calls you back (to save on phone bills).
|
|
|
|
· mgetty+fax handles FAX stuff, and provides an alternate ps_getty.
|
|
|
|
· ZyXEL is a control program for ZyXEL U-1496 modems. It handles
|
|
dialin, dialout, dial back security, FAXing, and voice mailbox
|
|
functions.
|
|
|
|
|
|
· SLIP and PPP software (if not in your Linux distribution) can be
|
|
found at ftp://metalab.unc.edu/pub/Linux/system/network/serial.
|
|
|
|
|
|
14.2. kermit and zmodem
|
|
|
|
For use of kermit with modems see the Modem-HOWTO. One can run zmodem
|
|
within the kermit program. To do this (for ttyS3), add the following
|
|
to your .kermrc file:
|
|
|
|
|
|
define rz !rz < /dev/ttyS3 > /dev/ttyS3
|
|
define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3
|
|
|
|
|
|
|
|
Be sure to put in the correct port your modem is on. Then, to use it,
|
|
just type rz or sz <filename> at the kermit prompt.
|
|
|
|
|
|
15. Serial Tips And Miscellany
|
|
|
|
15.1. Serial Modules
|
|
|
|
Often the serial driver is provided as a module(s) such as
|
|
generic_serial.ko. Drivers for USB serial ports and multiport cards
|
|
are often provided as modules. Linux should automatically load any
|
|
needed module, so in most cases you have nothing to do.
|
|
|
|
But sometimes you need to configure Linux to load certain modules or
|
|
gives parameters to the module or to the kernel.
|
|
|
|
Such parameters may be supplied to certain modules on the command line
|
|
for the kernel or in /etc/modules, /etc/modules.conf or
|
|
/etc/modprobe.conf. Since kernel 2.2 you don't edit the modprobe.conf
|
|
file but use the program update-modules to change it. The info that
|
|
is used to update modules.conf is put in /etc/modutils/.
|
|
|
|
The Debian/GNU Linux has a file named /etc/modutils/setserial which
|
|
runs the serial script in /etc/init.d/ every time the serial module is
|
|
loaded or unloaded. When the serial module is unloaded this script
|
|
will save the state of the module in /var/run/setserial.conf. Then if
|
|
the module loads again this saved state is restored. When the serial
|
|
module first loads at boot-time, there's nothing in
|
|
/var/run/setserial.conf so the state is obtained from
|
|
/etc/serial.conf. So there are two files that save the state. Other
|
|
distributions may do something similar.
|
|
|
|
Serial modules are found in subdirectories of
|
|
/lib/modules/.../kernel/drivers/. For multiport cards, look in the
|
|
serial subdirectory and/or char. For USB serial, look in the
|
|
usb/serial subdirectory. The module, parport_serial is for PCI cards
|
|
that contain both serial and parallel ports.
|
|
|
|
As a last resort, one may modify the serial driver by editing the
|
|
source code. Much of the serial driver is found in the file serial.c.
|
|
For info regarding writing of programs for the serial port see Serial-
|
|
Programming-HOWTO. It was revised in 1999 by Vern Hoxie but that
|
|
revision is not at LDP.
|
|
|
|
|
|
15.2. Kernel Configuration
|
|
|
|
15.3. Number of Serial Ports Supported
|
|
|
|
If you have more than 4 (or possibly 2) serial ports, then you must
|
|
insure that the kernel knows this. It can be done by configuring the
|
|
kernel when compiling or by a parameter given to the kernel when it
|
|
starts (boot-prompt or kernel command line).
|
|
|
|
The kernel configuration parameters:
|
|
CONFIG_SERIAL_8250_RUNTIME_UARTS=4 and CONFIG_SERIAL_8250_NR_UARTS=4
|
|
set the maximum number of ordinary serial ports (UARTs) equal to 4.
|
|
If you have more than 4 ordinary serial ports, then you need to change
|
|
the 4 to whatever. But you may override this via the kernel command
|
|
line for example: nr_uarts=16 (if serial support built into the
|
|
kernel) or 8250.nr_uarts=16 (if serial support is via a module). The
|
|
boot loader such as lilo or grub can be told to do this.
|
|
|
|
|
|
15.4. Serial Console (console on the serial port)
|
|
|
|
See the kernel documentation in: Documentation/serial-console.txt.
|
|
Kernel 2.4+ has better documentation. See also "Serial Console" in
|
|
Text-Terminal-HOWTO.
|
|
|
|
|
|
15.5. Line Drivers
|
|
|
|
For a text terminal, the RS-232 speeds are fast enough but the usable
|
|
cable length is often too short. Balanced technology could fix this.
|
|
The common method of obtaining balanced communication with a text
|
|
terminal is to install 2 line drivers in the serial line to convert
|
|
unbalanced to balanced (and conversely). They are a specialty item
|
|
and are expensive if purchased new.
|
|
15.6. Stopping the Data Flow when Printing, etc.
|
|
|
|
Normally flow control and/or application programs stop the flow of
|
|
bytes when its needed. But sometimes they don't. The problem is that
|
|
output to the serial port first passes thru the large serial buffer in
|
|
the PC's main memory. So if you want to abort printing, whatever is
|
|
in this buffer should be removed. When you tell an application
|
|
program to stop printing, it may not empty this buffer so printing
|
|
continues until it's empty. In addition, your printer has it's own
|
|
buffer which needs to be cleared. So telling the PC to stop printing
|
|
may not work due to these two buffers that continue to supply bytes
|
|
for the printer. It's a problem with printer software not knowing
|
|
about the serial port and that modem control lines need to be dropped
|
|
to stop the printer.
|
|
|
|
One way to insure that printing stops is to just turn off the printer.
|
|
With newer serial drivers, this works OK. The buffers are cleared and
|
|
printing doesn't resume. With older serial drivers, the PC's serial
|
|
buffer didn't clear and it would sometimes continue to print when the
|
|
printer was turned back on. To avoid this, you must wait a time
|
|
specified by setserial's closing_wait before turning the printer back
|
|
on again. You may also need to remove the print job from the print
|
|
queue so it won't try to resume.
|
|
|
|
|
|
15.7. Known IO Address Conflicts
|
|
|
|
15.7.1. Avoiding IO Address Conflicts with Certain Video Boards
|
|
|
|
The IO address of the IBM 8514 video board (and others) is allegedly
|
|
0x?2e8 where ? is 2, 4, 8, or 9. This may conflict (but shouldn't if
|
|
the serial port is well designed) with the IO address of ttyS3 at
|
|
0x02e8 if the serial port ignores the leading 0 hex digit when it
|
|
decodes the address (many do). That is bad news if you try to use
|
|
ttyS3 at this IO address. Another story is that Linux will not detect
|
|
your internal modem on ttyS3 but that you can use setserial to put
|
|
ttyS3 at this address and the modem will work fine.
|
|
|
|
15.7.2. IO address conflict with ide2 hard drive
|
|
|
|
The address of ttyS2 is 3e8-3ef while hard drive ide2 uses 3ee which
|
|
is in this range. So when booting Linux you may see a report of this
|
|
conflict. Most people don't use ide2 (the 3rd hard drive cable) and
|
|
may ignore this conflict message. You may have 2 hard drives on ide0
|
|
and two more on ide1 so most people don't need ide2.
|
|
|
|
|
|
15.8. Known Defective Hardware
|
|
|
|
15.8.1. Problem with AMD Elan SC400 CPU (PC-on-a-chip)
|
|
|
|
This has a race condition between an interrupt and a status register
|
|
of the UART. An interrupt is issued when the UART transmitter
|
|
finishes the transmission of a byte and the UART transmit buffer
|
|
becomes empty (waiting for the next byte). But a status register of
|
|
the UART doesn't get updated fast enough to reflect this. As a
|
|
result, the interrupt service routine rapidly checks and determines
|
|
(erroneously) that nothing has happened. Thus no byte is sent to the
|
|
port to be transmitted and the UART transmitter waits in vain for a
|
|
byte that never arrives. If the interrupt service routine had waited
|
|
just a bit longer before checking the status register, then it would
|
|
have been updated to reflect the true state and all would be OK.
|
|
|
|
There is a proposal to fix this by patching the serial driver. But
|
|
Should linux be patched to accommodate defective hardware, especially
|
|
if this patch may impair performance of good hardware?
|
|
16. Troubleshooting
|
|
|
|
See Modem-HOWTO for troubleshooting related to modems or getty for
|
|
modems. For a Text-Terminal much of the info here will be of value as
|
|
well as the troubleshooting info in Text-Terminal-HOWTO.
|
|
|
|
|
|
16.1. Serial Electrical Test Equipment
|
|
|
|
16.1.1. Breakout Gadgets, etc.
|
|
|
|
While a multimeter (used as a voltmeter) may be all that you need for
|
|
just a few serial ports, simple special test equipment has been made
|
|
for testing serial port lines. Some are called "breakout ... " where
|
|
breakout means to break out conductors from a cable. These gadgets
|
|
have a couple of connectors which connect to serial port connectors
|
|
(either at the ends of serial cables or at the back of a PC). Some
|
|
have test points for connecting a voltmeter. Others have LED lamps
|
|
which light when certain modem control lines are asserted (turned on).
|
|
The color of the light may indicate the polarity of the signal
|
|
(positive or negative voltage). Still others have jumpers so that you
|
|
can connect any wire to any wire. Some have switches.
|
|
|
|
Radio Shack sells (in 2002) a "RS-232 Troubleshooter" (formerly called
|
|
"RS-232 Line Tester") Cat. #276-1401. It checks TD, RD, CD, RTS, CTS,
|
|
DTR, and DSR. A green light means on (+12 v) while red means off (-12
|
|
v). They also sell a "RS-232 Serial Jumper Box" Cat. #276-1403.
|
|
This permits connecting the pins anyway you choose. Both these items
|
|
are under the heading of "Peripheral hookup helpers". Unfortunately,
|
|
they are not listed in the index to the printed catalog. They are on
|
|
the same page as the D type connecters so look in the index under
|
|
"Connectors, Computer, D-Sub". A store chain named "Active
|
|
Components" may have them.
|
|
|
|
|
|
16.1.2. Measuring voltages
|
|
|
|
Any voltmeter or multimeter, even the cheapest that sells for about
|
|
$10, should work fine. Trying to use other methods for checking
|
|
voltage is tricky. Don't use a LED unless it has a series resistor to
|
|
reduce the voltage across the LED. A 470 ohm resistor is used for a
|
|
20 ma LED (but not all LED's are 20 ma). The LED will only light for
|
|
a certain polarity so you may test for + or - voltages. Does anyone
|
|
make such a gadget for automotive circuit testing?? Logic probes may
|
|
be damaged if you try to use them since the TTL voltages for which
|
|
they are designed are only 5 volts. Trying to use a 12 V incandescent
|
|
light bulb is not a good idea. It won't show polarity and due to
|
|
limited output current of the UART it probably will not even light up.
|
|
|
|
To measure voltage on a female connector you may plug in a bent paper
|
|
clip into the desired opening. The paper clip's diameter should be no
|
|
larger than the pins so that it doesn't damage the contact. Clip an
|
|
alligator clip (or the like) to the paper clip to connect up. Take
|
|
care not to touch two pins at the same time with any metal object.
|
|
|
|
|
|
16.1.3. Taste voltage
|
|
|
|
As a last resort, if you have no test equipment and are willing to
|
|
risk getting shocked (or even electrocuted) you can always taste the
|
|
voltage. Before touching one of the test leads with your tongue, test
|
|
them to make sure that there is no high voltage on them. Touch both
|
|
leads (at the same time) to one hand to see if they shock you. Then
|
|
if no shock, wet the skin contact points by licking and repeat. If
|
|
this test gives you a shock, you certainly don't want to use your
|
|
tongue.
