mirror of https://github.com/tLDP/LDP
4257 lines
215 KiB
Plaintext
4257 lines
215 KiB
Plaintext
<!doctype linuxdoc system>
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<article>
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<title> Serial HOWTO
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<author>David S.Lawyer
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<tt><htmlurl url="mailto:dave@lafn.org" name="dave@lafn.org"></tt>
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original by Greg Hankins
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<date>v2.12, July 2001
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<!-- Change log:
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v2.12 July 2001 serial printing under LPRng
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v2.11 May 2001: stty 0 => hangup (was ok in v2.08. )
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v2.10 Feb 2001: EIA-485, frame errors on networks, gkermit, firewire
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v2.09 October 2000: link to Serial Driver homepage, ttySD etc.
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v2.08 June 2000: /proc/tty, fixed link to Gary's Encyclopedia.
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v2.07 May 2000: locking methods, clarity re uart protocol, sticky parity
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v2.06 2 March 2000: more on multiport, not 3-3 for null modem,
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butter -> buffer,
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v2.05 1 January 2000: Vern's & my url, ref to multiport modem cards
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v2.04 28 Nov. 1999: duplicate info removed from setserial
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v2.03 Nov. 1999: ttyS minor is 4, not 5
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v2.02 Oct. 1999: National Instruments card, removed m4_define TorS, lockfile
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error mesg
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v2.01 Aug. 1999. more on HSSI, irq=0, serial module, clarity,
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Computone update, New copyright, interrupts mis-set symptoms,
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m4-include files (PnP stuff, stty, etc.)
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v2.00 May 1999 holding reg. to shift reg.
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Added tech info. Removed modem and terminal info and put into Modem-HOWTO
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and Text-Terminal-HOWTO. Added info from Modem-HOWTO which is
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only about the serial port.
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v1.12 July 1998: reissue of old doc (v1.11). Added more info on Winmodems.
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-->
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<abstract>
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This document describes serial port features other than those which
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should be covered by Modem-HOWTO, PPP-HOWTO,
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Serial-Programming-HOWTO, or Text-Terminal-HOWTO. It does not cover
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the Universal Serial Bus (see the kernel documentation for USB).
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It lists info on multiport serial cards. It contains technical info
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about the serial port itself in more detail than found in the above
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HOWTOs and should be best for troubleshooting when the problem is
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the serial port itself. If you are dealing with a Modem, PPP (used
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for Internet access on a phone line), or a Text-Terminal, those
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HOWTOs should be consulted first. </abstract>
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<toc>
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<sect>Introduction
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<p> This HOWTO covers basic info on the Serial Port and multiport
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serial cards. Information specific to modems and text-terminals has
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been moved to Modem-HOWTO and Text-Terminal-HOWTO. Info on getty (the
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program that runs the login process or the like) has been also moved
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to these HOWTOs since mgetty and uugetty are best for modems while
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agetty is best for text-terminals. If you are dealing with a modem,
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text terminal, or printer, then you may not need to consult this
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HOWTO. But if you are using the serial port for some other device,
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using a multiport serial card, trouble-shooting the serial port
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itself, or want to understand more technical details of the serial
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port, then you may want to use this HOWTO as well as some of the other
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HOWTOs. (See <ref id="related_howtos" name="Related HOWTO's">) This
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HOWTO lists info on various multiport serial cards since they may be
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used for either modems or text-terminals. This HOWTO addresses Linux
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running on PCs (ISA or PCI buses), although it might be valid for
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other architectures.
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<sect1> Copyright, Disclaimer, & Credits
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<sect2>Copyright
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<p>
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Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2001 by David S.
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Lawyer <url url="mailto:dave@lafn.org">
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<!-- license.H begin -->
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Please freely copy and distribute (sell or give away) this document
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in 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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<enum>
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<item> If it's not a translation: Email a copy of your derivative work
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(in a format LDP accepts) to the author(s) and maintainer (could be
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the same person). If you don't get a response then email the LDP
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(Linux Documentation Project): submit@linuxdoc.org.
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<item>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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<item>Give due credit to previous authors and major contributors.
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</enum>
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If you're considering making a derived work other than a
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translation, it's requested that you discuss your plans with the
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current maintainer.
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<sect2>Disclaimer
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<p> While I haven't intentionally tried to mislead you, there are
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likely a number of errors in this document. Please let me know about
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them. Since this is free documentation, it should be obvious that I
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cannot be held legally responsible for any errors.
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<sect2>Trademarks.
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<p> Any brand names (starts with a capital letter) should be assumed to
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be a trademark). Such trademarks belong to their respective owners.
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<!-- copyright.H end -->
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<sect2>Credits
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<p> Most of the original Serial-HOWTO was written by Greg Hankins.
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<url url="mailto:gregh@twoguys.org">
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He also rewrote many contributions by others in order to maintain
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continuity in the writing style and flow. He wrote: ``Thanks to
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everyone who has contributed or commented, the list of people has
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gotten too long to list (somewhere over one hundred). Special thanks
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to Ted Ts'o for answering questions about the serial drivers.''
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Approximately half of v2.00 was from Greg Hankins HOWTO and the other
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half is by David Lawyer. Ted Ts'o has continued to be helpful.
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<sect1> New Versions of this Serial-HOWTO
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<p> New versions of the Serial-HOWTO will be available to
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browse and/or download at LDP mirror sites. For a list of mirror
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sites see: <url url="http://metalab.unc.edu/LDP/mirrors.html">.
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Various formats are available. If you only want to quickly check the
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date of the latest version look at <url
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url="http://www.linuxdoc.org/HOWTO/Serial-HOWTO.html"> and compare
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it to this version: v2.12 July 2001 . New in recent versions:<newline>
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v2.12 July 2001: serial printing under LPRng<newline>
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v2.11 stty 0 => hangup (was ok in v2.08)<newline>
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v2.10 EIA-485, frame errors on networks, gkermit, firewire
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<sect1> Related HOWTO's re the Serial Port <label id="related_howtos">
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<p> Modems, Text-Terminals, some printers, and other peripherals often
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use the serial port. Get these HOWTOs from the nearest mirror site as
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explained above.
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<itemize>
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<item><tt>Modem-HOWTO</tt> is about installing and configuring modems
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<item><tt>Printing-HOWTO</tt> has info for serial printers using old
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lpr command
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<item><tt>LPRng-HOWTO</tt> (not a LDP HOWTO, may come with software)
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has info for serial printing for "Next Generation" lpr
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<item><tt>Serial-Programming-HOWTO</tt> helps you write
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C programs (or parts of them) that read and write to the serial port
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and/or check/set its state. A new version has been written by Vern
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Hoxie but not submitted. A copy is at <ref id="vern_" name="Internet">.
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<item><tt>Text-Terminal-HOWTO</tt> 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 to
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control a server (via the serial port).
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</itemize>
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<sect1>Feedback
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<p>
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Please send me any questions, comments, suggestions, or additional
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material. I'm always eager to hear about what you think about this
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HOWTO. I'm also always on the lookout for improvements! Tell me
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exactly what you don't understand, or what could be clearer. You can
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reach me via email at <tt><htmlurl url="mailto:dave@lafn.org"> (David
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Lawyer)</tt>.
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<sect1> What is a Serial Port?
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<p> The conventional serial port (not the newer USB port, or HSSI
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port) is a very old I/O port. Almost all PC's have them. But Macs
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(Apple Computer) after mid 1998 (with colored cases) only have the USB
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port. It's possible, however, to put a conventional serial port
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device on the USB.
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The common specification for the conventional serial port is RS-232
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(or EIA-232). The connector for the serial port is often seen as one
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or two 9-pin connectors (in some cases 25-pin) on the back of a PC.
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But the serial port is is more than just that. It includes the
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associated electronics which must produce signals conforming to the
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EIA-232 specification. See <ref id="volt_shape" name="Voltage
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Waveshapes">. One pin is used to send out data bytes and another to
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receive data bytes. Another pin is a common signal ground. The other
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"useful" pins are used mainly for signalling purposes with a steady
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negative voltage meaning "off" and a steady positive voltage meaning
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"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 <ref id="uart_" name="What Are UARTs?"> These
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have improved over time and old models (several years old) are now
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obsolete.
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The serial port was originally designed for connecting modems but it's
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used to connect many other devices also such as mice, text-terminals, some
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printers, etc. to a computer. You just plug these devices into the
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serial port using the correct cable. Many internal modem cards have a
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built-in serial port so when you install one inside your PC it's as if
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you just installed another serial port in your PC.
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<sect> How the Hardware Transfers Bytes <label id="how_hdw_xfers">
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<p> Below is an introduction to the topic, but for a more advanced
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treatment of it see <ref id="fifo_" name="FIFOs">.
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<sect1> Transmitting
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<p> Transmitting is sending bytes out of the serial port away from the
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computer. Once you understand transmitting, receiving is easy to
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understand since it's similar. The first explanation given here will
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be grossly oversimplified. Then more detail will be added in later
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explanations. When the computer wants to send a byte out the serial
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port (to the external cable) the CPU sends the byte on the bus inside
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the computer to the I/O address of the serial port. The serial port
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takes the byte, and sends it out one bit at a time (a serial
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bit-stream) on the transmit pin of the serial cable connector. For what a
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bit (and byte) look like electrically see <ref id="volt_shape"
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name="Voltage Waveshapes">.
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Here's a replay of the above in a little more detail (but still very
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incomplete). Most of the work at the serial port is done by the UART
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chip (or the like). To transmit a byte, the serial device driver
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program (running on the CPU) sends a byte to the serial port"s I/O
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address. This byte gets into a 1-byte "transmit shift register" in
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the serial port. From this shift register bits are taken from the
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byte one-by-one and sent out bit-by-bit on the serial line. Then when
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the last bit has been sent and the shift register needs another byte
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to send it could just ask the CPU to send it another byte. Thus would
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be simple but it would likely introduce delays since the CPU might not
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be able to get the byte immediately. After all, the CPU is usually
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doing other things besides just handling the serial port.
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A way to eliminate such delays is to arrange things so that the CPU
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gets the byte before the shift register needs it and stores it in a
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serial port buffer (in hardware). Then when the shift register has
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sent out its byte and needs a new byte immediately, the serial port
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hardware just transfers the next byte from its own buffer to the shift
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register. No need to call the CPU to fetch a new byte.
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The size of this serial port buffer was originally only one byte, but
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today it is usually 16 bytes (more in higher priced serial ports).
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Now there is still the problem of keeping this buffer sufficiently
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supplied with bytes so that when the shift register needs a byte to
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transmit it will always find one there (unless there are no more bytes
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to send). This is done by contacting the CPU using an interrupt.
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First we'll explain the case of the old fashioned one-byte buffer,
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since 16-byte buffers work similarly (but are more complex). When the
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shift register grabs the byte out of the buffer and the buffer needs
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another byte, it sends an interrupt to the CPU by putting a voltage on
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a dedicated wire on the computer bus. Unless the CPU is doing
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something very important, the interrupt forces it to stop what it was
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doing and start running a program which will supply another byte to
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the port's buffer. The purpose of this buffer is to keep an extra
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byte (waiting to be sent) queued in hardware so that there will be no
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gaps in the transmission of bytes out the serial port cable.
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Once the CPU gets the interrupt, it will know who sent the interrupt
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since there is a dedicated interrupt wire for each serial port (unless
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interrupts are shared). Then the CPU will start running the serial
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device driver which checks registers at I/0 addresses to find out what
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has happened. It finds out that the serial's transmit buffer is empty
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and waiting for another byte. So if there are more bytes to send, it
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sends the next byte to the serial port's I/0 address. This next byte
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should arrive when the previous byte is still in the transmit shift
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register and is still being transmitted bit-by-bit.
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In review, when a byte has been fully transmitted out the transmit
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wire of the serial port and the shift register is now empty the
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following 3 things happen almost simultaneously:
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<enum>
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<item> The next byte is moved from the transmit buffer into
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the transmit shift register
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<item> The transmission of this new byte (bit-by-bit) begins
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<item> Another interrupt is issued to tell the device driver to send
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yet another byte to the now empty transmit buffer
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</enum>
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Thus we say that the serial port is interrupt driven. Each time the
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serial port issues an interrupt, the CPU sends it another byte. Once
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a byte has been sent to the transmit buffer by the CPU, then the CPU
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is free to pursue some other activity until it gets the next
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interrupt. The serial port transmits bits at a fixed rate which is
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selected by the user (or an application program). It's sometimes
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called the baud rate. The serial port also adds extra bits to each
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byte (start, stop and perhaps parity bits) so there are often 10 bits
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sent per byte. At a rate (also called speed) of 19,200 bits per
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second (bps), there are thus 1,920 bytes/sec (and also 1,920
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interrupts/sec).
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Doing all this is a lot of work for the CPU. This is true for many
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reasons. First, just sending one 8-bit byte at a time over a 32-bit
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data bus (or even 64-bit) is not a very efficient use of bus width.
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Also, there is a lot of overhead in handing each interrupt. When the
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interrupt is received, the device driver only knows that something
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caused an interrupt at the serial port but doesn't know that it's
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because a character has been sent. The device driver has to make
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various checks to find out what happened. The same interrupt could
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mean that a character was received, one of the control lines changed
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state, etc.
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A major improvement has been the enlargement of the buffer size of the
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serial port from 1-byte to 16-bytes. This means that when the CPU
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gets an interrupt it gives the serial port up to 16 new bytes to
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transmit. This is fewer interrupts to service but data must still be
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transferred one byte at a time over a wide bus. The 16-byte buffer is
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actually a FIFO (First In First Out) queue and is often called a FIFO.
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See <ref id="fifo_" name="FIFOs"> for details about the FIFO along
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with a repeat of some of the above info.
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<sect1> Receiving
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<p> Receiving bytes by a serial port is similar to sending them only
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it's in the opposite direction. It's also interrupt driven. For the
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obsolete type of serial port with 1-byte buffers, when a byte is fully
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received from the external cable it goes into the 1-byte receive
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buffer. Then the port gives the CPU an interrupt to tell it to pick
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up that byte so that the serial port will have room for storing the
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next byte which is currently being received. For newer serial ports
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with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
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after 14 bytes are in the receive buffer. The CPU then stops what it
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was doing, runs the interrupt service routine, and picks up 14 to 16
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bytes from the port. For an interrupt sent when the 14th byte has
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been received, there could be 16 bytes to get if 2 more bytes have
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arrived since the interrupt. But if 3 more bytes should arrive
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(instead of 2), then the 16-byte buffer will overrun. It also may
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pick up less than 14 bytes by setting it that way or due to timeouts.
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See <ref id="fifo_" name="FIFOs"> for more details.
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<sect1> The Large Serial Buffers
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<p> We've talked about small 16-byte serial port hardware
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buffers but there are also much larger buffers in main memory. When
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the CPU takes some bytes out of the receive buffer of the hardware, it
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puts them into a much larger (say 8k-byte) receive buffer in main
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memory. Then a program that is getting bytes from the serial port
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takes the bytes it's receiving out of that large buffer (using a
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"read" statement in the program). A similar situation exists for
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bytes that are to be transmitted. When the CPU needs to fetch some
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bytes to be transmitted it takes them out of a large (8k-byte)
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transmit buffer in main memory and puts them into the small 16-byte
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transmit buffer in the hardware.
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<sect> Serial Port Basics <label id="basics_">
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<!-- basics.H begin <sect> Serial Port and Modem Basics
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or <sect> Serial Port Basics In SS and MM -->
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<!-- Change log:
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Nov. '99: 2 serial drivers concurrently NG
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Sept. '00: data flow diagram
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Dec. '00 flow control +
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-->
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<!-- ifdef SERIAL_ -->
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<p> You don't have to understand the basics to use the serial port But
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understanding it may help to determine what is wrong if you run into
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problems. This section not only presents new topics but also repeats
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some of what was said in the previous section <ref id="how_hdw_xfers"
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name="How the Hardware Transfers Bytes"> but in greater detail.
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<sect1> What is a Serial Port ?
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<sect2> Intro to Serial
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<p> The serial port is an I/O (Input/Output) device.
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<!-- ifdef SERIAL_ -->
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An I/O device is just a way to get data into and out of a computer.
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There are many types of I/O devices such as serial ports, parallel
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ports, disk drive controllers, ethernet boards, universal serial
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buses, etc.
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<!-- ifdef SERIAL_ end -->
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Most PC's have one or two serial ports. Each has a 9-pin connector
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(sometimes 25-pin) on the back of the computer. Computer programs can
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send data (bytes) to the transmit pin (output) and receive bytes from
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the receive pin (input). The other pins are for control purposes and
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ground.
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The serial port is much more than just a connector. It converts the
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data from parallel to serial and changes the electrical representation
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of the data. Inside the computer, data bits flow in parallel (using
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many wires at the same time). Serial flow is a stream of bits over a
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single wire (such as on the transmit or receive pin of the serial
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connector). For the serial port to create such a flow, it must
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convert data from parallel (inside the computer) to serial on the
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transmit pin (and conversely).
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Most of the electronics of the serial port is found in a computer chip
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(or a part of a chip) known as a UART. For more details on UARTs
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see the section
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<ref id="uart_" name="What Are UARTS?">
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But you may want to finish this section first so that you will
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hopefully understand how the UART fits into the overall scheme of
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things.
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<sect2> Pins and Wires
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<p> Old PC's used 25 pin connectors but only about 9 pins were
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actually used so today most connectors are only 9-pin. Each of the 9
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pins usually connects to a wire. Besides the two wires used for
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transmitting and receiving data, another pin (wire) is signal ground.
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The voltage on any wire is measured with respect to this ground. Thus
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the minimum number of wires to use for 2-way transmission of data is
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3. Except that it has been known to work with no signal ground wire
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but with degraded performance and sometimes with errors.
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There are still more wires which are for control purposes (signalling)
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only and not for sending bytes. All of these signals could have been
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shared on a single wire, but instead, there is a separate dedicated
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wire for every type of signal. Some (or all) of these control wires
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are called "modem control lines". Modem control wires are either in
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the asserted state (on) of +12 volts or in the negated state (off) of
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-12 volts. One of these wires is to signal the computer to stop
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sending bytes out the serial port cable. Conversely, another wire
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signals the device attached to the serial port to stop sending bytes
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to the computer. If the attached device is a modem, other wires may
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tell the modem to hang up the telephone line or tell the computer that
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a connection has been made or that the telephone line is ringing
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(someone is attempting to call in). See section
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<ref id="pinout_" name="Pinout and Signals"> for more details.
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<!-- ifdef SERIAL_ -->
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<sect2> RS-232 or EIA-232, etc.
|
|
<p> 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 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
|
|
obsolete but is still widely used. EIA will be used in this howto.
|
|
Some documents use the full EIA/TIA designation. For info on other
|
|
(non-EIA-232) serial ports see the section <ref id="non_rs232"
|
|
name="Other Serial Devices (not async EIA-232)">
|
|
<!-- ifdef SERIAL_ end -->
|
|
|
|
<sect1> IO Address & IRQ
|
|
<p> 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 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 <ref
|
|
id="interrupt_" name="Interrupts">. For the PCI bus it doesn't work
|
|
exactly this way since the PCI bus has its own system of interrupts.
|
|
But since the PCI-aware BIOS sets up chips to map these PCI interrupts
|
|
to IRQs, it seemingly behaves just as described above except that
|
|
sharing of 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 to see if it matches the device's. If the
|
|
address matches, then the I/O device reads the data on the data bus.
|
|
|
|
<sect1> Names: ttyS0, ttyS1, etc.
|
|
<p> 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 <ref id="set_serial" name="What is Setserial">. This does
|
|
<tt/not/ set the I/O address and IRQ in the hardware itself (which is
|
|
set by jumpers or by plug-and-play software). Thus what 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 <ref id="ttySN_" name="Serial
|
|
Port Devices /dev/ttyS2, etc."> for more details>
|
|
|
|
<sect1> Interrupts <label id="interrupt_">
|
|
<p>
|
|
|
|
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 for 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 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, 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 (1 in old serial ports) 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 <ref id="flow_control" name="Flow Control"> to prevent
|
|
this.
|
|
|
|
Interrupts are also issued when the serial port has just sent out all
|
|
16 of its bytes from its small transmit 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 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 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.
|
|
|
|
|
|
|
|
|
|
|
|
<!-- ifdef SERIAL_ -->
|
|
<sect1> Data Flow (Speeds)
|
|
<p> 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
|
|
<ref id="flow_control" name="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.
|
|
<!-- ifdef SERIAL_ end -->
|
|
|
|
<sect1> Flow Control <label id="flow_control">
|
|
<p> Flow control means the ability to slow done 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 to allow a jump in instantaneous flow rates.
|
|
|
|
<sect2> Example of Flow Control
|
|
<p> For example, consider the case where you connect a 36.6k external
|
|
modem via a short cable to your serial port. The modem sends and
|
|
receives bytes over the phone line at 36.6k 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 the buffer, the level 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 would be true 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). The compression ratio is
|
|
3.43 (115.2/33.6) which is much higher than average. In this case the
|
|
modem is able to compress and the 115.2 bps PC-to-modem flow and send
|
|
the same data out on the phone line at 33.6bps. There's no need for
|
|
flow control here. But such a high compression ratio rarely happens
|
|
so that most of the time flow control is needed to slow down the flow
|
|
on the 115.2 bps PC-to-modem cable. The flow is stopped and started
|
|
so that the average flow is usually well under the "on" flow of 115.2
|
|
bps.
|
|
|
|
In the above example the modem was an external modem. But the same
|
|
situation exists (as of late 2000) 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 involve 3 buffers: 1. in the modem
|
|
2. in the UART chip (called FIFOs) 3. in main memory managed by the
|
|
serial driver. Flow control protects certain buffers from
|
|
overflowing. The small UART FIFO buffers are not protected in this
|
|
way but rely instead on a fast response to the interrupts they issue.
|
|
FIFO stand for "First In, First Out" which is the way it handles
|
|
bytes. All the 3 buffers use the FIFO rule but only one of them also
|
|
uses it as a name. This is the essence of flow control but there are
|
|
still some more details.
|
|
|
|
|
|
|
|
<!-- ifdef SERIAL_ -->
|
|
<sect2> Symptoms of No Flow Control
|
|
<p> Understanding flow-control theory can be of practical use. The
|
|
symptom of no flow control is chunks of data missing from files sent
|
|
without the benefit of flow control. This is because when overflow
|
|
happens, it's usually more than just a few bytes that overflow and are
|
|
lost. Often hundreds or even thousands of bytes get lost, and all in
|
|
contiguous chunks.