|
|
For the test for 12 V, Lick a finger and hold one test lead in it.
|
|
Put the other test lead on your tongue. If the lead on your tongue is
|
|
positive, there will be a noticeable taste. You might try this with
|
|
flashlight batteries first so you will know what taste to expect.
|
|
|
|
|
|
|
|
16.2. Serial Monitoring/Diagnostics
|
|
|
|
A few Linux programs will monitor the modem control lines and indicate
|
|
if they are positive (1) or negative (0). See section ``Serial
|
|
Monitoring/Diagnostics''
|
|
|
|
|
|
16.3. (The following subsections are in both the Serial and Modem
|
|
HOWTOs)
|
|
|
|
16.4. Serial Port Can't be Found
|
|
|
|
There are 3 possibilities:
|
|
|
|
1. Your port is disabled since both the BIOS and Linux failed to
|
|
enable it. It has no IO address.
|
|
|
|
2. Your port is enabled and has an IO address but it has no ttyS
|
|
device number (like ttyS14) assigned to that address so the port
|
|
can't be used. As a last resort, you may need to use "setserial"
|
|
to assign a ttyS number to it.
|
|
|
|
3. Your port does have a ttyS number assigned to it (like ttyS14) but
|
|
you don't know which physical connector it is (on the back of your
|
|
PC).
|
|
|
|
See ``Which Connector on the Back of my PC is ttyS1, etc?''
|
|
|
|
First check BIOS messages at boot-time (and possibly the BIOS menu for
|
|
the serial port). Then for the PCI bus type lspci -v. If this shows
|
|
something like "LPC Bridge" then your port is likely on the LPC bus
|
|
which is not well supported by Linux yet (but the BIOS might find it)
|
|
?? If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
|
|
and/or see Plug-and-Play-HOWTO. If the port happens to be enabled
|
|
then the following two paragraphs may help find the IO port:
|
|
|
|
|
|
16.4.1. Scanning/probing legacy ports
|
|
|
|
This is mainly for legacy non-PCI ports and ISA ports that are not
|
|
Plug-and-Play.
|
|
|
|
Using "scanport" (Debian only ??) will scan all enabled bus ports and
|
|
may discover an unknown port that could be a serial port (but it
|
|
doesn't probe the port). It could hang your PC. If you suspect that
|
|
your port may be at a certain address, you may try manually probing
|
|
with setserial, but it's a slow tedious task if you have several
|
|
addresses to probe. See ``Probing''.
|
|
|
|
|
|
16.5. Linux Creates an Interrupt Conflict (your PC has an ISA slot)
|
|
|
|
If your PC has a BIOS that handles ISA (and likely PCI too) then if
|
|
you find a IRQ conflict, it might be due to a shortage of free IRQs.
|
|
The BIOS often maintains a list of reserved IRQs, reserved for legacy
|
|
ISA cards. If too many are reserved, the BIOS may not be able to find
|
|
a free IRQ and will erroneously assign an IRQ to the serial port that
|
|
creates a conflict. So check to see if all the reserved IRQs are
|
|
really needed and if not, unreserve an IRQ that the serial port can
|
|
use. For more details, see Plug-and-Play-HOWTO.
|
|
|
|
|
|
16.6. Extremely Slow: Text appears on the screen slowly after long
|
|
delays
|
|
|
|
It's likely mis-set/conflicting interrupts. Here are some of the
|
|
symptoms which will happen the first time you try to use a modem,
|
|
terminal, or serial printer. In some cases you type something but
|
|
nothing appears on the screen until many seconds later. Only the last
|
|
character typed may show up. It may be just an invisible <return>
|
|
character so all you notice is that the cursor jumps down one line.
|
|
In other cases where a lot of data should appear on the screen, only a
|
|
batch of about 16 characters appear. Then there is a long wait of
|
|
many seconds for the next batch of characters. You might also get
|
|
"input overrun" error messages (or find them in logs).
|
|
|
|
|
|
|
|
For more details on the symptoms and why this happens see ``Interrupt
|
|
Problem Details'' and/or ``Interrupt Conflicts'' and/or ``Mis-set
|
|
Interrupts''. If it involves Plug-and-Play devices, see also the
|
|
Plug-and-Play-HOWTO.
|
|
|
|
As a quick check to see if it really is an interrupt problem, set the
|
|
IRQ to 0 with "setserial". This will tell the driver to use polling
|
|
instead of interrupts. If this seems to fix the "slow" problem then
|
|
you had an interrupt problem. You should still try to solve the
|
|
problem since polling uses excessive computer resources.
|
|
|
|
Checking to find the interrupt conflict may not be easy since Linux
|
|
supposedly doesn't permit any interrupt conflicts and will send you a
|
|
``/dev/ttyS?: Device or resource busy'' error message if it thinks you
|
|
are attempting to create a conflict. But a real conflict can be
|
|
created if "setserial" has told the kernel incorrect info. The kernel
|
|
has been lied to and thus doesn't think there is any conflict. Thus
|
|
using "setserial" will not reveal the conflict (nor will looking at
|
|
/proc/interrupts which bases its info on "setserial"). You still need
|
|
to know what "setserial" thinks so that you can pinpoint where it's
|
|
wrong and change it when you determine what's really set in the
|
|
hardware.
|
|
|
|
What you need to do is to check how the hardware is set by checking
|
|
jumpers or using PnP software to check how the hardware is actually
|
|
set. For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
|
|
"lspci" (if PCI bus). Compare this to how Linux (e.g. "setserial")
|
|
thinks the hardware is set.
|
|
|
|
|
|
16.7. Somewhat Slow: I expected it to be a few times faster
|
|
|
|
An obvious reason is that the baud rate is set too slow. It's claimed
|
|
that this once happened by trying to set the baud rate to a speed
|
|
higher than the hardware can support (such as 230400).
|
|
|
|
Another reason may be that whatever is on the serial port (such as a
|
|
modem, terminal, printer) doesn't work as fast as you thought it did.
|
|
|
|
|
|
Another possible reason is that you have an obsolete serial port: UART
|
|
8250, 16450 or early 16550 (or the serial driver thinks you do). See
|
|
|
|
``What Are UARTS?'' Use "setserial -g /dev/ttyS*". If it shows
|
|
anything less than a 16550A, this may be your problem. If you think
|
|
that "setserial" has it wrong check it out. See ``What is Setserial''
|
|
for more info. If you really do have an obsolete serial port, lying
|
|
about it to setserial will only make things worse.
|
|
|
|
|
|
16.8. The Startup Screen Shows Wrong IRQs for the Serial Ports
|
|
|
|
For non-PnP ports, Linux does not do any IRQ detection on startup.
|
|
When the serial module loads it only does serial device detection.
|
|
Thus, disregard what it says about the IRQ, because it's just assuming
|
|
the standard IRQs. This is done, because IRQ detection is unreliable,
|
|
and can be fooled. But if and when setserial runs from a start-up
|
|
script, it changes the IRQ's and displays the new (and hopefully
|
|
correct) state on on the startup screen. If the wrong IRQ is not
|
|
corrected by a later display on the screen, then you've got a problem.
|
|
|
|
So, even though I have my ttyS2 set at IRQ 5, I still see
|
|
|
|
|
|
ttyS02 at 0x03e8 (irq = 4) is a 16550A
|
|
|
|
|
|
|
|
at first when Linux boots. (Older kernels may show "ttyS02" as
|
|
"tty02" which is the same as ttyS2). You may need to use setserial to
|
|
tell Linux the IRQ you are using.
|
|
|
|
|
|
16.9. "Cannot open /dev/ttyS?: Device or resource busy
|
|
|
|
See ``/dev/tty? Device or resource busy''
|
|
|
|
|
|
16.10. "Cannot open /dev/ttyS?: Permission denied"
|
|
|
|
Check the file permissions on this port with "ls -l /dev/ttyS?"_ If
|
|
you own the ttyS? then you need read and write permissions: crw with
|
|
the c (Character device) in col. 1. It you don't own it then it will
|
|
work for you if it shows rw- in cols. 8 & 9 which means that everyone
|
|
has read and write permission on it. Use "chmod" to change
|
|
permissions. There are more complicated (and secure) ways to get
|
|
access like belonging to a "group" that has group permission. Some
|
|
programs change the permissions when they run but restore them when
|
|
the program exists normally. But if someone pulls the plug on your PC
|
|
it's an abnormal exit and correct permissions may not be restored.
|
|
|
|
|
|
16.11. "Cannot open /dev/ttyS?"
|
|
|
|
Unless stty is set for clocal, the CD pin may need to be asserted in
|
|
order to open a serial port. If the physical port is not connected to
|
|
anything, or if it's connected to something that is not powered on
|
|
(such an external modem) then there will be no voltage on CD from that
|
|
device. Thus the "cannot open" message. Either set clocal or connect
|
|
the serial port connector to something and power it on.
|
|
|
|
Even if a device is powered on and connected to a port, it may
|
|
sometimes prevent opening the port. An example of this is where the
|
|
device has negated CD and the CD pin on your PC is negated (negative
|
|
voltage).
|
|
|
|
|
|
16.12. "Operation not supported by device" for ttyS?
|
|
|
|
This means that an operation requested by setserial, stty, etc.
|
|
couldn't be done because the kernel doesn't support doing it.
|
|
Formerly this was often due to the "serial" module not being loaded.
|
|
But with the advent of PnP, it may likely mean that there is no modem
|
|
(or other serial device) at the address where the driver (and
|
|
setserial) thinks it is. If there is no modem there, commands (for
|
|
operations) sent to that address obviously don't get done. See ``What
|
|
is set in my serial port hardware?''
|
|
|
|
If the "serial" module wasn't loaded but "lsmod" shows you it's now
|
|
loaded it might be the case that it's loaded now but wasn't loaded
|
|
when you got the error message. In many cases the module will
|
|
automatically loaded when needed (if it can be found). To force
|
|
loading of the "serial" module it may be listed in the file:
|
|
/etc/modules.conf or /etc/modules. The actual module should reside
|
|
in: /lib/modules/.../misc/serial.o.
|
|
|
|
|
|
16.13. "Cannot create lockfile. Sorry"
|
|
|
|
Sometimes when it can't create a lockfile you get the erroneous
|
|
message: "... Device or resource busy" instead of the one above. When
|
|
a port is "opened" by a program a lockfile is created in /var/lock/.
|
|
Wrong permissions for the lock directory will not allow a lockfile to
|
|
be created there. Use "ls -ld /var/lock" to see if the permissions
|
|
are OK. Giving rwx permissions for the root owner and the group
|
|
should work, provided that the users that need to dialout belong to
|
|
that group. Others should have r-x permission. Even with this
|
|
scheme, there may be a security risk. Use "chmod" to change
|
|
permissions and "chgrp" to change groups. Of course, if there is no
|
|
"lock" directory no lockfile can be created there. For more info on
|
|
lockfiles see ``What Are Lock Files''
|
|
|
|
|
|
16.14. "Device /dev/ttyS? is locked."
|
|
|
|
This means that someone else (or some other process) is supposedly
|
|
using the serial port. There are various ways to try to find out what
|
|
process is "using" it. One way is to look at the contents of the
|
|
lockfile (/var/lock/LCK...). It should be the process id. If the
|
|
process id is say 100 type "ps 100" to find out what it is. Then if
|
|
the process is no longer needed, it may be gracefully killed by "kill
|
|
100". If it refuses to be killed use "kill -9 100" to force it to be
|
|
killed, but then the lockfile will not be removed and you'll need to
|
|
delete it manually. Of course if there is no such process as 100 then
|
|
you may just remove the lockfile but in most cases the lockfile should
|
|
have been automatically removed if it contained a stale process id
|
|
(such as 100).