|
|
<!-- ifdef SERIAL_ end -->
|
|
|
|
<sect2> Hardware vs. Software Flow Control
|
|
<p> If feasible it's best to use "hardware" flow control that uses two
|
|
dedicated "modem control" wires to send the "stop" and "start"
|
|
signals.
|
|
|
|
|
|
Software flow control uses the main receive and transmit 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, 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.
|
|
|
|
|
|
<sect1> Data Flow Path; Buffers
|
|
<p> Much has been explained about this including flow
|
|
control, a pair of 16-byte FIFO buffers (in the UART), and a pair of
|
|
larger buffers inside a device connected to the serial port (such as a
|
|
modem. But there is still another pair of buffers. These are large
|
|
buffers (perhaps 8k) in main memory also known as serial port buffers.
|
|
When an application program sends bytes to the serial port
|
|
|
|
they first get stashed in the the transmit serial port buffer in main
|
|
memory. The pair consists of both this transmit buffer and 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.
|
|
|
|
<verb>
|
|
application 8k-byte 16-byte 1k-byte tele-
|
|
BROWSER ------- MEMORY -------- UART --------- MODEM -------- phone
|
|
program buffer buffer buffer line
|
|
</verb>
|
|
|
|
The serial device driver takes out say 16 bytes from this transmit buffer,
|
|
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) 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 device
|
|
connected to the serial port has stopped, an application program
|
|
may keep sending bytes to the 8k transmit buffer until it becomes
|
|
fill.
|
|
|
|
When it gets fill, 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. It may
|
|
switch to running other processes while it's waiting at a flow control
|
|
stop. The above was a little oversimplified since there is another
|
|
alternative of having the application program itself do something else
|
|
while it is waiting to "write".
|
|
|
|
|
|
|
|
<!-- ifdef SERIAL_ -->
|
|
<sect1> Complex Flow Control Example
|
|
<p> 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 with a modem to access a BBS. For
|
|
this I use the application program "minicom" which deals with 2 serial
|
|
ports: one connected to a modem and another connected to the
|
|
text-terminal. What I type at the text terminal goes into the first
|
|
serial port to minicom, then from minicom out the second serial port
|
|
to the modem, and then onto the telephone line to the BBS. The
|
|
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. What happens when such a "stop" is issued? Let's
|
|
consider a case where the "stop" is long enough to get thru to the BBS
|
|
and stop the program at the BBS 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 BBS which is being sent simultaneously to both my
|
|
text-terminal and a to 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 the first serial port on my PC. The device driver
|
|
detects it and stops sending bytes from the 8k serial buffer (in main
|
|
memory) to the terminal. Now 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 BBS. This modem stops
|
|
sending bytes out of its buffer and when its buffer gets fill, another
|
|
stop signal is sent to the serial port of the BBS. At the BBS, the
|
|
8-k (or whatever) buffer fills up and the program at the BBS can't
|
|
write to it anymore and thus temporarily halts.
|
|
|
|
Thus a stop signal from a text terminal has halted a programs on a BBS
|
|
computer. What a complex 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.
|
|
|
|
If the terminal speed limitation is the bottleneck in the flow from
|
|
the BBS to the terminal, then its flow control "stop" is actually
|
|
stopping the program that is sending from the BBS as explained above.
|
|
But you may ask: How can a "stop" last so long that 11 buffers (some
|
|
of them large) all get filled up? 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 it 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 BBS neatly as
|
|
described above. It may take a few "stops" from the terminal to
|
|
result in one "stop" at the BBS. 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 BBS 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 BBS had a flow path to the hard-disk without bottlenecks,
|
|
the overflow due to the terminal happened on this path and chunks of
|
|
text were lost and never even made it to the hard-disk. Note that the
|
|
flow to the hard-disk passed thru my modem and since the overflow
|
|
happened there, bytes intended for the hard-disk were lost.
|
|
<!-- ifdef SERIAL_ end -->
|
|
|
|
<sect1> Serial Driver Module
|
|
<p> 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 <tt/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 <tt/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
|
|
<ref id="ser_module" name="Serial Module">
|
|
See <ref id="set_serial" name="What is Setserial"> for more info on
|
|
<tt/setserial/.
|
|
|
|
<!-- basics.H end -->
|
|
|
|
|
|
<sect> Is the Serial Port Obsolete?
|
|
<sect1> Introduction
|
|
<p> The answer is yes, but ... The serial port is somewhat obsolete
|
|
but it's still needed, especially for Linux. The serial port has many
|
|
shortcomings but almost all new PC's seem to come with them them.
|
|
Linux supports ordinary telephone modems only if they work thru a
|
|
serial port.
|
|
|
|
The serial port must pass data between the computer and the external
|
|
cable. Thus it has two interfaces and both of these interfaces are
|
|
slow. First we'll consider the interface via external cable to the
|
|
outside world.
|
|
|
|
<sect1> EIA-232 Cable Is Low Speed & Short Distance
|
|
<p> The conventional EIA-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 low speed. All of the 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 EIA-232 is. See <ref id="non_232"
|
|
name="Successors to EIA-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 110 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.
|
|
|
|
<sect1> Inefficient Interface to the Computer
|
|
|
|
<p> To communicate with the computer, any I/O device needs to have
|
|
an address so that the computer can write to it and read from it. For
|
|
this purpose many I/O devices (such as serial ports) use a special
|
|
type of address known as an I/O addresses (sometimes called an I/O
|
|
port). It's actually a range of addresses and the lower address in
|
|
this range is the base address. If someone only says (or writes)
|
|
"address" it likely really means "base address"
|
|
|
|
Instead of using I/O, addresses some I/O devices read and write
|
|
directly from/to main memory. This provides more bandwidth since the
|
|
conventional serial I/O system only moves a byte at a time. There are
|
|
various ways to read/write directly to main memory. One way is called
|
|
shared memory I/O (where the shared memory is usually on the same card
|
|
as the I/O device). Other methods are DMA (direct memory access) on
|
|
the ISA bus and what is about the same as DMA (only much faster):
|
|
"bus mastering" on the PCI bus. These methods are a lot faster than
|
|
those used for the serial port. Thus the conventional serial port
|
|
with its interrupt driven (every 14 bytes) interface and single bytes
|
|
transfers on a bus which could accommodate 4 (or 8) bytes at a time is
|
|
not suited for very high speed I/O.
|
|
|
|
<sect> Multiport Serial Boards/Cards/Adapters
|
|
<sect1> Intro to Multiport Serial
|
|
|
|
<p> Multiport serial cards install in slots in a PC on the ISA or PCI
|
|
bus. Instead of being called "... cards" 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 terminals and/or modems to
|
|
serial ports. They are still used this way but today most modems are
|
|
internal digital modems (often called something else) and they don't
|
|
usually use multiport serial cards.
|
|
|
|
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.
|
|
|
|
<sect1> Modem Limitations
|
|
<p>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). Modem banks that connect to multiport cards do
|
|
exist, and some have a card that can access multiplexed digital phone
|
|
lines. Thus one can use a multiport card with a few 56k digital
|
|
modems.
|
|
|
|
For both analog and digital modem there is one modem on each serial
|
|
port so there needs to be an external cable (modem bank to multiport)
|
|
for each modem. This can lead to a large number of cables. So it's
|
|
less clutter (and cheaper) to use internal modems without a multiport
|
|
card. It's somewhat analogous to the lower cost of an internal modem
|
|
for a desktop PC as compared to the higher cost (and more cabling) for
|
|
an external modem. See Modem-HOWTO: Modem Pools, Digital Modems.
|
|
|
|
<sect1> Dumb vs. Smart Cards
|
|
<p>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 put into the kernel or activate by
|
|
putting a #define in the source code (or the like).
|
|
|
|
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.
|
|
|
|
<sect1> Getting/Enabling a Driver
|
|
<sect2> Introduction
|
|
<p>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. Support for dumb boards is likely to the built
|
|
into the kernel while smart boards usually need a module.
|
|
|
|
<sect2> Driver is built into the kernel (mostly dumb boards)
|
|
<p>A pre-compiled kernel is not likely to have multiport support
|
|
built in (especially after kernel 2.4). So you probably need to compile
|
|
it yourself. In kernel 2.4 you should select "CONFIG_SERIAL_EXTENDED
|
|
when configuring the kernel (just before you compile). If you select
|
|
this there will be still more choices presented to you. Even after
|
|
you do this you may need to edit the resulting source code a
|
|
little (depending on the card).
|
|
|
|
<sect2> Modules (mostly for smart boards)
|
|
<p>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, but the kernel may automatically do
|
|
this for you if a program is trying to use a device on the smart board
|
|
(provided there exists a table showing which module to load for the
|
|
device). This table may be in /etc/modules.conf and/or be internal to
|
|
the kernel. Also certain parameters may need to be passed to the
|
|
driver (via lilo's append command or via /etc/modules.conf). For
|
|
kernel 2.4 the modules should be found in
|
|
<tt>/lib/modules/.../kernel/drivers/char.</tt>
|
|
|
|
<sect2> Getting info on multiport boards
|
|
<p> 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/kernel-doc... 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).
|
|
|
|
<sect1>Making multiport devices in the /dev directory <label id="make_multi">
|
|
<p>
|
|
The serial ports your multiport board uses depends on what kind of board
|
|
you have. Some have their own device names like /dev/ttyE27 or
|
|
/dev/ttyD2, etc. Ones that use the standard names like /dev/ttyS14
|
|
may be listed in detail in <tt>rc.serial</tt> or in
|
|
<tt>0setserial</tt>. These files may be included in a setserial or
|
|
serial package. You may need to create these devices (unless an
|
|
installation script does it for you). Either use the <tt/mknod/
|
|
command, or the <tt/MAKEDEV/ script. Devices (in the /dev directory)
|
|
for ttyS type serial ports are made by adding ``64 + port number''.
|
|
So, if you wanted to create devices for <tt>ttyS17</tt>, you would
|
|
type:
|
|
|
|
<tscreen><verb>
|
|
linux# mknod -m 666 /dev/ttyS17 c 4 81
|
|
</verb></tscreen>
|
|
Note the "major" number is always 4 for ttyS devices (and 5 for the
|
|
obsolete cua devices). Also ``64 + 17 = 81''. Using the <tt/MAKEDEV/
|
|
script, you would type:
|
|
|
|
<tscreen><verb>
|
|
linux# cd /dev
|
|
linux# ./MAKEDEV ttyS17
|
|
</verb></tscreen>
|
|
|
|
For the names and numbers of other types of serial ports other than
|
|
ttyS.. see devices.txt in the kernel documentation. Besides the
|
|
listing of various brands of multiports found in this HOWTO there is
|
|
<url url="http://members.aa.net/~swear/pedia/serialcards.html"
|
|
name="Gary's Encyclopedia - Serial Cards">. It's not as complete, but
|
|
may have some different links.
|
|
|
|
<sect1>Standard PC Serial Cards
|
|
|
|
<p> In olden days PCs came with a serial card installed. Later on the
|
|
serial function was put on the hard-drive interface card. Today, one
|
|
or two serial ports are usually built into the motherboard. Most of
|
|
them (as of 2001) use a 16550 but some use 16650 (32-byte FIFOs).
|
|
But one may still buy the old PC serial cards if they need 1-4 more
|
|
serial ports. These are for ttyS0-ttyS3 (COM1 - COM4). 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 a PCI one,
|
|
make sure Linux supports it.
|
|
|
|
Here's a list of a few popular brands:
|
|
<itemize>
|
|
<item>Byte Runner (may order directly, shows prices) <url
|
|
url="http://www.byterunner.com">
|
|
<item> SIIG <url url="http://www.siig.com/io">
|
|
<item> Dolphin <url url="http://www.dolphinfast.com/sersol/">
|
|
</itemize>
|
|
<p>
|
|
Note: due to address conflicts, you may not be able to use COM4 and
|
|
IBM8514 video card (or some others) simultaneously. See <ref
|
|
id="8514_" name="Avoiding IO Address Conflicts with Certain Video
|
|
Boards">
|
|
|
|
<sect1>Dumb Multiport Serial Boards (with standard UART chips)
|
|
<p> They are also called "serial adapters". They often have a
|
|
special method of sharing interrupts which requires that you compile
|
|
support for them into the kernel.<newline>
|
|
|
|
* => The file that ran setserial in Debian shows some details of configuring
|
|
# => See note below for this board
|
|
<itemize>
|
|
<item>AST FourPort and clones (4 ports) * #
|
|
<item>Accent Async-4 (4 ports) *
|
|
<item>Arnet Multiport-8 (8 ports)
|
|
<item>Bell Technologies HUB6 (6 ports)
|
|
<item>Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports;
|
|
See the Boca mini-howto revised in 2001) * #
|
|
<item>Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
|
|
sharing ?? Uses odd-ball 10-cond RJ45-like connectors)
|
|
<item>Boca 2by4 (4 serial ports, 2 parallel ports)
|
|
<item>Byte Runner <url url="http://www.byterunner.com">
|
|
<item>Computone ValuePort V4-ISA (AST FourPort compatible) *
|
|
<item>Digi PC/8 (8 ports) #
|
|
<item>Dolphin <url url="http://www.dolphinfast.com/sersol/">
|
|
<item>Globetek <url url="http://www.globetek.com/">
|
|
<item>GTEK BBS-550 (8 ports; See the mini-howto)
|
|
<item>Hayes ESP (after kernel 2.1.15)
|
|
<item>HUB-6 See Bell Technologies.
|
|
<item>Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible) *
|
|
<item>Moxa C104, Moxa C104+ (AST FourPort compatible) *
|
|
<item><url
|
|
url="http://digital.natinst.com/manuals.nsf/web%2Fbyproductcurrent?OpenView&Start=1&Count=500&Expand=15.1#15.1" name="NI-SERIAL"> by National Instruments
|
|
<item>PC-COMM (4 ports) <item><url url="http://www.sealevel.com"
|
|
name="Sealevel Systems">
|
|
COMM-2 (2 ports), COMM-4 (4 ports) and COMM-8 (8 ports)
|
|
<item>SIIG I/O Expander 2S IO1812 (4 ports) #
|
|
<item>STB-4COM (4 ports)
|
|
<item>Twincom ACI/550
|
|
<item>Usenet Serial Board II (4 ports) *
|
|
<item>VScom (uses same driver as ByteRunner)
|
|
</itemize>
|
|
<p>
|
|
In general, Linux will support any serial board which uses a 8250,
|
|
16450, 16550, 16550A, 16650, etc. UART. See the latest man page for
|
|
"setserial" for a more complete list.
|
|
|
|
Notes:
|
|
|
|
AST Fourport: You might need to specify <tt/skip_test/ in <tt/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.
|
|
|
|
<sect1>Intelligent Multiport Serial Boards
|
|
<p>
|
|
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 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 *.o but these may not work for all
|
|
models shown. 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). To check on the latest serial
|
|
driver go to <url url="http://serial.sourceforge.net/" name="Linux
|
|
Serial Driver home page">
|
|
|
|
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). But all major brands and
|
|
websites should be shown here so it something is missing let me know.
|
|
Go the 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.
|
|
|
|
<itemize>
|
|
<item>Chase Research (UK based, ISA/PCI cards)<newline>
|
|
webpage: <tt><url url="www.chaser.com"></tt><newline>
|
|
driver status: for 2.2 kernel. Supported by Chase.
|
|
|
|
<item>Comtrol RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)<newline>
|
|
webpage: <tt><htmlurl url="http://www.comtrol.com"
|
|
name="http://www.comtrol.com"></tt><newline>
|
|
driver status: supported by Comtrol. rocket.o<newline>
|
|
driver location: <tt><htmlurl
|
|
url="ftp://tsx-11.mit.edu/pub/linux/packages/comtrol"
|
|
name="ftp://tsx-11.mit.edu/pub/linux/packages/comtrol"></tt>
|
|
|
|
<item>Computone IntelliPort II (ISA, PCI and EISA busses up to 64
|
|
ports)<newline>
|
|
webpage: <url url="http://www.computone.com"><newline>
|
|
driver location:
|
|
<url url="ftp://ftp.computone.com/PUB/Products/IntelliPortII/Linux/">,
|
|
patch at <url
|
|
url="http://www.wittsend.com/computone/linux-2.2.10-ctone.patch.gz"><newline>
|
|
mailing list: <url url="mailto:majordomo@lazuli.wittsend.com"> with
|
|
"subscribe linux-computone" in body<newline>
|
|
note: Old ATvantage and Intelliport cards are not supported by Computone
|
|
|
|
<item> Connecttech<newline>
|
|
website: <tt><url url="http://www.connecttech.com/porducts/products.html"></tt><newline>
|
|
driver location: <url url="ftp://ftp.connecttech.com/pub/linux/">
|
|
|
|
<item>Cyclades<newline>
|
|
Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),<newline>
|
|
Cyclom-Z (MIPS R3000; 8 - 64 ports)<newline>
|
|
website: <tt><url url="http://www.cyclades.com/products.html"></tt><newline>
|
|
driver status: supported by Cyclades<newline>
|
|
driver location: <tt><htmlurl url="ftp://ftp.cyclades.com/pub/cyclades"
|
|
name="ftp://ftp.cyclades.com/pub/cyclades"></tt> and included in Linux
|
|
kernel since version 1.1.75: cyclades.o
|
|
|
|
<item>Decision PCCOM (2-8 ports; ISA and PCI; AKA PC COM)<newline>
|
|
ISA:<newline>
|
|
contact: <tt><url url="mailto:info@cendio.se"></tt><newline>
|
|
driver location: <tt><htmlurl url="ftp://ftp.signum.se/pub/pccom8"
|
|
name="ftp://ftp.signum.se/pub/pccom8"></tt><newline>
|
|
PCI:<newline>
|
|
drivers: <url url="http://www.decision.com.tw"> Click "download"<newline>
|
|
driver status: Support in serial driver 5.03. For an earlier driver,
|
|
there exists a patch for kernel 2.2.16 at <url
|
|
url="http://www.qualica.com/serial/">
|
|
|
|
<item>Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),<newline>
|
|
PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),<newline>
|
|
PC/Xr (16MHz IDT3041; 4 or 8 ports),<newline>
|
|
PC/Xem (20MHz IDT3051; 8 - 64 ports)<newline>
|
|
website: <tt><url url="http://www.dgii.com"></tt><newline>
|
|
driver status: supported by Digi<newline>
|
|
driver location: <tt><htmlurl
|
|
url="ftp://ftp.dgii.com/drivers/linux"
|
|
name="ftp://ftp.dgii.com/drivers/linux"></tt> and
|
|
included in Linux kernel since version 2.0. epca.o
|
|
|
|
<item>Digi COM/Xi (10MHz 80188; 4 or 8 ports)<newline>
|
|
contact: Simon Park, <tt><htmlurl url="mailto:si@wimpol.demon.co.uk"
|
|
name="si@wimpol.demon.co.uk"></tt><newline>
|
|
driver status: ?<newline>
|
|
note: Simon is often away from email for months at a time due to
|
|
his job. Mark Hatle, <url url="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.
|
|
|
|
<item>Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.equinox.com"
|
|
name="http://www.equinox.com"></tt><newline>
|
|
driver status: supported by Equinox<newline>
|
|
driver location: <tt><htmlurl
|
|
url="ftp://ftp.equinox.com/library/sst"
|
|
name="ftp://ftp.equinox.com/library/sst"></tt>
|
|
|
|
<item>Globetek<newline>
|
|
website: <url url="http://www.globetek.com/products.shtml"><newline>
|
|
driver location: <url url="http://www.globetek.com/media/files/linux.tar.gz">
|
|
|
|
<item>GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),<newline>
|
|
SmartCard (24MHz Dallas DS80C320; 8 ports),<newline>
|
|
BlackBoard-8A (16C654 UARTs; 8 ports),<newline>
|
|
PCSS (15/24MHz 8032; 8 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.gtek.com" name="http://www.gtek.com">
|
|
</tt><newline>
|
|
driver status: supported by GTEK<newline>
|
|
driver location: <tt><htmlurl url="ftp://ftp.gtek.com/pub"
|
|
name="ftp://ftp.gtek.com/pub"></tt>
|
|
|
|
<item>Hayes ESP (COM-bic; 1 - 8 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.nyx.net/˜arobinso"
|
|
name="http://www.nyx.net/˜arobinso"></tt><newline>
|
|
driver status: Supported by Linux kernel (1998) since v. 2.1.15.
|
|
esp.o. Setserial 2.15+ supports. Also supported by author<newline>
|
|
driver location: <tt><htmlurl url="http://www.nyx.net/˜arobinso"
|
|
name="http://www.nyx.net/˜arobinso"></tt>
|
|
|
|
<item>Intelligent Serial Interface by Multi-Tech Systems<newline>
|
|
PCI: 4 or 8 port. ISA 8 port. DTE speed 460.8k<newline>
|
|
webpage: <url url="http://www.multitech.com/products/">
|
|
|
|
<item>Maxpeed SS (Toshiba; 4, 8 and 16 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.maxpeed.com"
|
|
name="http://www.maxpeed.com"></tt><newline>
|
|
driver status: supported by Maxpeed<newline>
|
|
driver location: <tt><htmlurl url="ftp://maxpeed.com/pub/ss"
|
|
name="ftp://maxpeed.com/pub/ss"></tt>
|
|
|
|
<item> Microgate SyncLink ISA and PCI high speed multiprotocol
|
|
serial. Intended for synchronous HDLC.<newline>
|
|
website: <url
|
|
url="http://ww/microgate.com/products/sllinux/hdlcapi.htm"><newline>
|
|
driver status: supported by Microgate: synclink.o
|
|
|
|
<item>Moxa C218 (12MHz 80286; 8 ports),<newline>
|
|
Moxa C320 (40MHz TMS320; 8 - 32 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.moxa.com"
|
|
name="http://www.moxa.com"></tt><newline>
|
|
driver status: supported by Moxa<newline>
|
|
driver locations: <tt><url
|
|
url="http://www.moxa.com/support/download/download.php3>"></tt>
|
|
<tt><url url="ftp://ftp.moxa.com/drivers/linux" ></tt>
|
|
(also from Taiwan at www.moxa.com.tw/...) where ... is the same as
|
|
above)
|
|
|
|
<item>SDL RISCom/8 (Cirrus Logic CD180; 8 ports)<newline>
|
|
website: <tt><htmlurl url="http://www.sdlcomm.com"
|
|
name="http://www.sdlcomm.com"></tt><newline>
|
|
driver status: supported by SDL<newline>
|
|
driver location: <tt><htmlurl url="ftp://ftp.sdlcomm.com/pub/drivers"
|
|
name="ftp://ftp.sdlcomm.com/pub/drivers"</tt>
|
|
|
|
<item> Specialix SX (25MHz T225; 8? - 32 ports),<newline>
|
|
SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)<newline>
|
|
webpage: <url url="www.specialix.com/products/io/serialio.htm"><newline>
|
|
driver status: Supported by Specialix<newline>
|
|
driver location: <url url="http://www.BitWizard.nl/specialix/"><newline>
|
|
old driver location: <url
|
|
url="ftp://metalab.unc.edu/pub/Linux/kernel/patches/serial">
|
|
|
|
<item>Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and<newline>
|
|
EasyConnection (8 - 32 ports) - each with
|
|
Cirrus Logic CD1400 UARTs,<newline>
|
|
Stallion (8MHz 80186 CPU; 8 or 16 ports),<newline>
|
|
Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),<newline>
|
|
ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),<newline>
|
|
EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)<newline>
|
|
contact: <tt><htmlurl url="mailto:sales@stallion.com"
|
|
name="sales@stallion.com"></tt> or
|
|
<tt><htmlurl url="http://www.stallion.com"
|
|
name="http://www.stallion.com"></tt><newline>
|
|
driver status: supported by Stallion<newline>
|
|
driver location: <tt><htmlurl
|
|
url="ftp://ftp.stallion.com/drivers/ata5/Linux"
|
|
name="ftp://ftp.stallion.com/drivers/ata5/Linux"></tt> and
|
|
included in linux kernel since 1.3.27
|
|
|
|
<item>System Base
|
|
website: <url url="http://www.sysbas.com/">
|
|
</itemize>
|
|
|
|
<p>
|
|
A review of Comtrol, Cyclades, Digi, and Stallion products was printed
|
|
in the June 1995 issue of the <EM/Linux Journal/. The article is
|
|
available at <tt><htmlurl url="http://www.ssc.com/lj/issue14"
|
|
name="http://www.ssc.com/lj/issue14"></tt>.