|
|
|
|
|
|
16.15. "/dev/tty? Device or resource busy"
|
|
|
|
This means that the device you are trying to access (or use) is
|
|
supposedly busy (in use) or that a resource it needs (such as an IRQ)
|
|
is supposedly being used by another device and can't be shared. This
|
|
message is easy to understand if it only means that the device is busy
|
|
(in use). But it sometimes means that a needed resource is already in
|
|
use (busy). What makes it even more confusing is that in some cases
|
|
neither the device nor the resources that it needs are actually
|
|
"busy".
|
|
|
|
In olden days, if a PC was shutdown by just turning off the power, a
|
|
bogus lockfile might remain and then later on one would get this bogus
|
|
message and not be able to use the serial port. Software today is
|
|
supposed to automatically remove such bogus lockfiles, but as of 2006
|
|
there is still a problem with the "wvdial" dialer program related to
|
|
lockfiles. If wvdial can't create a lockfile because it doesn't have
|
|
write permission in the /var/lock/ directory, you will see this
|
|
erroneous message. See ``Cannot create lockfile. Sorry''
|
|
The following example is where interrupts can't be shared (at least
|
|
one of the interrupts is on the ISA bus). The ``resource busy'' part
|
|
often means (example for ttyS2) ``You can't use ttyS2 since another
|
|
device is using ttyS2's interrupt.'' The potential interrupt conflict
|
|
is inferred from what "setserial" thinks. A more accurate error
|
|
message would be ``Can't use ttyS2 since the setserial data (and
|
|
kernel data) indicates that another device is using ttyS2's
|
|
interrupt''. If two devices use the same IRQ and you start up only
|
|
one of the devices, everything is OK because there is no conflict yet.
|
|
But when you next try to start the second device (without quitting the
|
|
first device) you get a "... busy" error message. This is because the
|
|
kernel only keeps track of what IRQs are actually in use and actual
|
|
conflicts don't happen unless the devices are in use (open). The
|
|
situation for I/O address (such as 0x3f8) conflict is similar.
|
|
|
|
This error is sometimes due to having two serial drivers: one a module
|
|
and the other compiled into the kernel. Both drivers try to grab the
|
|
same resources and one driver finds them "busy".
|
|
|
|
There are two possible cases when you see this message:
|
|
|
|
1. There may be a real resource conflict that is being avoided.
|
|
|
|
2. Setserial has it wrong and the only reason ttyS2 can't be used is
|
|
that setserial erroneously predicts a conflict.
|
|
|
|
What you need to do is to find the interrupt setserial thinks ttyS2 is
|
|
using. Look at /proc/tty/driver/serial. You should also be able to
|
|
find it with the "setserial" command for ttyS2.
|
|
|
|
Bug in old versions: Prior to 2001 there was a bug which wouldn't let
|
|
you see it with "setserial". Trying to see it would give the same
|
|
"... busy" error message.
|
|
|
|
To try to resolve this problem reboot or: exit or gracefully kill all
|
|
likely conflicting processes. If you reboot: 1. Watch the boot-time
|
|
messages for the serial ports. 2. Hope that the file that runs
|
|
"setserial" at boot-time doesn't (by itself) create the same conflict
|
|
again.
|
|
|
|
If you think you know what IRQ say ttyS2 is using then you may look at
|
|
/proc/interrupts to find what else (besides another serial port) is
|
|
currently using this IRQ. You might also want to double check that
|
|
any suspicious IRQs shown here (and by "setserial") are correct (the
|
|
same as set in the hardware). A way to test whether or not it's a
|
|
potential interrupt conflict is to set the IRQ to 0 (polling) using
|
|
"setserial". Then if the busy message goes away, it was likely a
|
|
potential interrupt conflict. It's not a good idea to leave it
|
|
permanently set at 0 since it will put more load on the CPU.
|
|
|
|
|
|
16.16. "Input/output error" from setserial, stty, pppd, etc.
|
|
|
|
This means that communication with the serial port isn't working
|
|
right. It could mean that there isn't any serial port at the IO
|
|
address that setserial thinks your port is at. It could also be an
|
|
interrupt conflict (or an IO address conflict). It also may mean that
|
|
the serial port is in use (busy or opened) and thus the attempt to
|
|
get/set parameters by setserial or stty failed. It will also happen
|
|
if you make a typo in the serial port name such as typing "ttys"
|
|
instead of "ttyS".
|
|
|
|
|
|
|
|
16.17. "LSR safety check engaged"
|
|
|
|
LSR is the name of a hardware register. It usually means that there
|
|
is no serial port at the address where the driver thinks your serial
|
|
port is located. You need to find your serial port and possibly
|
|
configure it. See ``Locating the Serial Port: IO address IRQs''
|
|
and/or ``What is Setserial''
|
|
|
|
|
|
16.18. Overrun errors on serial port
|
|
|
|
This is an overrun of the hardware FIFO buffer and you can't increase
|
|
its size. Bug note (reported in 2002): Due to a bug in some kernel
|
|
2.4 versions, the port number may be missing and you will only see
|
|
"ttyS" (no port number). But if devfs notation such as "tts/2" was
|
|
being used, there was no bug. See ``Higher Serial Thruput''.
|
|
|
|
|
|
|
|
16.19. Port gets characters only sporadically
|
|
|
|
There could be some other program running on the port. Use "top"
|
|
(provided you've set it to display the port number) or type "ps
|
|
-alxw". Look at the results to see if the port is being used by
|
|
another program. Be on the lookout for the gpm mouse program which
|
|
often runs on a serial port.
|
|
|
|
|
|
16.20. Troubleshooting Tools
|
|
|
|
These are some of the programs you might want to use in
|
|
troubleshooting:
|
|
|
|
· "lsof /dev/ttyS*" will list serial ports which are open.
|
|
|
|
· "setserial" shows and sets the low-level hardware configuration of
|
|
a port (what the driver thinks it is). See ``What is Setserial''
|
|
|
|
· "stty" shows and sets the configuration of a port (except for that
|
|
handled by "setserial"). See the section ``Stty''
|
|
|
|
· "modemstat" or "statserial" or "watch head /proc/tty/driver/serial"
|
|
will show the current state of various modem signal lines (such as
|
|
DTR, CTS, etc.). The one in /proc also shows byte flow and errors.
|
|
|
|
· "irqtune" will give serial port interrupts higher priority to
|
|
improve performance.
|
|
|
|
· "hdparm" for hard-disk tuning may help some more.
|
|
|
|
· "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.
|
|
|
|
· "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
|
|
PnP devices on the ISA bus.
|
|
|
|
· Some "files" in the /proc tree (such as ioports, interrupts, and
|
|
tty/driver/serial).
|
|
|
|
|
|
|
|
16.21. Almost all characters are wrong; Many missing or many extras
|
|
|
|
Perhaps a baud mismatch. If one port sends at twice the speed that
|
|
the other port is set to receive, then every two characters sent will
|
|
be received as one character. Each bit of this received character
|
|
will be a sample of two bits of what is sent and will be wrong. Also,
|
|
only half the characters sent seem to get received. For flow in the
|
|
reverse direction, it's just the opposite. Twice as many characters
|
|
get received than were sent. A worse mismatch will produce even worse
|
|
results.
|
|
|
|
A speed mismatch is not likely to happen with a modem since the modem
|
|
autodetects the speed. One cause of a mismatch may be due to serial
|
|
port hardware that has been set to run at very fast speeds. It may
|
|
actually operate at a speed say 8 times that of which you (or an
|
|
application) set it via software. See ``Very High Speeds''
|
|
|
|
|
|
17. Interrupt Problem Details
|
|
|
|
While the section ``Troubleshooting'' lists problems by symptom, this
|
|
section explains what will happen if interrupts are set incorrectly.
|
|
This section helps you understand what caused the symptom, what other
|
|
symptoms might be due to the same problem, and what to do about it.
|
|
|
|
|
|
17.1. Types of interrupt problems
|
|
|
|
The "setserial" program will show you how serial driver thinks the
|
|
interrupts are set. If the serial driver (and setserial) has it right
|
|
then everything regarding interrupts should be OK. Of course a
|
|
/dev/ttyS must exist for the device and Plug-and-Play (or jumpers)
|
|
must have set an address and IRQ in the hardware. Linux will not
|
|
knowingly permit an interrupt conflict and you will get a "Device or
|
|
resource busy" error message if you attempt to do something that would
|
|
create a conflict.
|
|
|
|
Since the kernel tries to avoid interrupt conflicts and gives you the
|
|
"resource busy" message if you try to create a conflict, how can
|
|
interrupt conflicts happen? Easy. "setserial" may have it wrong and
|
|
erroneously predicts no conflict when there will actually be a real
|
|
conflict based on what is set in the hardware. When this happens
|
|
there will be no "... busy" message but a conflict will physically
|
|
happen. Performance is likely to be extremely slow. Both devices
|
|
will send identical interrupt signals on the same wire and the CPU
|
|
will erroneously think that the interrupts only come from one device.
|
|
This will be explained in detail in the following sections.
|
|
|
|
Linux doesn't complain when you assign two devices the same IRQ
|
|
provided that neither device is in use. As each device starts up
|
|
(initializes), it asks Linux for permission to use its hardware
|
|
interrupt. Linux keeps track of which interrupt is assigned to whom,
|
|
and if your interrupt is already in use, you'll see this "... busy"
|
|
error message. Thus if two devices use the same IRQ and you start up
|
|
only one of the devices, everything is OK. But when you next try to
|
|
start the second device (without quitting the first device) you get
|
|
"... busy" error message.
|
|
|
|
|
|
17.2. Symptoms of Mis-set or Conflicting Interrupts
|
|
|
|
The symptoms depend on whether or not you have a modern serial port
|
|
with FIFO buffers or an obsolete serial port without FIFO buffers.
|
|
It's important to understand the symptoms for the obsolete ones also
|
|
since sometimes modern ports seem to behave that way.
|
|
|
|
For the obsolete serial ports, only one character gets thru every
|
|
several seconds. This is so slow that it seems almost like nothing is
|
|
working (especially if the character that gets thru is invisible (such
|
|
a space or newline). For the modern ports with FIFO buffers you will
|
|
likely see bursts of up to 16 characters every several seconds.
|
|
|
|
If you have a modem on the port and dial a number, it seemingly may
|
|
not connect since the CONNECT message may not make it thru. But after
|
|
a long wait it may finally connect and you may see part of a login
|
|
message (or the like). The response from your side of the connection
|
|
may be so delayed that the other side gives up and disconnects you,
|
|
resulting in a NO CARRIER message.
|
|
|
|
If you use minicom, a common test to see if things are working is to
|
|
type the simplest "AT" command and see if the modem responds. Typing
|
|
just at<enter> should normally (if interrupts are OK) result in an
|
|
immediate "OK" response from the modem. With bad interrupts you type
|
|
at<enter> and may see nothing. But then after 10 seconds or so you
|
|
see the cursor drop down one line. What is going on is that the FIFO
|
|
is behaving like it can only hold one byte. The "at" you typed caused
|
|
it to overrun and both letters were lost. But the final <enter>
|
|
eventually got thru and you "see" this invisible character by noticing
|
|
that the cursor jumped down one line. If you were to type a single
|
|
letter and then wait about 10 seconds, you should see it echo back to
|
|
the screen. This is fine if your typing speed is less that one word
|
|
per minute :-)
|
|
|
|
|
|
17.3. Mis-set Interrupts
|
|
|
|
If you don't understand what an interrupt does see ``Interrupts''. If
|
|
a serial port has one IRQ set in the hardware but a different one set
|
|
in the device driver, the device driver will not catch any interrupts
|
|
sent by the serial port. Since the serial port uses interrupts to
|
|
call its driver to service the port (fetching bytes from its 16-byte
|
|
receive buffer or putting another 16-bytes in its transmit buffer) one
|
|
might expect that the serial port would not work at all.