|
|
|
|
<sect1> Unsupported Multiport Boards
|
|
<p> The following boards don't mention any Linux support as of 1 Jan.
|
|
2000. Let me know if this changes.
|
|
<itemize>
|
|
<item> Aurora (PCI only) <url url="www.auroratech.com">
|
|
</itemize>
|
|
|
|
<sect>Configuring the Serial Port
|
|
<!-- configure.H begin (in MM, SS)
|
|
<sect>Configuring the Serial Port
|
|
Change-log:
|
|
-->
|
|
|
|
<sect1> PCI Bus Support Underway <label id="PCI_ser_conf">
|
|
<p>
|
|
Although most PCI modems are "winmodems" without a
|
|
Linux driver (and will not work under Linux), other PCI serial cards
|
|
(usually modem cards) will often work OK under Linux. Some need no
|
|
special support in the serial driver and work fine under Linux once
|
|
setserial is used to configure them. Other PCI cards need special
|
|
support in the kernel. Some of these cards are supported by kernel
|
|
2.4 (and in later versions of the 2.3 series). Kernel 2.2 has no such
|
|
support. If your modem (or serial port) happens to be supported, then
|
|
you shouldn't need to do anything to PnP configure it. The new serial
|
|
driver will read the id number digitally stored on the card to
|
|
determine how to support the card. It should assign the I/O address
|
|
to it, determine it's IRQ, etc. So you don't need to use "setserial"
|
|
for it.
|
|
|
|
If you have a
|
|
|
|
PCI modem card you should be looking at Modem-HOWTO and not this
|
|
Serial-HOWTO. If you just have a PCI serial port card (with no modem
|
|
on the card) but it will not work because the latest serial driver doesn't support
|
|
it, you can help in attempting to create a driver for it. To do this
|
|
you'll need to contact the maintainer of the serial driver, Theodore
|
|
(Ted) Y. Ts'o.
|
|
You will need to email Ted Ts'o a
|
|
copy of the output of "lspci -vv" with full information about the
|
|
model and manufacturer of the PCI modem (or serial port). Then he
|
|
will try to point you to a test driver which might work for it. You
|
|
will then need to get it, compile it and possibly recompile your
|
|
kernel. Then you will test the driver to see if it works OK for you
|
|
and report the results to Ted Ts'o. If you are willing to do all the
|
|
above (and this is the latest version of this HOWTO) then email the
|
|
needed info to him at: <url url="mailto:tytso@mit.edu">.
|
|
|
|
PCI modems are not well standardized. Some use main memory for
|
|
communication with the PC. It you see 8-digit hexadecimal addresses
|
|
it's not likely to work with Linux. Some require special enabling of
|
|
the IRQ. The output of "lspci" can help determine if one can be
|
|
supported. If you see a 4-digit IO port and no long memory address,
|
|
the modem might work by just telling "setserial" the IO port and the
|
|
IRQ. Some people have gotten a 3COM 3CP5610 PCI Modem to work that
|
|
way.
|
|
|
|
<sect1> Configuring Overview <label id="irq_io_conf">
|
|
<p> In many cases, configuring will happen automatically and you have
|
|
nothing to do. But sometimes you need to configure (or just want to
|
|
check out the configuration). If so, first you need to know about the
|
|
two parts to configuring the serial port under Linux:
|
|
|
|
The first part (low-level configuring) is assigning it an IO address,
|
|
IRQ, and name (such as ttyS2). This IO-IRQ pair must be set in both
|
|
the hardware and told to the serial driver. We might just call this
|
|
"io-irq" configuring for short. The <tt/setserial/ is used to tell
|
|
the driver. PnP methods, jumpers, etc, are used to set 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.
|
|
|
|
The second part (high-level configuring) is assigning it a speed (such
|
|
as 38.4k bits/sec), selecting flow control, etc. This is often done
|
|
by communication programs such as PPP, minicom, or by getty (which you
|
|
may run on the port so that others may log into your computer).
|
|
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 with the <tt/stty/ program. <tt/stty/ is also useful
|
|
to view the current status if you're having problems.
|
|
See also the section <ref id="stty_" name="Stty">
|
|
When Linux starts, some effort is made to detect and configure
|
|
(low-level) a few serial ports. Exactly what happens depends on your
|
|
BIOS, hardware, Linux distribution, etc. If the serial ports work OK,
|
|
there may be no need for you to do any configuring. Application
|
|
programs often do the high-level configuring but you may need to
|
|
supply them with the required information. With Plug-and-Play serial
|
|
ports (often built into an internal modem), the situation has become
|
|
more complex. Here are cases when you need to do low-level
|
|
configuring (set IRQ and IO addresses):
|
|
|
|
<itemize>
|
|
<item> Plan to use more than 2 serial ports
|
|
<item> Installing a new serial port (such as an internal modem)
|
|
<item> Having problems with serial port(s)
|
|
</itemize>
|
|
|
|
For kernel 2.2+ you may be able to use more that 2 serial ports
|
|
without low-level configuring by sharing interrupts. This only works
|
|
if the serial hardware supports it and may be no easier than low-level
|
|
configuring. See <ref id="int_share-2.2" name="Interrupt sharing and
|
|
Kernels 2.2+">
|
|
|
|
The low-level configuring (setting the IRQ and IO address) seems to
|
|
cause people more trouble (than high-level), although for many it's
|
|
fully automatic and there is no configuring to be done. Thus most all
|
|
of this section is on that topic. Until the serial driver knows the
|
|
correct IRQ and IO address the port will not work at all. It may not
|
|
even be found by Linux. Even if it can be found, it may work
|
|
extremely slow if the IRQ is wrong. See <ref id="slow_"
|
|
name="Extremely Slow: Text appears on the screen slowly after long
|
|
delays">.
|
|
|
|
In the Wintel world, the IO address and IRQ 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 review,
|
|
the low-level configuring consists of putting two values (an IRQ
|
|
number and IO address) into two places:
|
|
|
|
<enum>
|
|
<item> the device driver (often by running "<tt/setserial/" at
|
|
boot-time)
|
|
<item> memory registers of the serial port hardware itself
|
|
</enum>
|
|
|
|
You may watch the start-up (= boot-time) messages. They are usually
|
|
correct. But if you're having problems, there's a good chance that
|
|
some of these messages don't show the true configuration of the
|
|
hardware (and they are not supposed to). See <ref id="boot_mesgs"
|
|
name="I/O Address & IRQ: Boot-time messages">.
|
|
|
|
<sect1> Common mistakes made re low-level configuring
|
|
<p> Here are some common mistakes people make:
|
|
<itemize>
|
|
<item>setserial command: They run it (without the "autoconfig" and
|
|
auto_irq options) and think it has checked out the hardware (it
|
|
hasn't).
|
|
<item>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.
|
|
<item>/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).
|
|
<item>/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.
|
|
</itemize>
|
|
|
|
<sect1> I/O Address & IRQ: Boot-time messages <label id="boot_mesgs">
|
|
<p> 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. Use Shift-PageUp to scroll back to the messages after
|
|
they have flashed by. Shift-PageDown will scroll in the opposite
|
|
direction. The <tt/dmesg/ command may be used at any time to view
|
|
some of the messages but it often misses important ones. Here's an
|
|
example of the start-up messages (as of mid 1999). Note that ttyS00
|
|
is the same as /dev/ttyS0.
|
|
|
|
<tscreen><verb>
|
|
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
|
|
ttyS00 at 0x03f8 (irq = 4) is a 16550A
|
|
ttyS01 at 0x02f8 (irq = 3) is a 16550A
|
|
ttyS02 at 0x03e8 (irq = 4) is a 16550A
|
|
|
|
Later you see what was saved, but it's not necessarily correct either:
|
|
|
|
Loading the saved-state of the serial devices...
|
|
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
|
|
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
|
|
/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A
|
|
</verb></tscreen>
|
|
|
|
Note that there is a slight disagreement: The first message shows
|
|
ttyS2 at irq=4 while the second shows it at irq=5. Your may only have
|
|
the first message. In most cases the last message is the correct one.
|
|
But if your 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 <tt/setserial/ program being run at
|
|
boot-time. 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 someone incorrectly put this into a
|
|
configuration file (or the like). The fact that Linux sometimes gets
|
|
IRQs wrong is because it doesn't by default probe for IRQs. It just
|
|
assumes the "standard" ones (first message) or accepts what you told
|
|
it when you configured it (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 serial ports 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. Your may check them manually with <tt/setserial/
|
|
using the <tt/autoconfig/ and <tt/auto_irq/ options but this isn't
|
|
guaranteed to be correct either.
|
|
|
|
The data shown by the BIOS messages (which you see at first) is what
|
|
is set in the hardware. If your serial port is Plug-and-Play PnP then
|
|
it's possible that the <tt/isapnp/ 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). 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, Linux will not
|
|
find them 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. The PC hardware (a PnP BIOS) may automatically low-level
|
|
configure this. PnP configuring will be explained later.
|
|
|
|
<sect1> What is the current IO address and IRQ of my Serial Port ?
|
|
<label id="what_is_io_irq">
|
|
<p> If your serial port seems to work OK, then you may type "setserial
|
|
-g /dev/ttyS*", look at /proc/tty/driver/serial, or inspect the
|
|
start-up messages. If you serial port doesn't work (or is very slow)
|
|
then you need to read further.
|
|
|
|
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. They both should be the same. If they're not it spells
|
|
trouble since the driver has incorrect info on the physical serial
|
|
port. 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 <ref id="slow_" name="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 I0-IRQ
|
|
pairs are identical you must find out how they are set in both the
|
|
driver and the hardware.
|
|
|
|
<sect2> What does the device driver think?
|
|
<p> This is easy to find out. Just look at the start-up messages or
|
|
type "setserial -g /dev/ttyS*". If everything works OK then what
|
|
it tells you is likely also set in the hardware. 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.
|
|
|
|
<tt>/proc/ioports</tt> will show the IO addresses that the drivers are using.
|
|
<tt>/proc/interrupts</tt> 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.
|
|
<tt>/proc/tty/driver/serial</tt> shows most 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 in the case of bytes) working. 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 and it implies
|
|
that the driver is using the wrong interrupt. 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.
|
|
|
|
<sect2> What is set in my serial port hardware ? <label
|
|
id="io-irq_in_hdw"
|
|
<p> How do you find out what IO address and IRQ are actually set in
|
|
the device hardware? 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 pciutils can and starting with
|
|
kernel 2.4, these will be built into the serial driver.
|
|
|
|
One crude method is try probing with setserial using the "autoconfig"
|
|
option. You'll need to guess the addresses to probe at. See <ref
|
|
id="set_serial" name="What is Setserial">. For a PCI serial port, use
|
|
the "lspci" command (for kernels <2.2 look at /proc/pci). If your
|
|
serial port is is Plug-and-Play see the next two subsections.
|
|
|
|
For a port set with jumpers, its how the jumpers were set. 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.
|
|
|
|
<sect2> What is set in my PnP serial port hardware ?
|
|
<p> 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. If you have an ISA PnP port, it
|
|
can reach a state where it doesn't have any IO address or IRQ and is
|
|
in effect disabled. It should still be possible to find the port
|
|
using the <tt/pnpdump/ program.
|
|
|
|
For Plug-and-Play (PnP) on the ISA bus one may try the <tt/pnpdump/
|
|
program (part of <tt/isapnptools/). If you use the --dumpregs option
|
|
then it should tell you the actual IO address and IRQ set in the port.
|
|
The address it "trys" is not the device's IO address, but a special
|
|
|
|
For PnP ports checking on how it's configured under DOS/Windows may not
|
|
be of much help. Windows stores its configuration info in its
|
|
Registry which is not used by Linux. It may supply the BIOS's
|
|
non-volatile memory with some info but it may not be kept in sync with
|
|
the current Window configuration in the Registry ?? 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 running
|
|
Linux) then Linux should use whatever configuration is in the BIOS's
|
|
non-volatile memory.
|
|
|
|
<sect1>Choosing Serial IRQs <label id="choose_IRQ">
|
|
|
|
<p> If you have Plug-and-Play ports then either a PnP BIOS or the
|
|
serial driver may configure all your devices for you and then you may
|
|
not need to choose any IRQs. PnP determines what it thinks is best
|
|
and assigns them. But if you use the tools in Linux for Plug-and-Play
|
|
(isapnp and pcitools) 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).
|
|
|
|
<sect2> IRQ 0 is not an IRQ
|
|
<p> 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. This is quite inefficient but can be tried if there
|
|
is an interrupt conflict or mis-set interrupt. The advantage of
|
|
assigning this 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.
|
|
|
|
<sect2> Interrupt sharing and Kernels 2.2+ <label id="int_share-2.2">
|
|
<p> The general rule is that every device should use a unique IRQ and
|
|
not share them. But there are situations where sharing is permitted
|
|
such as with most multi-port boards. Even when it is permitted, it
|
|
may not be 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 nice to allocate every device its own interrupt.
|
|
|
|
Prior to kernel 2.2, serial IRQs could be shared with each other only
|
|
for most multiport boards. Starting with kernel 2.2 serial IRQs may
|
|
be sometimes shared between all 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). Thus even if you have 2.2, it may be best
|
|
to avoid sharing.
|
|
|
|
<sect2> What IRQs to choose?
|
|
|
|
<p> The serial hardware often has only a limited number of IRQs it can
|
|
be set at. Also you don't want IRQ conflicts. So there may not be
|
|
much of a choice. Your PC may normally come with <tt/ttyS0/ and
|
|
<tt/ttyS2/ at IRQ 4, and <tt/ttyS1/ and <tt/ttyS3/ at IRQ 3. Looking
|
|
at <tt>/proc/interrupts</tt> 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 of 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.
|
|
|
|
<tscreen><verb>
|
|
/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
|
|
</verb></tscreen>
|
|
<p>
|
|
Standard IRQ assignments:
|
|
<verb>
|
|
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 coprocessor
|
|
IRQ 14 Hard disk controller 1
|
|
IRQ 15 Hard disk controller 2
|
|
</verb>
|
|
<p>
|
|
|
|
There is really no Right Thing to do when choosing interrupts. Just
|
|
make sure it 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, double-check <tt>/proc/interrupts</tt> when
|
|
programs that use interrupts are being run and make sure there are no
|
|
conflicts.
|
|
|
|
<sect1> Choosing Addresses --Video card conflict with ttyS3
|
|
<label id="choose_address">
|
|
<p> 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 <tt/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 <tt/ttyS3/ (ISA bus) at
|
|
this IO address.
|
|
|
|
For the ISA bus you should try to use the default addresses shown
|
|
below. Addresses shown 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:
|
|
|
|
<tscreen><verb>
|
|
ttyS0 address 0x3f8
|
|
ttyS1 address 0x2f8
|
|
ttyS2 address 0x3e8
|
|
ttyS3 address 0x2e8
|
|
</verb></tscreen>
|
|
|
|
Suppose there is an address conflict (as reported by <tt>setserial -g
|
|
/dev/ttyS*</tt>) 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.
|
|
|
|
<sect1> Set IO Address & IRQ in the hardware (mostly for PnP)
|
|
<label id="io-irq_methods">
|
|
|
|
<p> After it's set in the hardware don't forget to insure that it also
|
|
gets set in the driver by using <tt/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:
|
|
|
|
<itemize>
|
|
<item> Using a PnP BIOS CMOS setup menu
|
|
(usually only for external
|
|
devices
|
|
on ttyS0 (Com1) and ttyS1 (Com2))
|
|
<item> Letting a PnP BIOS automatically configure a PnP serial port
|
|
See <ref id="bios_conf" name="Using a PnP BIOS to I0-IRQ Configure">
|
|
<item> Doing nothing if you have both a PnP serial port and a PnP
|
|
Linux operating system (see Plug-and-Play-HOWTO).
|
|
<item> Using <tt/isapnp/ for a PnP serial port non-PCI)
|
|
<item> Using pciutils (pcitools) for the PCI bus
|
|
</itemize>
|
|
|
|
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 was designed to automate this io-irq configuring,
|
|
but for Linux at present, it has made life more complicated. The
|
|
standard kernels for Linux don't support plug-and-play very well. If
|
|
you use a patch to the Linux kernel to covert it to a plug-and-play
|
|
operating system, then all of the above should be handled
|
|
automatically by the OS. But when you want to use this to automate
|
|
configuring devices other that the serial port, you may find that
|
|
you'll still have to configure the drivers manually since many Linux
|
|
drivers are not written to support a Linux PnP OS. If you use
|
|
<tt/isapnptools/ or the BIOS for configuring plug-and-play this will
|
|
only put the two values into the registers of the serial port section
|
|
of the modem card and you will likely still need to set up setserial.
|
|
None of this is easy or very well documented as of early 1999. See
|
|
Plug-and-Play-HOWTO and the isapnptools FAQ.
|
|
|
|
<sect2> Using a PnP BIOS to I0-IRQ Configure <label id="bios_conf">
|
|
<p> While the explanation of how to use a PnP OS 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 and for an "Award"
|
|
BIOS it was found under "chipset features setup" There is often
|
|
little to choose from. Unless otherwise indicated in a menu, these
|
|
first two ports normally get set at the standard IO addresses and
|
|
IRQs. See <ref id="dev_nos" name="Serial Port Device Names &
|
|
Numbers">
|
|
|
|
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 you probably don't
|
|
have) or if thinks you don't have a PnP OS it may fully configure all
|
|
the PnP devices but not configure the device drivers. This is what
|
|
you want but it's not always easy to figure out exactly what the PnP
|
|
BIOS has done.
|
|
|
|
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 change its PnP
|
|
configuration to accommodate 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 <ref id="io-irq_in_hdw" name="What is set in my
|
|
serial port hardware?"
|
|
|
|
<sect1> Giving the IRQ and IO Address to Setserial
|
|
<p> 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 <ref
|
|
id="sets_boot_time" name="Boot-time Configuration">
|
|
<!-- configure.H end-->
|
|
|
|
|
|
<sect1> High-level Configuring: stty, etc.
|
|
<p> As a rule, your application program will do most (or all) of this.
|
|
The command which does it is <tt/stty/. See <ref id="stty_"
|
|
name="Stty">
|
|
|
|
<sect2> Configuring Flow Control: Hardware Flow Control is Best
|
|
<p> See <ref id="flow_control" name="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 wires for it in the cable between
|
|
the serial port and the device. If the device is on a card, 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 for you. It might even set
|
|
hardware flow control by default. Like the IRQ and IO address, it
|
|
must be set both in the serial driver and the hardware connected to
|
|
the serial port. How it's set into the hardware is hardware
|
|
dependent. Often 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 <tt/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:
|
|
|
|
<tscreen><verb>
|
|
stty crtscts < /dev/ttyS2
|
|
or for stty version >= 1.17:
|
|
stty -F /dev/ttyS2 crtscts
|
|
</verb></tscreen>
|
|
|
|
<tt/crtscts/ stands for a Control setting to use the RTS and CTS pins of
|
|
the serial port for hardware flow control. The upper case
|
|
letters of the last sentence spell: <tt/crtscts/.
|
|
|
|
<sect> Serial Port Devices /dev/ttyS2, etc. <label id="ttySN_">
|
|
<!-- device_dir.H begin
|
|
in Modem and Serial HOWTOs
|
|
<sect> Serial Port Devices /dev/ttyS2, etc. -->
|
|
|
|
<p> For creating devices in the device directory see:
|
|
|
|
<ref id="create_dev" name="Creating Devices In the /dev directory">
|
|
|
|
<sect1> Devfs (The new Device File System)
|
|
<p> This is a new type of device interface to Linux. It's optional
|
|
starting with kernel 2.4. It's more efficient than the conventional
|
|
interface and makes it easy to deal with a huge number of devices.
|
|
The device names have all changed as well. But there's an option to
|
|
continue using the old names. For a detailed description of it see:
|
|
<url url="http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html">
|
|
|
|
The name changes (if used) are: 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 should still be in the /dev directory although optionally
|
|
one may put them elsewhere.
|
|
|
|
<sect1>Serial Port Device Names & Numbers <label id="dev_nos">
|
|
<p> Devices in Linux have major and minor numbers (unless you use the
|
|
new devfs). The serial port ttySx (x=0,1,2, etc.) has major number 4.
|
|
You may see this (and the minor numbers too) by typing: "ls -l ttyS*"
|
|
in the /dev directory.
|
|
|
|
There formerly was an alternate name for each serial port. For
|
|
example, ttyS2 would have cua2 as an alternate name. You may still
|
|
have the cua devices in your /dev directory but they are now
|
|
deprecated. Their drivers behave slightly different than for the ttyS
|
|
ones. Device Obsolete">."') See the Modem-HOWTO
|
|
section: "cua Device Obsolete">.
|
|
|
|
Dos/Windows use the COM name while the <tt/setserial/ program uses
|
|
tty00, tty01, etc. Don't confuse these with dev/tty0, dev/tty1, etc.