|
|
|
|
But it still may work anyway --sort of. Why? Well, besides the
|
|
interrupt method of servicing the port there's an undocumented slow
|
|
polling method that doesn't need interrupts. The way it works is that
|
|
every so often the device driver checks the serial port to see if it
|
|
needs anything such as if it has some bytes that need fetching from
|
|
its receive buffer. If interrupts don't work, the serial driver falls
|
|
back to this polling method. But this polling method was not intended
|
|
to be used a substitute for interrupts. It's so slow that it's not
|
|
practical to use and may cause buffer overruns. Its purpose may have
|
|
been to get things going again if just one interrupt is lost or fails
|
|
to do the right thing. It's also useful in showing you that
|
|
interrupts have failed. Don't confuse this slow polling method with
|
|
the fast polling method that operates on ports that have their IRQs
|
|
set to 0.
|
|
|
|
For the 16-byte transmit buffer, 16 bytes will be transmitted and then
|
|
it will wait until the next polling takes place (several seconds
|
|
later) before the next 16 bytes are sent out. Thus transmission is
|
|
very slow and in small chunks. Receiving is slow too since bytes that
|
|
are received by the receive buffer are likely to remain there for
|
|
several seconds until it is polled.
|
|
|
|
This explains why it takes so long before you see what you typed.
|
|
When you type say AT to a modem, the AT goes out the serial port to
|
|
the modem. The modem then echos the AT back thru the serial port to
|
|
the screen. Thus the AT characters have to pass twice thru the serial
|
|
port. Normally this happens so fast that AT seems to appear on the
|
|
screen at the same time you hit the keys on the keyboard. With slow
|
|
polling delays at the serial port, you don't see what you typed until
|
|
perhaps 15 seconds later. Even then, you don't often see all you
|
|
typed but only the first several characters.
|
|
|
|
What about overruns of the 16-byte receive buffer? This will happen
|
|
with an external modem since the modem just sends to the serial port
|
|
at high speed which is likely to overrun the 16-byte buffer. But for
|
|
an internal modem, the serial port is on the same card and it's likely
|
|
to check that this 16-byte receive buffer has room for more bytes
|
|
before putting received bytes into it. In this case there will be no
|
|
overrun of this receive buffer, but text will just appear on your
|
|
screen in 16-byte chunks spaced at intervals of several seconds.
|
|
|
|
Even with an external modem you might not get overruns. If just a few
|
|
characters (under 16) are sent you don't get overruns since the buffer
|
|
likely has room for them. But attempts to send a larger number of
|
|
bytes from your modem to your screen may result in overruns. However,
|
|
more than 16 (with no gaps) can get thru without overruns if the
|
|
timing is right. For example, suppose a burst of 32 bytes is sent
|
|
into the port from the external cable. The polling might just happen
|
|
after the first 16 bytes came in so it would pick up these 16 bytes
|
|
OK. Then there would be space for the next 16 bytes so that entire 32
|
|
bytes gets thru OK. While this scenario is not very likely, similar
|
|
cases where 17 to 31 bytes make thru are more likely. But it's even
|
|
more likely that only an occasional 16-byte chunk will get thru with
|
|
possible loss of data.
|
|
|
|
If you have an obsolete serial port with only a 1-byte buffer (or it's
|
|
been incorrectly set to work like a 1-byte buffer) then the situation
|
|
will be much worse than described above and only one character will
|
|
occasionally make it thru the port. Every character received causes
|
|
an overrun (and is lost) except for the last character received. This
|
|
character is likely to be just a line-feed since this is often the
|
|
last character to be transmitted in a burst of characters sent to your
|
|
screen. Thus you may type AT<return> to the modem but never see AT on
|
|
the screen. All you see several seconds later is that the cursor
|
|
drops down one line (a line feed). This has happened to me with a
|
|
16-byte FIFO buffer that was behaving like a 1-byte buffer.
|
|
|
|
When a communication program starts up, it expects interrupts to be
|
|
working. It's not geared to using this slow polling-like mode of
|
|
operation. Thus all sorts of mistakes may be made such as setting up
|
|
the serial port and/or modem incorrectly. It may fail to realize when
|
|
a connection has been made. If a script is being used for login, it
|
|
may fail (caused by timeout) due to the polling delays.
|
|
|
|
|
|
17.4. Interrupt Conflicts
|
|
|
|
When two devices have the same IRQ number it's called sharing
|
|
interrupts. Under some conditions this sharing works out OK.
|
|
Starting with kernel version 2.2, ISA serial ports may, if the
|
|
hardware is designed for this, share interrupts with other serial
|
|
ports. Devices on the PCI bus may share the same IRQ interrupt with
|
|
other devices on the PCI bus (provided the software supports this).
|
|
In other cases where there is potential for conflict, there should be
|
|
no problem if no two devices with the same IRQ are ever "in use" at
|
|
the same time. More precisely, "in use" really means "open" (in
|
|
programmer jargon). In cases other than the exceptions mentioned
|
|
above (unless special software and hardware permit sharing), sharing
|
|
is not allowed and conflicts arise if sharing is attempted.
|
|
|
|
Even if two processes with conflicting IRQs run at the same time, one
|
|
of the devices will likely have its interrupts caught by its device
|
|
driver and may work OK. The other device will not have its interrupts
|
|
caught by the correct driver and will likely behave just like a
|
|
process with mis-set interrupts. See ``Mis-set Interrupts'' for more
|
|
details.
|
|
|
|
|
|
17.5. Resolving Interrupt Problems
|
|
|
|
If you are getting a very slow response as described above, then one
|
|
test is to change the IRQ to 0 (uses fast polling instead of
|
|
interrupts) and see if the problem goes away. Note that the polling
|
|
due to IRQ=0 is orders of magnitude faster than the slow "polling" due
|
|
to bad interrupts. If IRQ=0 seems to fix the problem, then there was
|
|
likely something wrong with the interrupts. Using IRQ=0 is very
|
|
resource intensive and is only a temporary fix. You should try to
|
|
find the cause of the interrupt problem and not permanently use IRQ=0.
|
|
|
|
Check /proc/interrupts to see if the IRQ is currently in use by
|
|
another process. If it's in use by another serial port you could try
|
|
"top" (type f and then enable the TTY display) or "ps -e" to find out
|
|
which serial ports are in use. If you suspect that setserial has a
|
|
wrong IRQ then see ``What is the current IO address and IRQ of my
|
|
Serial Port ?''
|
|
|
|
|
|
18. What Are UARTs? How Do They Affect Performance?
|
|
|
|
18.1. Introduction to UARTS
|
|
|
|
UARTs (Universal Asynchronous Receiver Transmitter) are serial chips
|
|
on your PC motherboard (or on an internal modem card). The UART
|
|
function may also be done on a chip that does other things as well.
|
|
On older computers like many 486's, the chips were on the disk IO
|
|
controller card. Still older computer have dedicated serial boards.
|
|
|
|
When PCs all had parallel bus architecture, the UART's purpose was to
|
|
convert bytes from the PC's parallel bus to a serial bit-stream. The
|
|
cable going out of the serial port is serial and has only one wire for
|
|
each direction of flow. The serial port sends out a stream of bits,
|
|
one bit at a time. Conversely, the bit stream that enters the serial
|
|
port via the external cable was converted to parallel bytes that the
|
|
computer can understand. UARTs deal with data in byte sized pieces,
|
|
which is conveniently also the size of ASCII characters.
|
|
|
|
Say you have a terminal hooked up to a serial port on your PC. When
|
|
you type a character, the terminal gives that character to its
|
|
transmitter (also a UART). The transmitter sends that byte out onto
|
|
the serial line, one bit at a time, at a specific rate. On the PC
|
|
end, the receiving UART takes all the bits and reconstruct the byte
|
|
(parallel on older PCs) and puts it in a buffer. For newer PCs that
|
|
might have a PCI-e serial port, the UART doesn't need to convert
|
|
parallel-to-serial since the PCI-e "bus" is already a serial line.
|
|
But the PCI-e line carries an encoded signal which must be decoded and
|
|
then greatly slowed down to the speed of the RS-232 serial line.
|
|
|
|
Along with converting between serial and parallel, the UART does some
|
|
other things as a byproduct (side effect) of its primary task. The
|
|
voltage used to represent bits is also converted (changed). Extra
|
|
bits (called start and stop bits) are added to each byte before it is
|
|
transmitted. See the Serial-HOWTO section ``Voltage Waveshapes'' for
|
|
details. Also, while the flow rate (in bytes/sec) on the parallel bus
|
|
inside the computer is very high, the flow rate out the UART on the
|
|
serial port side of it is much lower. The UART has a fixed set of
|
|
rates (speeds) which it can use at its serial port interface.
|
|
|
|
|
|
|
|
18.2. Two Types of UARTs
|
|
|
|
There are two basic types of UARTs: dumb UARTS and FIFO UARTS. Dumb
|
|
UARTs are the 8250, 16450, early 16550, and early 16650. They are
|
|
obsolete but if you understand how they work it's easy to understand
|
|
how the modern ones work with FIFO UARTS ( late 16550, 16550A, and
|
|
higher numbers). Note that the driver for all of them is still
|
|
labeled a "8250" driver in Linux where you may see it in compile
|
|
options if you compile your own kernel, etc.
|
|
|
|
There is some confusion regarding 16550. Early models had a bug and
|
|
worked properly only as 16450's (no FIFO). Later models with the bug
|
|
fixed were named 16550A but many manufacturers did not accept the name
|
|
change and continued calling it a 16550. Most all 16550's in use
|
|
today are like 16550A's. Linux will report it as being a 16550A even
|
|
though your hardware manual (or a label note) says it's a 16550. A
|
|
similar situation exists for the 16650 (only it's worse since the
|
|
manufacturer allegedly didn't admit anything was wrong). Linux will
|
|
report a late 16650 as being a 16650V2. If it reports it as 16650 it
|
|
is bad news and only is used as if it had a one-byte buffer.
|
|
|
|
|
|
18.3. FIFOs
|
|
|
|
To understand the differences between dumb and FIFO (First In, First
|
|
Out queue discipline) first let's examine what happens when a UART has
|
|
sent or received a byte. The UART itself can't do anything with the
|
|
data passing thru it, it just receives and sends it. For the obsolete
|
|
dumb UARTS, the CPU gets an interrupt from the serial device every
|
|
time a byte has been sent or received. The CPU then moves the
|
|
received byte out of the UART's buffer and into memory somewhere, or
|
|
gives the UART another byte to send. The obsolete 8250 and 16450
|
|
UARTs only have a 1 byte buffer. That means, that every time 1 byte
|
|
is sent or received, the CPU is interrupted. At low transfer rates,
|
|
this is OK. But, at high transfer rates, the CPU gets so busy dealing
|
|
with the UART, that is doesn't have time to adequately tend to other
|
|
tasks. In some cases, the CPU does not get around to servicing the
|
|
interrupt in time, and the byte is overwritten, because they are
|
|
coming in so fast. This is called an "overrun" or "overflow".