|
|
which are used for the console (your PC monitor) but are not serial
|
|
ports. The table below is for the "standard" case (but yours could be
|
|
different).
|
|
|
|
<tscreen><verb>
|
|
IO devfs
|
|
dos major minor major minor address name
|
|
COM1 /dev/ttyS0 4, 64; /dev/cua0 5, 64 3F8 /dev/tts/0
|
|
COM2 /dev/ttyS1 4, 65; /dev/cua1 5, 65 2F8 /dev/tts/1
|
|
COM3 /dev/ttyS2 4, 66; /dev/cua2 5, 66 3E8 /dev/tts/2
|
|
COM4 /dev/ttyS3 4, 67; /dev/cua3 5, 67 2E8 /dev/tts/3
|
|
</verb></tscreen>
|
|
|
|
<sect1> Universal Serial Bus Ports
|
|
<p> Although the USB is not covered in this HOWTO, the serial ports on
|
|
the USB are: /dev/ttyUSB0 /dev/ttyUSB1, etc.
|
|
|
|
<sect1> Link ttySN to /dev/modem
|
|
<p> On some installations, two extra devices will be created,
|
|
<tt>/dev/modem</tt> for your modem and <tt>/dev/mouse</tt> for a
|
|
mouse. Both of these are symbolic links to the appropriate serial
|
|
device in <tt>/dev</tt> which you specified during the installation
|
|
Except if you have a bus mouse, then <tt>/dev/mouse</tt> will point to
|
|
the bus mouse device).
|
|
|
|
Formerly (in the 1990s) the use of <tt>/dev/modem</tt> was discouraged
|
|
since lock files might not realize that it was really say
|
|
<tt>/dev/ttyS2</tt>. The newer lock file system doesn't fall into
|
|
this trap so it's now OK to use such links.
|
|
|
|
<!-- device_dir.H end -->
|
|
|
|
|
|
<sect1> Which Connector on the Back of my PC is ttyS1, etc?
|
|
<sect2> Inspect the connectors
|
|
<p> 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 PnP configured yet). A report from setserial (at
|
|
boot-time or run by you from the command line) should help you
|
|
identify the non-modem port.
|
|
|
|
If you have two serial connectors it may be more difficult.
|
|
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 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 CMOS setup menu). But 1 or A are more likely to be ttyS0 with 2 or
|
|
B ttyS1.
|
|
|
|
<sect2> Send bytes to the port
|
|
<p> 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) it's 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 positive 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 "<tt>stty -F /dev/ttyS1 -a</tt>" 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 and perhaps
|
|
use a scrap of paper to prevent the mini clips from accidentally
|
|
touching the metal of the connector. 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 (Alt-F2 for
|
|
example) 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
|
|
<ref id="ser_elect_test" name="Serial Electrical Test Equipment"> for
|
|
more info.
|
|
|
|
<sect2> Connect a device to the connector
|
|
<p> 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 if connected.
|
|
|
|
<sect2> Missing connectors
|
|
<p> 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 serial DB
|
|
connector. Someone may build a PC from this and omit the connector.
|
|
There may be a "serial" label on the motherboard but no ribbon cable
|
|
connects to the pins next to this label. To use this port you must
|
|
get a ribbon cable/connector. I've seen different wiring arrangements
|
|
for such ribbon cables so beware.
|
|
|
|
<sect1>Creating Devices In the /dev directory <label id="create_dev">
|
|
<p>
|
|
If you don't have a device "file" that you need, you will have to
|
|
create it with the <tt/mknod/ command or with the MAKEDEV shell
|
|
script. Example, suppose you needed to create <tt/ttyS0/:
|
|
<tscreen><verb>
|
|
linux# mknod -m 666 /dev/ttyS0 c 4 64
|
|
</verb></tscreen>
|
|
You can use the <tt/MAKEDEV/ script, which lives in <tt>/dev</tt>.
|
|
See the man page for it. This simplifies the making of devices. For
|
|
example, if you needed to make the devices for <tt>ttyS0</tt> you
|
|
would type:
|
|
<tscreen><verb>
|
|
linux# cd /dev
|
|
linux# ./MAKEDEV ttyS0
|
|
</verb></tscreen>
|
|
This handles the devices creation and should set the correct permissions.
|
|
For making multiport devices see <ref id="make_multi" name="Making
|
|
multiport devices in the /dev directory">.
|
|
|
|
<sect>Interesting Programs You Should Know About
|
|
<p> 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.
|
|
|
|
<sect1> Serial Monitoring/Diagnostics Programs <label id="serial_mon">
|
|
<p> 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).
|
|
<itemize>
|
|
<item> The "file": /proc/tty/driver/serial lists those that are positive
|
|
<item> 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.
|
|
<item> statserial (Info displayed on entire screen)
|
|
<item> serialmon (Doesn't monitor RTS, CTS, DSR but logs other
|
|
functions)
|
|
</itemize>
|
|
You may already have them. If not, download them from <url url=
|
|
"http://metalab.unc.edu/pub/Linux/system/serial/" name="Serial
|
|
Software">. As of June 1998, I know of no diagnostic program in Linux
|
|
for the serial port.
|
|
|
|
<sect1> Changing Interrupt Priority
|
|
<p>
|
|
<itemize>
|
|
<item> <tt/irqtune/ will give serial port interrupts higher
|
|
priority to improve performance.
|
|
<item> <tt/hdparm/ for hard-disk tuning may help some more.
|
|
</itemize>
|
|
|
|
<sect1>What is Setserial ? <label id="set_serial">
|
|
<!-- setserial.H begin (in MM TT SS)
|
|
<sect1>What is Setserial ? <label id="set_serial">
|
|
Change Log:
|
|
May 2000: <sect2> IRQs near end ttyS0 -> ttyS1 + clarity
|
|
Nov. 2000: auto_irq may work on the 2nd try
|
|
Dec. 2000: saving state of serial module
|
|
June 2001 OK to use setserial with Laptops
|
|
-->
|
|
<p> This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There
|
|
are some minor differences, depending on which HOWTO it appears in.
|
|
|
|
<sect2> Introduction
|
|
<p> If you have a Laptop (PCMCIA) don't use <tt/setserial/ until you
|
|
read <ref id="laptops_" name="Laptops: PCMCIA">. <tt/setserial/ is a
|
|
program which allows you to tell the device driver software the I/O
|
|
address of the serial port, which interrupt (IRQ) is set in the port's
|
|
hardware, what type of UART you have, etc. Since theres a good chance
|
|
that the serial ports will be automatically detected and set, many
|
|
people never need to use <tt/setserial/. In any case setserial will
|
|
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) tries to use setserial.
|
|
|
|
Setserial can also show how the driver is currently set. In addition,
|
|
it can be made to probe the hardware and try to determine the UART
|
|
type and IRQ, but this has severe limitations. See <ref
|
|
id="probing_ss" name="Probing">. Note that it can't set the IRQ or
|
|
the port address in the hardware of PnP serial ports (but the
|
|
plug-and-play features of the serial driver may do this).
|
|
|
|
If you only have one or two built-in serial ports, they will usually
|
|
get set up correctly without using setserial. Otherwise, if you add
|
|
more serial ports (such as a modem card) you will likely need to deal
|
|
with setserial. Besides the man page for <tt/setserial/, check out
|
|
info in <tt>/usr/doc/setserial.../</tt> or
|
|
<tt>/usr/share/doc/setserial</tt>. It should tell you how setserial
|
|
is handled in your distribution of Linux.
|
|
|
|
<tt/Setserial/ is often run automatically at boot-time by a start-up
|
|
shell-script for the purpose of assigning IRQs, etc. to the driver.
|
|
Setserial will only work if the serial module is loaded (or if the
|
|
equivalent was compiled into your kernel). If the serial module gets
|
|
unloaded later on, the changes previously made by <tt/setserial/ will
|
|
be forgotten by the kernel. But recent (2000) distributions may
|
|
contain scripts that save and restore this. If not, then
|
|
<tt/setserial/ must be run again to reestablish them. In addition to
|
|
running via a start-up script, something akin to <tt/setserial/ also
|
|
runs earlier when the serial module is loaded (or the like). Thus
|
|
when you watch the start-up messages on the screen it may look like it
|
|
ran twice, and in fact it has.
|
|
|
|
Setserial can set 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 needed at slow baud rates of 1200 or lower. It's
|
|
also needed at higher speeds if there are a lot of "flow control"
|
|
waits. See "closing_wait" in the setserial man page.
|
|
|
|
|
|
Setserial does not set either IRQ's nor I/O addresses in the serial
|
|
port hardware itself. That is done either by jumpers or by
|
|
plug-and-play. You must tell setserial the identical values that have
|
|
been set in the hardware. Do not just invent some values that you
|
|
think would be nice to use and then tell them to setserial. However,
|
|
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 <tt/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:
|
|
<tscreen><verb>
|
|
setserial -g /dev/ttyS*
|
|
</verb></tscreen>
|
|
you'll see some info about how that device driver is configured for
|
|
your ports. Note that where it says <tt>"UART: unknown"</tt> it
|
|
probably means that no uart exists. In other words you probably have
|
|
no such serial port and the other info shown about the port is
|
|
meaningless and should be ignored. If you really do have such a
|
|
serial port, setserial doesn't recognize it and that needs to be
|
|
fixed.
|
|
|
|
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 without complaint. They will also be officially
|
|
registered with the kernel as seen by the "scanport" command.
|
|
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 will not accept
|
|
it. But if you use 3e9 it will accept it. Unfortunately 3e9 is
|
|
already 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.
|
|
|
|
While assignments made by setserial are lost when the PC is powered
|
|
off, a configuration file may restore them (or a previous
|
|
configuration) when the PC is started up again. In newer versions,
|
|
what you change by setserial may get automatically saved to a
|
|
configuration file. In older versions, the configuration file only
|
|
changes if you edit it manually so the configuration always remains
|
|
the same from boot to boot. See <ref id="ss_conf_script"
|
|
name="Configuration Scripts/Files">
|
|
|
|
<sect2> Probing <label id="probing_ss">
|
|
|
|
<p> With appropriate options, <tt/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 <ref id="probing_ss" name="Probing">
|
|
|
|
The purpose of this 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 <tt>pnpdump
|
|
--dumpregs</tt>. To try to detect the physical hardware use the -v
|
|
(verbose) and <tt/autoconfig/ command to <tt/setserial/. If the
|
|
resulting message shows a uart type such as 16550A, then you're OK.
|
|
If instead it shows "<tt/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
|
|
"<tt/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 may be saved and put into the configuration file
|
|
<tt>/etc/serial.conf</tt> which will be used next time you start
|
|
Linux. At boot-time when the serial module loads (or the like), a
|
|
probe for UARTs is made automatically and reported on the screen. But
|
|
the IRQs shown may be wrong. The second report of the same is the
|
|
result of a script which usually does no probing and thus provides no
|
|
reliable information as to how the hardware is actually set. It only
|
|
shows configuration data someone wrote into the script or data that
|
|
got saved in /etc/serial.conf.
|
|
|
|
It may be that two serial ports both have the same IO address set in
|
|
the hardware. Of course this is not permitted 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
|
|
<tt/setserial/ to give the IRQ a ficticious value.
|
|
|
|
<sect2> Boot-time Configuration <label id="sets_boot_time">
|
|
<p> When the kernel loads the serial module (or if the "module
|
|
equivalent" is built into the kernel) then only <tt/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). You see
|
|
this as a boot-time message just like as if <tt/setserial/ had been
|
|
run.