|
|
|
|
FIFO UARTs help solve this problem. The 16550A (or 16550) FIFO chip
|
|
comes with 16 byte FIFO buffers. This means that it can receive up to
|
|
14 bytes (or send 16 bytes) before it has to interrupt the CPU. Not
|
|
only can it wait for more bytes, but the CPU then can transfer all (14
|
|
to 16) bytes at a time. This is a significant advantage over the
|
|
obsolete UARTs, which only had 1 byte buffers. The CPU receives less
|
|
interrupts, and is free to do other things. Data is rarely lost.
|
|
Note that the interrupt threshold of FIFO buffers (trigger level) may
|
|
be set at less than 14. 1, 4 and 8 are other possible choices. As of
|
|
late 2000 there was no way the Linux user could set these directly
|
|
(setserial can't do it). While many PC's only have a 16550 with
|
|
16-byte buffers, better UARTS have even larger buffers.
|
|
|
|
Note that the interrupt is issued slightly before the buffer gets full
|
|
(at say a "trigger level" of 14 bytes for a 16-byte buffer). This
|
|
allows room for a couple more bytes to be received before the
|
|
interrupt service routine is able to actually fetch all these bytes.
|
|
The trigger level may be set to various permitted values by kernel
|
|
software. A trigger level of 1 will be almost like an obsolete UART
|
|
(except that it still has room for 15 more bytes after it issues the
|
|
interrupt).
|
|
|
|
Now consider the case where you're on the Internet. It's just sent
|
|
you a short webpage of text. All of this came in thru the serial
|
|
port. If you had a 16-byte buffer on the serial port which held back
|
|
characters until it had 14 of them, some of the last several
|
|
characters on the screen might be missing as the FIFO buffer waited to
|
|
get the 14th character. But the 14th character doesn't arrive since
|
|
you've been sent the entire page (over the phone line) and there are
|
|
no more characters to send to you. It could be that these last
|
|
characters are part of the HTML formatting, etc. and are not
|
|
characters to display on the screen but you don't want to lose format
|
|
either.
|
|
|
|
There is a "timeout" to prevent the above problem. The "timeout"
|
|
works like this for the receive UART buffer: If characters arrive one
|
|
after another, then an interrupt is issued only when say the 14th
|
|
character reaches the buffer. But if a character arrives and the next
|
|
character doesn't arrive soon thereafter, then an interrupt is issued
|
|
anyway. This results in fetching all of the characters in the FIFO
|
|
buffer, even if only a few (or only one) are present. There is also
|
|
"timeout" for the transmit buffer as well.
|
|
|
|
|
|
18.4. Why FIFO Buffers are Small
|
|
|
|
You may wonder why the FIFO buffers are not larger. After all, memory
|
|
is cheap and it wouldn't cost much more to use buffers in the kilo-
|
|
byte range. The reason is flow control. Flow control stops the flow
|
|
of data (bytes) on serial line when necessary. If a stop signal is
|
|
sent to serial port, then the stop request is handled by software
|
|
(even if the flow control is "hardware"). The serial port hardware
|
|
knows nothing about flow control.
|
|
|
|
If the serial port buffer contains 64 bytes ready to send when it
|
|
receives a flow control signal to stop sending, it will send out the
|
|
64 bytes anyway in violation of the stop request. There is no
|
|
stopping it since it doesn't know about flow control. If the buffer
|
|
was large, then many more bytes would be sent in violation of flow
|
|
control's request to stop.
|
|
|
|
|
|
18.5. UART Model Numbers
|
|
|
|
Here's a list of some UARTs. TL is Trigger Level
|
|
|
|
· 8250, 16450, early 16550: Obsolete with 1-byte buffers
|
|
|
|
· 16550, 16550A, 16C552: 16-byte buffers, TL=1,4,8,14; 115.2 kbps
|
|
standard, many support 230.4 or 460.8 kbps
|
|
|
|
· 16650: 32-byte buffers. 460.8 kbps
|
|
|
|
· 16750: 64-byte buffer for send, 56-byte for receive. 921.6 kbps
|
|
|
|
· 16850, 16C850: 128-byte buffers. 460.8 kbps or 1.5 mbps
|
|
|
|
· 16950
|
|
|
|
· Hayes ESP: 1k-byte buffers.
|
|
|
|
For V.90 56k modems, it may be a several percent faster with a 16650
|
|
(especially if you are downloading large uncompressed files). The
|
|
main advantage of the 16650 is its larger buffer size as the extra
|
|
speed isn't needed unless the modem compression ratio is high. Some
|
|
56k internal modems may come with a 16650 ??
|
|
|
|
Non-UART, and intelligent multiport boards use DSP chips to do
|
|
additional buffering and control, thus relieving the CPU even more.
|
|
For example, the Cyclades Cyclom, and Stallion EasyIO boards use a
|
|
Cirrus Logic CD1400 RISC UART, and many boards use 80186 CPUs or even
|
|
special RISC CPUs, to handle the serial IO.
|
|
Many 486 PCs (old) and all Pentiums (or the like) should have at least
|
|
16550As (usually called just 16550's) with FIFOs. Some better
|
|
motherboards today (2000) even have 16650s. For replacing obsolete
|
|
UARTs with newer ones in pre 1990 hardware see the Appendix: Obsolete
|
|
...
|
|
|
|
|
|
19. Pinout and Signals
|
|
|
|
19.1. Pinout of 9-pin and 25-pin serial connectors
|
|
|
|
The pin numbers are often engraved in the plastic of the connector but
|
|
you may need a magnifying glass to read them. Note DCD is sometimes
|
|
labeled CD. The numbering of the pins on a female connector is read
|
|
from right to left, starting with 1 in the upper right corner (instead
|
|
of 1 in the upper left corner for the male connector as shown below).
|
|
--> direction is out of PC.
|
|
|
|
|
|
|
|
___________ ________________________________________
|
|
\1 2 3 4 5/ Looking at pins \1 2 3 4 5 6 7 8 9 10 11 12 13/
|
|
\6 7 8 9/ on male connector \14 15 16 17 18 19 20 21 22 23 24 25/
|
|
------ -----------------------------------
|
|
Pin # Pin # Acronym Full-Name Direction What-it-May-Do/Mean
|
|
9-pin 25-pin
|
|
3 2 TxD Transmit Data --> Transmits bytes out of PC
|
|
2 3 RxD Receive Data <-- Receives bytes into PC
|
|
7 4 RTS Request To Send --> RTS/CTS flow control
|
|
8 5 CTS Clear To Send <-- RTS/CTS flow control
|
|
6 6 DSR Data Set Ready <-- I'm ready to communicate
|
|
4 20 DTR Data Terminal Ready--> I'm ready to communicate
|
|
1 8 DCD Data Carrier Detect<-- Modem connected to another
|
|
9 22 RI Ring Indicator <-- Telephone line ringing
|
|
5 7 SG Signal Ground
|
|
|
|
9-Pin DB9 Connector 25-Pin DB-25 Connector
|
|
1 DCD Carrier Detect 1 Chassis Ground
|
|
2 RxD Receive Data 2 TxD Transmit Data
|
|
3 TxD Transmit Data 3 RxD Receive Data
|
|
4 DTR Data Terminal Ready 4 RTS Request To Send
|
|
5 SG Signal Ground 5 CTS Clear To Send
|
|
6 DSR Data Set Ready 6 DSR Data Set Ready
|
|
7 RTS Request To Send 7 SG Signal Ground
|
|
8 CTS Clear To Send 8 DCD Carrier Detect
|
|
9 RI Ring Indicator 20 DTR Data Terminal Ready
|
|
22 RI Ring Indicator
|
|
|
|
|
|
|
|
19.2. Signals May Have No Fixed Meaning
|
|
|
|
Only 3 of the 9 pins have a fixed assignment: transmit, receive and
|
|
signal ground. This is fixed by the hardware and you can't change it.
|
|
But the other signal lines are controlled by software and may do (and
|
|
mean) almost anything at all. However they can only be in one of two
|
|
states: asserted (+12 volts) or negated (-12 volts). Asserted is "on"
|
|
and negated is "off". For example, Linux software may command that
|
|
DTR be negated and the hardware only carries out this command and puts
|
|
-12 volts on the DTR pin. A modem (or other device) that receives
|
|
this DTR signal may do various things. If a modem has been configured
|
|
a certain way it will hang up the telephone line when DTR is negated.
|
|
In other cases it may ignore this signal or do something else when DTR
|
|
is negated (turned off).
|
|
It's like this for all the 6 signal lines. The hardware only sends
|
|
and receives the signals, but what action (if any) they perform is up
|
|
to the Linux software and the configuration/design of devices that you
|
|
connect to the serial port. However, most pins have certain functions
|
|
which they normally perform but this may vary with the operating
|
|
system and the device driver configuration. Under Linux, one may
|
|
modify the source code to make these signal lines behave differently
|
|
(some people have).
|
|
|
|
|
|
19.3. Cabling Between Serial Ports
|
|
|
|
A cable from a serial port always connects to another serial port. An
|
|
external modem or other device that connects to the serial port has a
|
|
serial port built into it. For modems, the cable is always straight
|
|
thru: pin 2 goes to pin 2, etc. The modem is said to be DCE (Data
|
|
Communications Equipment) and the computer is said to be DTE (Data
|
|
Terminal Equipment). Thus for connecting DTE-to-DCE you use straight-
|
|
thru cable. For connecting DTE-to-DTE you must use a null-modem cable
|
|
(also called a crossover cable). There are many ways to wire such
|
|
cable (see examples in Text-Terminal-HOWTO subsection: "Direct Cable
|
|
Connection")
|
|
|
|
There are good reasons why it works this way. One reason is that the
|
|
signals are unidirectional. If pin 2 sends a signal out of it (but is
|
|
unable to receive any signal) then obviously you can't connect it to
|
|
pin 2 of the same type of device. If you did, they would both send
|
|
out signals on the same wire to each other but neither would be able
|
|
to receive any signal. There are two ways to deal with this
|
|
situation. One way is to have a two different types of equipment
|
|
where pin 2 of the first type sends the signal to pin 2 of the second
|
|
type (which receives the signal). That's the way it's done when you
|
|
connect a PC (DTE) to a modem (DCE). There's a second way to do this
|
|
without having two different types of equipment: Connect pin sending
|
|
pin 2 to a receiving pin 3 on same type of equipment. That's the way
|
|
it's done when you connect 2 PCs together or a PC to a terminal (DTE-
|
|
to-DTE). The cable used for this is called a null-modem cable since
|
|
it connects two PCs without use of a modem. A null-modem cable may
|
|
also be called a cross-over cable since the wires between pins 2 and 3
|
|
cross over each other (if you draw them on a sheet of paper). The
|
|
above example is for a 25 pin connector but for a 9-pin connector the
|
|
pin numbers 2 and 3 are just the opposite.
|
|
|
|
The serial pin designations were originally intended for connecting a
|
|
dumb terminal to a modem. The terminal was DTE (Data Terminal
|
|
Equipment) and the modem was DCE (Data Communication Equipment).
|
|
Today the PC is usually used as DTE instead of a terminal (but real
|
|
terminals may still be used this way). The names of the pins are the
|
|
same on both DTE and DCE. The words: "receive" and "transmit" are
|
|
from the "point of view" of the PC (DTE). The transmit pin from the
|
|
PC transmits to the "transmit" pin of the modem (but actually the
|
|
modem is receiving the data from this pin so from the point of view of
|
|
the modem it would be a receive pin).
|
|
|
|
The serial port was originally intended to be used for connecting DTE
|
|
to DCE which makes cabling simple: just use a straight-thru cable.
|
|
Thus when one connects a modem one seldom needs to worry about which
|
|
pin is which. But people wanted to connect DTE to DTE (for example a
|
|
computer to a terminal) and various ways were found to do this by
|
|
fabricating various types of special null-modem cables. In this case
|
|
what pin connects to what pin becomes significant.
|
|
|
|
|
|
|
|
19.4. RTS/CTS and DTR/DSR Flow Control
|
|
|
|
This is "hardware" flow control. Flow control was previously
|
|
explained in the ``Flow Control'' subsection but the pins and voltage
|
|
signals were not. Linux only supports RTS/CTS flow control at present
|
|
(but a special driver may exist for a specific application which
|
|
supports DTR/DSR flow control). Only RTS/CTS flow control will be
|
|
discussed since DTR/DSR flow control works the same way. To get
|
|
RTS/CTS flow control one needs to either select hardware flow control
|
|
in an application program or use the command:
|
|
stty -F /dev/ttyS2 crtscts (or the like). This enables RTS/CTS
|
|
hardware flow control in the Linux device driver.
|
|
|
|
Then when a DTE (such as a PC) wants to stop the flow into it, it
|
|
negates RTS. Negated "Request To Send" (-12 volts) means "request NOT
|
|
to send to me" (stop sending). When the PC is ready for more bytes it
|
|
asserts RTS (+12 volts) and the flow of bytes to it resumes. Flow
|
|
control signals are always sent in a direction opposite to the flow of
|
|
bytes that is being controlled. DCE equipment (modems) works the same
|
|
way but sends the stop signal out the CTS pin. Thus it's RTS/CTS flow
|
|
control using 2 lines.
|
|
|
|
On what pins is this stop signal received? That depends on whether we
|
|
have a DCE-DTE connection or a DTE-DTE connection. For DCE-DTE it's a
|
|
straight-thru connection so obviously the signal is received on a pin
|
|
with the same name as the pin it's sent out from. It's RTS-->RTS (PC
|
|
to modem) and CTS<--CTS (modem to PC). For DTE-to-DTE the connection
|
|
is also easy to figure out. The RTS pin always sends and the CTS pin
|
|
always receives. Assume that we connect two PCs (PC1 and PC2)
|
|
together via their serial ports. Then it's RTS(PC1)-->CTS(PC2) and
|
|
CTS(PC1)<--RTS(PC2). In other words RTS and CTS cross over. Such a
|
|
cable (with other signals crossed over as well) is called a "null
|
|
modem" cable. See ``Cabling Between Serial Ports''
|
|
|
|
What is sometimes confusing is that there is the original use of RTS
|
|
where it means about the opposite of the previous explanation above.
|
|
This original meaning is: I Request To Send to you. This request was
|
|
intended to be sent from a terminal (or computer) to a modem which, if
|
|
it decided to grant the request, would send back an asserted CTS from
|
|
its CTS pin to the CTS pin of the computer: You are Cleared To Send to
|
|
me. Note that in contrast to the modern RTS/CTS bi-directional flow
|
|
control, this only protects the flow in one direction: from the
|
|
computer (or terminal) to the modem. This original use appears to be
|
|
little used today on modern equipment (including modems).
|
|
|
|
|
|
19.4.1. The DTR and DSR Pins
|
|
|
|
Just like RTS and CTS, these pins are paired. For DTE-to-DTE
|
|
connections they are likely to cross over. There are two ways to use
|
|
these pins. One way is to use them as a substitute for RTS/CTS flow
|
|
control. The DTR pin is just like the RTS pin while the DSR pin
|
|
behaves like the CTS pin. Although Linux doesn't support DTR/DSR flow
|
|
control, it can be obtained by connecting the RTS/CTS pins at the PC
|
|
to the DSR/DTR pins at the device that uses DTR/DSR flow control. DTR
|
|
flow control is the same as DTR/DSR flow control but it's only one-way
|
|
and only uses the DTR pin at the device. Many text terminals and some
|
|
printers use DTR/DSR (or just DTR) flow control. In the future, Linux
|
|
may support DTR/DSR flow control. The software has already been
|
|
written but it's not clear when (or if) it will incorporated into the
|
|
serial driver.
|
|
|
|
The normal use of DTR and DSR (not for flow control) is as follows: A
|
|
device asserting DTR says that its powered on and ready to operate.
|
|
For a modem, the meaning of a DTR signal from the PC depends on how
|
|
the modem is configured. Negating DTR is sometimes called "hanging
|
|
up" but it doesn't always do this. One way to "hang up" (negate DTR)
|
|
is to set the baud rate to 0 using the command "stty 0". Trying to do
|
|
this from a "foreign" terminal may not work due to the two-interface
|
|
problem. See ``Two interfaces at a terminal''. For internal modem-
|
|
serial_ports it worked OK with a port using minicom but didn't work if
|
|
the port was using wvdial. Why?
|
|
|
|
|
|
19.5. Preventing a Port From Opening
|
|
|
|
If "stty -clocal" (or getty is used with the "local" flag negated)
|
|
then a serial port can't open until DCD gets an assert (+12 volts)
|
|
signal.
|
|
|
|
|
|
20. Voltage Waveshapes
|
|
|
|
20.1. Voltage for a Bit
|
|
|
|
At the RS-232 serial port, voltages are bipolar (positive or negative
|
|
with respect to ground) and should be about 12 volts in magnitude
|
|
(some are 5 or 10 volts). For the transmit and receive pins +12
|
|
volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit
|
|
(sometimes called "mark"). This is known as inverted logic since
|
|
normally a 0-bit is both false and negative while a one is normally
|
|
both true and positive. Although the receive and transmit pins are
|
|
inverted logic, other pins (modem control lines) are normal logic with
|
|
a positive voltage being true (or "on" or "asserted") and a negative
|
|
voltage being false (or "off" or "negated"). Zero voltage has no
|
|
meaning (except it usually means that the unit is powered off).
|
|
|
|
A range of voltages is allowed. The specs say the magnitude of a
|
|
transmitted signal should be between 5 and 15 volts but must never
|
|
exceed 25 V. Any voltage received under 3 V is undefined (but some
|
|
devices will accept a lower voltage as valid). One sometimes sees
|
|
erroneous claims that the voltage is commonly 5 volts (or even 3
|
|
volts) but it's usually 11-12 volts. If you are using a EIA-422
|
|
(RS-422) port on a Mac computer as an RS-232 (requires a special
|
|
cable) or EIA-423 (RS-423) then the voltage will actually be only 5 V.
|
|
The discussion here assumes 12 V.
|
|
|
|
Note that normal computer logic normally is just a few volts (5 volts
|
|
was once the standard) so that if you try to use test equipment
|
|
designed for testing 3-5 volt computer logic (TTL) on the 12 volts of
|
|
a serial port, it may damage the test equipment.
|
|
|
|
|
|
20.2. Voltage Sequence for a Byte
|
|
|
|
The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is
|
|
being sent. To start a byte it jumps to +12 V (space) for the start
|
|
bit and remains at +12 V for the duration (period) of the start bit.
|
|
Next comes the low-order bit of the data byte. If it's a 0-bit
|
|
nothing changes and the line remains at +12 V for another bit-period.
|
|
If it's a 1-bit the voltage jumps from +12 to -12 V. After that comes
|
|
the next bit (-12 V if a 1 or +12 V if a 0), etc., etc. After the
|
|
last data bit, a parity bit may be sent and then a -12 V (mark) stop
|
|
bit. Then the line remains at -12 V (idle) until the next start bit.
|
|
Note that there is no return to 0 volts and thus there is no simple
|
|
way (except by a synchronizing signal) to tell where one bit ends and
|
|
the next one begins for the case where 2 consecutive bits are the same
|
|
polarity (both zero or both one).
|
|
|
|
A 2nd stop bit would also be -12 V, just the same as the first stop
|
|
bit. Since there is no signal to mark the boundaries between these
|
|
bits, the only effect of the 2nd stop bit is that the line must remain
|
|
at -12 V idle twice as long. The receiver has no way of detecting the
|
|
difference between a 2nd stop bit and a longer idle time between
|
|
bytes. Thus communications works OK if one end uses one stop bit and
|
|
the other end uses 2 stop bits, but using only one stop bit is
|
|
obviously faster. In rare cases 1 1/2 stop bits are used. This means
|
|
that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
|
|
50% wider than normal).
|
|
|
|
|
|
20.3. Parity Explained
|
|
|
|
Characters are normally transmitted with either 7 or 8 bits of data.
|
|
An additional parity bit may (or may not) be appended to this
|
|
resulting in a byte length of 7, 8 or 9 bits. Some terminal emulators
|
|
and older terminals do not allow 9 bits. Some prohibit 9 bits if 2
|
|
stop bits are used (since this would make the total number of bits too
|
|
large: 12 bits total after adding the start bit).
|
|
|
|
The parity may be set to odd, even or none (mark and space parity may
|
|
be options on some terminals or other serial devices). With odd
|
|
parity, the parity bit is selected so that the number of 1-bits in a
|
|
byte, including the parity bit, is odd. If a such a byte gets
|
|
corrupted by a bit being flipped, the result is an illegal byte of
|
|
even parity. This error will be detected and if it's an incoming byte
|
|
to the terminal an error-character symbol will appear on the screen.
|
|
Even parity works in a similar manner with all legal bytes (including
|
|
the parity bit) having an even number of 1-bits. During set-up, the
|
|
number of bits per character usually means only the number of data
|
|
bits per byte (7 for true ASCII and 8 for various ISO character sets).
|
|
|
|
A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
|
|
0). For mark parity, the parity bit is always a one-bit. For space
|
|
parity it's always a zero-bit. Mark or space parity (also known as
|
|
"sticky parity") only wastes bandwidth and should be avoided if
|
|
feasible. The stty command can't set sticky parity but it's supported
|
|
by serial hardware and can be dealt with by programming in C. "No
|
|
parity" means that no parity bit is added. For terminals that don't
|
|
permit 9 bit bytes, "no parity" must be selected when using 8 bit
|
|
character sets since there is no room for a parity bit.
|
|
|
|
|
|
20.4. Forming a Byte (Framing)
|
|
|
|
In serial transmission of bytes via RS-232 ports, the low-order bit is
|
|
always sent first (the bit-order). Serial ports on PC's use
|
|
asynchronous communication where there is a start bit and a stop bit
|
|
to mark the beginning and end of a byte. This is called framing and
|
|
the framed byte is sometimes called a frame. As a result a total of
|
|
9, 10, or 11 bits are sent per byte with 10 being the most common.
|
|
8-N-1 means 8 data bits, No parity, 1 stop bit. This adds up to 10
|
|
bits total when one counts the start bit. One stop bit is almost
|
|
universally used. At 110 bits/sec (and sometimes at 300 bits/sec) 2
|
|
stop bits were once used but today the 2nd stop bit is used only in
|
|
very unusual situations (or by mistake since it still works OK that
|
|
way but wastes bandwidth).
|
|
|
|
Don't confuse this type of framing with the framing used for a packet
|
|
of bytes on a network. The serial port just frames every byte. For a
|
|
network, many bytes are framed into a packet (sometimes called a
|
|
frame). For a network frame, instead of a start bit, there is a
|
|
sequence of bytes called a header. On a network that uses serial
|
|
ports (with modems), a report of a frame error usually refers to a
|
|
multi-byte frame and not the serial port frame of a single byte.
|
|
|
|
|
|
|
|
20.5. How "Asynchronous" is Synchronized
|
|
|
|
The RS-232 serial port as implemented on PC is asynchronous which in
|
|
effect means that there is no "clock" signal sent with "ticks" to mark
|
|
when each bit is sent.. There are only two states of the transmit (or
|
|
receive) wire: mark (-12 V) or space (+12 V). There is no state of 0
|
|
V. Thus a sequence of 1-bits is transmitted by just a steady -12 V
|
|
with no markers of any kind between bits. For the receiver to detect
|
|
individual bits it must always have a clock signal which is in
|
|
synchronization with the transmitter clock. Such a clock would
|
|
generate a "tick" in synchronization with each transmitted (or
|
|
received) bit.
|
|
|
|
For asynchronous transmission, synchronization is achieved by framing
|
|
each byte with a start bit and a stop bit (done by hardware). The
|
|
receiver listens on the negative line for a positive start bit and
|
|
when it detects one it starts its clock ticking. It uses this clock
|
|
tick to time the reading of the next 7, 8 or 9 bits. (It actually is
|
|
a little more complex than this since several samples of a bit are
|
|
normally taken and this requires additional timing ticks.) Then the
|
|
stop bit is read, the clock stops and the receiver waits for the next
|
|
start bit. Thus async is actually synchronized during the reception
|
|
of a single byte but there is no synchronization between one byte and
|
|
the next byte.
|
|
|
|
|
|
21. Other Serial Devices (not async RS-232)
|
|
|
|
21.1. Successors to RS-232
|
|
|
|
A number of EIA (or RS) standards have been established for higher
|
|
speeds and longer distances using twisted-pair (balanced) technology.
|
|
Balanced transmission make possible higher speeds, and can be a
|
|
thousand times faster than unbalanced RS-232. For a given speed, the
|
|
distance (maximum cable length) may be many times longer with twisted
|
|
pair. But PC's, prior to about 2004??, kept being made with the
|
|
quasi-obsolete RS-232 since it works OK with modems and mice since the
|
|
cable length is short.
|
|
|
|
High speed serial ports (over 460.8 kbps) will often support both
|
|
RS-232 and EIA-485/EIA-422 (RS-485/RS-422) modes . (Note that for
|
|
non-RS-232 I've used the "EIA" designation instead of the more
|
|
commonly used "RS" but they both mean the same thing.) At such high
|
|
speeds RS-232 is not of much use (except for a very short cable).
|
|
|
|
|
|
21.2. EIA-422-A (balanced) and EIA-423-A (unbalanced)
|
|
|
|
EIA-423 is just like the unbalanced RS-232 except that the voltage is
|
|
only 5 volts. Since this falls within RS-232 specs it can be
|
|
connected to a RS-232 port. Its specs call for somewhat higher speeds
|
|
than the RS-232 (but this may be of little help on a long run where
|
|
it's the unbalance that causes interference). Since EIA-423 is not
|
|
much of an improvement over RS-232, it is seldom used except on old
|
|
Mac computers.
|
|
|
|
EIA-422 is twisted pair (known as "balanced" or "differential) and is
|
|
(per specs) exactly 100 times as fast as EIA-423 (which in turn is
|
|
somewhat faster than RS-232). Apple's Mac computer used it prior to
|
|
mid-1998 with its RS-232/EIA-422 port. The Mac used a small round
|
|
"mini-DIN-8" connector and named these serial ports as "modem port",
|
|
"printer port", and/or "GeoPort".
|
|
|
|
Mac also provided conventional RS-232 but at only at 5 volts (which is
|
|
still legal RS-232). To make it work like at RS-232 one must use a
|
|
special cable which (signal) grounds RxD+ (one side of a balanced
|
|
pair) and use RxD- as the receive pin. While TxD- is used as the
|
|
transmit pin, for some reason TxD+ should not be grounded. See
|
|
Macintosh Communications FAQ <http://www.modemshop.com/csm-comm-
|
|
faq.html>. However, due to the fact that Macs (and upgrades for them)
|
|
cost more than PC's, they are not widely as host computers for Linux.
|
|
|
|
|
|
21.3. EIA-485
|
|
|
|
This is like EIA-422 (balanced = differential). It is half-duplex.
|
|
It's not just point-to-point but is like ethernet or the USB since all
|
|
devices (nodes) on it share the same "bus". It may be used for a
|
|
multidrop LAN (up to 32 nodes or more). Unfortunately, Linux
|
|
currently doesn't support this and you can only use it under Linux
|
|
only for point-to-point where it behaves like RS-232. So read further
|
|
only if you are curious about how its features would work if only
|
|
Linux supported them.
|
|
|
|
Since many nodes share the same twisted pair, there's a need to use
|
|
the electrical tri-state mode. Thus, besides the 0 and 1 binary
|
|
states, there is also an open circuit state to permit other nodes to
|
|
use the twisted pair line. Instead of a transmitter keeping a 1-state
|
|
voltage on the line during line idle, the line is open circuited and
|
|
all nodes just listen (receive mode).
|
|
|
|
The most common architecture is master/slave. The master polls the
|
|
slaves to see if they have anything to send. A slave can only
|
|
transmit just after it's been polled. But EIA-485 is just an
|
|
electrical specification and doesn't specify any protocol for the
|
|
master/slave interaction. In fact, it doesn't even specify that there
|
|
must be a master and slaves. So various protocols have been used.
|
|
Based on a discussion of 485 on the linux-serial mailing list in March
|
|
2003, it seems likely that none of these master/slave protocols are
|
|
currently supported by Linux.
|
|
|
|
There is an alternative implementation where two pair of wires are
|
|
used for sending data. One pair is only for the Master to send to the
|
|
Slaves. Since no one transmits on this line except the master, there
|
|
is no need for it to be tri-state. Thus the Master may just be RS-232
|
|
but the slaves must still be EIA-485. See
|
|
<http://www.hw.cz/english/docs/rs485/rs485.html> for more details.
|
|
|
|
|
|
21.4. EIA-530
|
|
|
|
EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
|
|
(balanced) was intended to be a replacement for RS-232 but few have
|
|
been installed. It uses the same 25-pin connector as RS-232.
|
|
|
|
|
|
21.5. EIA-612/613
|
|
|
|
The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin
|
|
connector and goes up to about 50 Mbits/s but the distance is limited
|
|
to only several meters. For Linux there are PCI cards supporting
|
|
HSSI. The companies that sell the cards often provide (or point you
|
|
to) a Linux driver. A howto or the like is needed for this topic.
|
|
|
|
|
|
21.6. The Universal Serial Bus (USB)
|
|
|
|
The Universal Serial Bus (USB) is being built into PCI chips. Newer
|
|
PC's have them. It was originally 12 Mbps but is now 480 Mbps over a
|
|
twisted pair with a 4-pin connector (2 wires are power supply). It
|
|
also is limited to short distances of at most 5 meters (depends on
|
|
configuration). Linux supports the bus, although not all devices that
|
|
can plug into the bus are supported.
|
|
|
|
It is synchronous and transmits in special packets like a network.
|
|
Just like a network, it can have several devices physically attached
|
|
to it, including serial ports. Each device on it gets a time-slice of
|
|
exclusive use for a short time. A device can also be guaranteed the
|
|
use of the bus at fixed intervals. One device can monopolize it if no
|
|
other device wants to use it. It's not simple to describe in detail.
|
|
|
|
For serial ports on the USB bus, there are numerous configuration
|
|
options to use when compiling the kernel. They all start with:
|
|
CONFIG_USB_SERIAL. Each one is usually for a certain brand/model of
|
|
serial port, although generic is also an option. See the
|
|
Configuration Help file in the kernel documentation.
|
|
|
|
For documentation, see the USB directory in /usr/share/doc/kernel ...
|
|
and look at the file: usb-serial.txt. The modules that support usb
|
|
serial devices are found in the modules tree:
|
|
kernel/drivers/usb/serial. It would be nice to have a HOWTO on the
|
|
USB. See also <http://www.linux-usb.org> and/or
|
|
<http://www.qbik.ch/usb/>.
|
|
|
|
|
|
21.7. Firewire
|
|
|
|
Firewire (IEEE 1394) is something like the USB only faster (800 Mbps
|
|
is planned). The protocol on the bus is claimed to be more efficient
|
|
than USB's. It uses two twisted pair for data plus two power
|
|
conductors (6 conductors in all). A variants uses only 4 conductors.
|
|
You may compile firewire support into the Linux kernel. Like USB,
|
|
it's also limited to short distances.
|
|
|
|
|
|
21.8. MIDI
|
|
|
|
Sound cards often have a 15-pin game port connector used for MIDI.
|
|
They are for connecting a musical keyboard to a PC so that you can
|
|
create musical recordings. You could also connect a MIDI sound
|
|
system. The MIDI standard uses 31250 baud (1M/32) which is not
|
|
available on an ordinary serial port. Some MIDI devices are designed
|
|
so that they can be connected directly to an ordinary serial port.
|
|
|
|
Besides the 15-pin connector, a 5-pin DIN connector is also a MIDI
|
|
standard but the flow of sound is only one way thru it so for
|
|
bidirectional sound you need 2 of them. Breakout cables often have a
|
|
15-pin connector on one end and 2 or more 5-pin connectors on the
|
|
other end. The /dev/midi00 is for MIDI.
|
|
|
|
|
|
21.9. Synchronization & Synchronous
|
|
|
|
Beside the asynchronous RS-232 (and others) there are a number of
|
|
synchronous serial port standards. In fact RS-232 includes
|
|
synchronous specifications but they aren't normally implemented for
|
|
serial ports on PC's. But first we'll explain what a synchronous
|
|
means.
|
|
|
|
|
|
21.9.1. Defining Asynchronous vs Synchronous
|
|
|
|
Asynchronous (async) means "not synchronous". In practice, an async
|
|
signal is what the async serial port sends and receives which is a
|
|
stream of bytes with each byte framed by a start and stop bit.
|
|
Synchronous (sync) is most everything else. But this doesn't explain
|
|
the basic concepts.
|
|
|
|
In theory, synchronous means that bytes are sent out at a constant
|
|
rate one after another in step with a clock signal tick. There is
|
|
often a separate wire or channel for sending the clock signal. The
|
|
clock signal might also be embedded in the transmitted bytes.
|
|
Asynchronous bytes may be sent out erratically with various time
|
|
intervals between bytes (like someone typing characters at a
|
|
keyboard).
|
|
|
|
When a file is being sent thru the async serial port, the flow of
|
|
bytes will likely be at the speed of the port (say 115.2k) which is a
|
|
constant rate. This flow may frequently start and stop due to flow
|
|
control. Is this sync or async? Ignoring the flow control stops, it
|
|
might seem like sync since it's a steady flow. But it's not because
|
|
there is no clock signal and the bytes could have been sent
|
|
erratically since they are framed by start/stop bits.
|
|
|
|
Another case is where data bytes (without any start-stop bits) are put
|
|
into packets with possible erratic spacing between one packet and the
|
|
next. This is called sync since the bytes within each packet are
|
|
transmitted synchronously.
|
|
|
|
|
|
21.9.2. Synchronous Communication
|
|
|
|
Did you ever wonder what all the unused pins are for on a 25-pin
|
|
connector for the serial port? Most of them are for use in
|
|
synchronous communication which is seldom implemented in chips for
|
|
PC's. There are pins for sync timing signals as well as for a sync
|
|
reverse channel. The RS-232 spec provides for both sync and async but
|
|
PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip
|
|
such as a 16450, 16550A, or 16650 and can't deal with sync. For sync
|
|
one needs a USRT chip or the equivalent where the "S" stands for
|
|
Synchronous. A USART chip supports both synchronous and asynchronous.
|
|
Since sync is a niche market, a sync serial port is likely to be quite
|
|
expensive.
|
|
|
|
SCC stands for "Serial Communication Controller" or "Serial Controller
|
|
Chip". It's likely old terminology and since it doesn't say "sync" or
|
|
"async" it might support both.
|
|
|
|
Besides the sync part of the RS-232, there are various other EIA
|
|
synchronous standards. For RS-232, 3 pins of the connector are
|
|
reserved for clock (or timing) signals. Sometimes it's a modem's task
|
|
to generate some timing signals making it impossible to use
|
|
synchronous communications without a synchronous modem (or without a
|
|
device called a "synchronous modem eliminator" which provides the
|
|
timing signals).
|
|
|
|
Although few serial ports are sync, synchronous communication does
|
|
often take place over telephone lines using modems which use V.42
|
|
error correction. This strips off the start/stop bits and puts the
|
|
data bytes in packets resulting in synchronous operation over the
|
|
phone line.
|
|
|
|
|
|
22. Other Sources of Information
|
|
|
|
22.1. Books
|
|
|
|
|
|
1. Axleson, Jan: Serial Port Complete, Lakeview Research, Madison, WI,
|
|
1998.
|
|
|
|
2. Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
|
|
Computer Society Press, Los Alamitos, CA, 1996.
|
|
|
|
3. Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
|
|
|
|
4. Campbell, Joe: C Programmer's Guide to Serial Communications, 2nd
|
|
ed., Unknown Publisher, 1993.
|
|
|
|
5. Levine, Donald: POSIX Programmer's Guide
|
|
<http://www.ora.com/catalog/posix/>, O'Reilly, 1991.
|
|
|
|
6. Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
|
|
Hungry Minds, 2000.
|
|
|
|
7. Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
|
|
|
|
8. Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.
|
|
|
|
9. Stevens, Richard W.: Advanced Programming in the UNIX Environment
|
|
<http://heg-
|
|
school.aw.com/cseng/authors/stevens/advanced/advanced.nclk>, (ISBN
|
|
0-201-56317-7; Addison-Wesley)
|
|
|
|
10.
|
|
Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
|
|
Chapter 7: Serial Ports
|
|
|
|
Notes re books:
|
|
|
|
1. "... Complete" has hardware details (including register) but the
|
|
programming aspect is Window oriented.
|
|
|
|
2. "Physical Layer ..." covers much more than just RS-232.
|
|
|
|
|
|
22.2. Serial Software
|
|
|
|
If it's not available in your Linux distribution try:
|
|
Serial Software <http://www.ibiblio.org/pub/Linux/system/serial/> for
|
|
Linux software for the serial ports including getty and port monitors.
|
|
Serial Communications
|
|
<http://www.ibiblio.org/pub/Linux/system/serial/> for communication
|
|
programs.
|
|
|
|
|
|
· irqtune will give serial port interrupts higher priority to improve
|
|
performance. Using hdparm for hard-disk tuning may help some more.
|
|
|
|
· modemstat and statserial show the current state of various modem
|
|
control lines. See ``Serial Monitoring/Diagnostics''
|
|
|
|
|
|
22.3. Related Linux Documents
|
|
|
|
|
|
· man pages for: setserial and stty
|
|
|
|
· Low-Level Terminal Interface
|
|
<http://www.gnu.org/manual/glibc/html_chapter/libc_12.html> part of
|
|
"GNU C Library Reference manual" (in libc (or glibc) docs package).
|
|
It covers the detailed meaning of "stty" commands, etc.
|
|
|
|
· Modem-HOWTO: modems on the serial port
|
|
|
|
· PPP-HOWTO: help with PPP (using a modem on the serial port)
|
|
|
|
· Printing-HOWTO: for setting up a serial printer
|
|
|
|
|
|
· Serial-Programming-HOWTO: for some aspects of serial-port
|
|
programming
|
|
|
|
· Text-Terminal-HOWTO: how they work and how to install and configure
|
|
|
|
· UPS-HOWTO: setting up UPS sensors connected to your serial port
|
|
|
|
· UUCP-HOWTO: for information on setting up UUCP
|
|
|
|
|
|
22.4. Serial Mailing List
|
|
|
|
The Linux serial mailing list. To join, send email to
|
|
majordomo@vger.kernel.org, with ``subscribe linux-serial'' in the
|
|
message body. If you send ``help'' in the message body, you get a
|
|
help message. The server also serves many other Linux lists. Send
|
|
the ``lists'' command for a list of mailing lists.
|
|
|
|
|
|
22.5. Internet
|
|
|
|
|
|
· Serial port - Wikipedia <http://en.wikipedia.org/wiki/Serial_port>
|
|
|
|
· Linux Serial Driver home page <http://serial.sourceforge.net/>
|
|
Includes info about PCI support.
|
|
|
|
· Serial-Programming-HOWTO (not yet available from the Linux
|
|
Documentation Project). It's now on my website:
|
|
<http://www.lafn.org/~dave/linux/Serial-Programming-HOWTO.txt> See
|
|
also: Serial-Programming-HOWTO by Peter Baumann
|
|
<http://www.lafn.org/~dave/linux/Serial-Programming-HOWTO-B.txt>
|
|
|
|
· Terminal IO by Vern Hoxie
|
|
<http://www.lafn.org/~dave/linux/terminalIO.html> Termios man page
|
|
revision by Vern Hoxie
|
|
<http://www.lafn.org/~dave/linux/termios.txt>
|
|
|
|
· A white paper discussing serial communications and multiport serial
|
|
boards was available from Cyclades at http://www.cyclades.com.
|
|
|
|
· Serial and UART Tutorial (FreeBSD)
|
|
<http://www.freebsd.org/doc/en_US.ISO8859-1/articles/serial-uart/>
|
|
|
|
|
|
23. Appendix A: Very Obsolete Hardware/Software
|
|
|
|
23.1. Replacing pre 1990 UARTS
|
|
|
|
Many 486 PCs (old) and all Pentiums (or the like) should have modern
|
|
16550As (usually called just 16550's) with FIFOs. If you have
|
|
something really old (pre 1990), the chip may unplug so that you may
|
|
be able to upgrade by finding a plug-in 16550A chip and replacing your
|
|
old existing 16450 UART. If the functionality has been built into
|
|
another type of chip, you are out of luck. If the UART is socketed,
|
|
then upgrading would be easy if you could find a replacement. The new
|
|
and old are pin-to-pin compatible. It may be more feasible to just
|
|
buy a new serial card on the Internet (few retail stores stock them as
|
|
of 2000) or find a used one.
|
|
|
|
|
|
23.2. Two Ports with the Same IO address
|
|
|
|
Modern kernels should not allow the opening of ports with the same IO
|
|
address. But one may probe for ports even though they are not open.
|
|
If two ports have the same IO address then old fashioned probing by
|
|
sending commands to the address will erroneously indicate only one
|
|
port. But modem device detection at boot-time should discover both
|
|
ports and report the conflict. In olden days, all sorts of errors
|
|
were reported/observed for devices illegally attempting to use the
|
|
same IO address. See ``Probing''.
|
|
|
|
|
|
23.3. Configuring by modifying source code
|
|
|
|
In the past, to get a certain serial port supported, one might need to
|
|
modify the C source code, perhaps by adding a #define to it. Today,
|
|
the use of parameters for modules or the kernel, or the use of
|
|
configuration options should handle all cases (except possible for
|
|
antique hardware ??).
|
|
|
|
|
|
23.4. Modems on Multiport Cards Obsolete for Sending at 56k
|
|
|
|
For a modem to transmit at nearly 56k requires that it be a special
|
|
digital modem and have a digital connection to a digital phone line
|
|
(such as a T1 line). Modems used with serial cards (the modems may
|
|
either be on the serial card or on another card) normally have no such
|
|
digital connection so they can't be used at the 56k speed, and thus
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are obsolete unless one doesn't need to send at 56k. In other words
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they are obsolete for ISP servers but might be OK for small business
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or home use.
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A partial exception to the above are modem banks that connect to
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multiport serial cards where the modem bank can access multiplexed
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digital phone lines. Thus one could use a multiport serial card with
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a few 56k digital modems for sending at 56k. For both analog and
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digital modems there is one modem on each serial port so there needs
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to be an external cable (modem bank to multiport) for each modem.
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This can lead to a large number of cables. So it's less clutter (and
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cheaper) to use internal modems without a multiport card. This makes
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even this "exception" obsolete for high volume work. It's somewhat
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analogous to the lower cost of an internal modem for a desktop PC as
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compared to the higher cost (and more cabling) for an external modem.
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See Modem-HOWTO: Modem Pools, Digital Modems.
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23.5. Lock-Files if you used the depreciated devfs
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The abandoned device-filesystem (devfs) has the /dev directory with
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subdirectories. As of late 2001, there were problems with lockfiles.
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For example, the lockfile mechanism considered dev/usb/tts/0 and
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/dev/tts/0 to be the same device with name "0". Ditto for all other
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devices that had the same "leaf" name. Also, if some applications use
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the old name for a device and other applications use the devfs name
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for the same device, then the lockfiles will have different names.
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But the serial driver should know they are the same.
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23.6. Devfs (The deprecated Device File System. History)
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Kernel 2.4 introduced the now obsolete optional "device file system"
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(devfs) with a whole new set of names for everything. Some people
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|
used it but the majority probably didn't. But in 2003-4, it was
|
|
claimed that devfs had unsolvable problems and starting with kernel
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2.6.12 it was replaced with "udev" (kernels prior to 2.6.12 also could
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|
use udev but with some problems). Although udev doesn't provide all
|
|
the functionality of devfs, it does handle hot plugging. Also, the
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|
use of udev isn't required to run Linux so some people don't use it.
|
|
But many distributions install it by default.
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Devfs was a good idea and was claimed to be more efficient than udev.
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|
But unfortunately, the author of devfs didn't maintain it for long and
|
|
it allegedly became not too well maintained. So for better or worse
|
|
we now have udev instead although the debate of devfs vs. udev still
|
|
continues. For a detailed description of devfs see:
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|
<http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html> Also see the
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|
kernel documentation tree: filesystems/devfs.
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|
The names of devices for the devfs can be used in udev, but usually
|
|
are not and may not be simple to activate. Here's the devfs names for
|
|
serial devices: ttyS1 becomes tts/1, ttyUSB1 becomes /usb/tts/1, and
|
|
ttyACM1 is /usb/acm/1. Note that the number 1 above is just an
|
|
example. It could be replaced by 0, 2, 3, 4, etc. Some more examples
|
|
of udev names: ttyS2 becomes tts/2 (Serial port), tty3 becomes vc/3
|
|
(Virtual Console), ptyp1 becomes pty/m1 (PTY master), ttyp2 becomes
|
|
pty/s2 (PTY slave). "tts" looks like a directory which contains
|
|
devices "files": 0, 1, 2, etc. All of these new names were put in the
|
|
/dev directory although optionally one may put them elsewhere.
|
|
|
|
For devfs, device names in the /dev directory are created
|
|
automatically by the corresponding driver. Thus, if serial support
|
|
comes from a module and that module isn't loaded yet, there will not
|
|
be any serial devices in the /dev directory. This can be confusing:
|
|
you physically have serial ports but don't see them in the /dev
|
|
directory. However, if a device name is requested (attempt to open
|
|
it) by a communication program and the serial module isn't loaded, the
|
|
kernel is supposed to try to find a driver for it and create a name
|
|
for it in the /dev directory.
|
|
|
|
This works OK if it finds a driver. But suppose there is no driver
|
|
found for it. For example, if you try to use "setserial" to configure
|
|
a port that the driver failed to detect, it claims there is no such
|
|
port. How does one create a devfs port in this case?
|
|
|
|
For multiport devices for example, /dev/ttyF9 becomes /dev/ttf/9, or
|
|
in a later version /dev/tts/F9. Substitute for F (or f) whatever
|
|
letter(s) your multiport board uses for this purpose. A multiport
|
|
driver is supposed to create a devfs name similar to the above and put
|
|
it into the /dev directory
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END OF Serial-HOWTO
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