|
|
|
|
To correct possible errors in IRQs (or for other reasons) there may be
|
|
a file somewhere that runs <tt/setserial/ again. Unfortunately, if
|
|
this file has some IRQs wrong, the kernel will still have incorrect
|
|
info about the IRQs. This file should run early at boot-time before
|
|
any process uses the serial port. In fact, your distribution may have
|
|
set things up so that the setserial program runs automatically from a
|
|
start-up script at boot-time. More info about how to handle this
|
|
situation for your particular distribution might be found in file
|
|
named "setserial..." or the like located in directory /usr/doc/ or
|
|
/usr/share/doc/.
|
|
|
|
Before modifying a configuration file, you can test out a "proposed"
|
|
<tt/setserial/ command by just typing it on the command line. In some
|
|
cases the results of this use of <tt/setserial/ will automatically get
|
|
saved in /etc/serial.conf when you shutdown. So if it worked OK (and
|
|
solved your problem) then there's no need to modify any configuration
|
|
file. See <ref id="new_config" name="New configuration method using
|
|
/etc/serial.conf">.
|
|
|
|
<sect2> Configuration Scripts/Files <label id="ss_conf_script">
|
|
<p> Your objective is to modify (or create) a script file in the /etc
|
|
tree that runs setserial at boot-time. Most distributions provide
|
|
such a file (but it may not initially reside in the /etc tree). In
|
|
addition, setserial 2.15 and higher often have an /etc/serial.conf
|
|
file that is used by the above script so that you don't need to
|
|
directly edit the script that runs setserial. In addition just using
|
|
setserial on the command line (2.15+) may ultimately alter this
|
|
configuration file.
|
|
|
|
So prior to version 2.15 all you do is edit a script. After 2.15 you
|
|
may need to either do one of three things: 1. edit a script. 2. edit
|
|
<tt>/etc/serial.conf</tt> or 3. run "setserial" on the command line
|
|
which may result in <tt>/etc/serial.conf</tt> automatically being
|
|
edited. Which one of these you need to do depends on both your
|
|
particular distribution, and how you have set it up.
|
|
|
|
<sect2> Edit a script (required prior to version 2.15)
|
|
<label id="old_sets_script">
|
|
<p> Prior to setserial 2.15 (1999) there was no /etc/serial.conf file
|
|
to configure setserial. Thus you need to find the file that runs
|
|
"setserial" at boot time and edit it. If it doesn't exist, you need
|
|
to create one (or place the commands in a file that runs early at
|
|
boot-time). If such a file is currently being used it's likely
|
|
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. You might use "locate" to try to find such a file. For
|
|
example, you could type: locate "*serial*".
|
|
|
|
The script <tt>/etc/rc.d/rc.serial</tt> was commonly used in the past.
|
|
The Debian distribution used <tt>/etc/rc.boot/0setserial</tt>.
|
|
Another file once used was <tt>/etc/rc.d/rc.local</tt> but it's
|
|
not a good idea to use this since it may not be run early enough.
|
|
It's been reported that other processes may try to open the serial
|
|
port before rc.local runs resulting in serial communication failure.
|
|
Today it's most likely in /etc/init.d/ but it isn't normally intended
|
|
to be edited.
|
|
|
|
If such a file is supplied, it should contain a number of
|
|
commented-out examples. By uncommenting some of these and/or
|
|
modifying them, you should be able to set things up correctly. Make
|
|
sure that you are using a valid path for <tt/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 is a lot faster than doing repeated reboots to get
|
|
it right.
|
|
|
|
For versions >= 2.15 (provided your distribution implemented the
|
|
change, Redhat didn't) 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 <ref id="new_config"
|
|
name="New configuration method using /etc/serial.conf">.
|
|
|
|
If you want setserial to automatically determine the uart and the IRQ
|
|
for ttyS3 you would add something like:
|
|
|
|
<tscreen><verb>
|
|
/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
|
|
</verb></tscreen>
|
|
Do this for every serial port you want to auto configure. Be sure to
|
|
give a device name that really does exist on your machine. In some
|
|
cases this will not work right due to the hardware. If you know what
|
|
the uart and irq actually are, you may want to assign them explicitly
|
|
with "setserial". For example:
|
|
|
|
<tscreen><verb>
|
|
/sbin/setserial /dev/ttyS3 irq 5 uart 16550A skip_test
|
|
</verb></tscreen>
|
|
|
|
|
|
<sect2> New configuration method using /etc/serial.conf
|
|
<label id="new_config">
|
|
<p> Prior to setserial version 2.15, the way to configure setserial
|
|
was to manually edit the shell-script that ran setserial at boot-time.
|
|
See <ref id="old_sets_script" name="Edit a script (after version 2.15:
|
|
perhaps not)">. Starting with version 2.15 (1999) of <tt/setserial/
|
|
this shell-script is not edited but instead gets its data from a
|
|
configuration file: <tt>/etc/serial.conf</tt>. Furthermore you may
|
|
not even need to edit serial.conf because using the "setserial"
|
|
command on the command line may automatically cause serial.conf to be
|
|
edited appropriately.
|
|
|
|
This was intended so that you don't need to edit any file in order to
|
|
set up (or change) what setserial does each time that Linux is booted.
|
|
But there are serious pitfalls because it's not really "setserial"
|
|
that edits serial.conf. Confusion is compounded because different
|
|
distributions handle this differently. In addition, you may modify it
|
|
so that it works differently.
|
|
|
|
What often happens is this: When you shut down your PC the script
|
|
that runs "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 <tt>serial.conf</tt> file. 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.
|
|
|
|
If 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 at least one distribution, the removal of
|
|
"###AUTOSAVE###" from the first line is automatically done after the
|
|
first time you shutdown just after installation. The serial.conf file
|
|
should contain some comments to explain this.
|
|
|
|
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 <tt/setserial/ to set it to
|
|
"uart none". 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.
|
|
|
|
BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
|
|
only the setserial parameters displayed by "setserial -Gg /dev/ttyS*"
|
|
get saved but the other parameters don't get saved. Use the -a flag
|
|
to "setserial" to see all parameters. This will only affect a small
|
|
minority of users since the defaults for the parameters not saved are
|
|
usually OK for most situations. It's been reported as a bug and may
|
|
be fixed by now.
|
|
|
|
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
|
|
<tt>/etc/init.d/{set}serial stop</tt>. 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".
|
|
|
|
<sect2> IRQs
|
|
|
|
<p> By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
|
|
ttyS3 share IRQ 3. But actually sharing serial interrupts (using them
|
|
in running programs) is not permitted unless you: 1. have kernel 2.2
|
|
or better, and 2. you've complied in support for this, and 3. your
|
|
serial hardware supports it. See
|
|
|
|
<ref id="int_share-2.2" name="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 an internal modem and retain ttyS0 and ttyS1,
|
|
then you should attempt to find an unused IRQ and set it both on 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
|
|
may be one you can use for a modem. To set the IRQ in hardware you
|
|
may need to use isapnp, a PnP BIOS, or patch Linux to make it PnP. To
|
|
help you determine which spare IRQ's you might have, type "man
|
|
setserial" and search for say: "IRQ 11".
|
|
|
|
<sect2> Laptops: PCMCIA <label id="laptops_">
|
|
<p>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
|
|
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.
|
|
|
|
<!-- setserial.H end -->
|
|
|
|
|
|
<sect1> Stty <label id="stty_">
|
|
<!-- stty.H begin <sect1> Stty <label id="stty_">
|
|
In Serial and Text-Terminal -->
|
|
<sect2> Introduction
|
|
<p> <tt/stty/ does much of the configuration of the serial port but
|
|
since application programs (and the getty program) often handle it,
|
|
you may not need to use it much. It's handy if your 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. Don't try to learn all the setting unless you want to
|
|
become a serial guru. Most of the defaults should work OK and some of
|
|
the settings are needed only for certain obsolete dumb terminals made
|
|
in the 1970's.
|
|
|
|
<tt/stty/ is documented in the man pages with a more detailed account
|
|
in the info pages. Type <tt>"man stty"</tt> or <tt>"info stty"</tt>.
|
|
|
|
Whereas <tt/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 at 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 <tt/stty/ man or info
|
|
page for more details. Also see the man page: <tt/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 is useful. 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.
|
|
|
|
<sect2> Flow control options
|
|
<p> 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 restarts 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 the the PC's port will be overrun. So you seldom
|
|
actually need to enable ixoff. But it's often enabled "just in case".
|
|
|
|
<sect2> Using stty at a "foreign" terminal
|
|
<p> Using <tt/stty/ to configure the terminal that you are currently
|
|
using is easy. Doing it for a different (foreign) terminal or serial
|
|
port may be impossible. For example, let's say you are at the PC monitor
|
|
(tty1) and want to use <tt/stty/ to deal with the serial port ttyS2.
|
|
Prior to about 2000 you needed to use the redirection operator "<".
|
|
After 2000 (provided your version of setserial is >= 1.17 and stty >=
|
|
2.0) there is a better method using the -F option. This will work
|
|
when the old redirection method fails. Even with the latest versions
|
|
be warned that if there is a terminal on ttyS2 and a shell is running
|
|
on that terminal, then what you see will likely be deceptive and
|
|
trying to set it will not work. See <ref id="2_term_interfaces"
|
|
name="Two interfaces at a terminal"> to understand it.
|
|
|
|
The new method is ``stty -F /dev/ttyS2 ...'' (or --file instead of F).
|
|
If ... is -a it displays all the stty settings. The old redirection
|
|
method (which still works in later versions) is to type ``stty ... <
|
|
/dev/ttyS2''. If the new method works but the old one hangs, it
|
|
implies that the port is hung due to a modem control line not being
|
|
asserted. Thus the old method is still useful for troubleshooting.
|
|
See the following subsection for details.
|
|
|
|
<sect3> Old redirection method
|
|
<p> 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 a 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.
|
|
|
|
The old redirection method 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. Some people tried to use
|
|
``stty ... > /dev/ttyS2'' to set the terminal. This will not do it.
|
|
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.
|
|
|
|
<sect2> Two interfaces at a terminal <label id="2_term_interfaces">
|
|
<p> 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 anything to this cooked
|
|
mode but what was typed in raw mode gets executed 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 since you
|
|
are starting the command-line editor). The settings for the "raw"
|
|
mode are based only on the basic 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 another
|
|
(foreign) terminal 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 terminal you tried to "set".
|
|
But if you type a stty command at your terminal (without the -F option
|
|
or redirection) 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.
|
|
|
|
<sect2> Where to put the stty command ? <label id="stty_where">
|
|
<p> Should you need to have <tt/stty/ set up the serial interface each
|
|
time the computer starts up then you need to put the <tt/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.
|
|
<!-- stty.H end -->
|
|
|
|
|
|
<sect1> What is isapnp ?
|
|
<p> <tt/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.
|
|
|
|
<sect1> What is slattach?
|
|
<p> It's "serial line attach". It puts the serial line into a
|
|
networking mode. You can thus network two computers together via a
|
|
serial line using, for example, the slip protocol. But for the ppp
|
|
protocol, you need to start pppd on the serial line.
|
|
|
|
<sect> Speed (Flow Rate) <label id="speed_">
|
|
|
|
<p> 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 <ref id="stty_" name="Stty">
|
|
|
|
<sect1> Can't Set a High Enough Speed
|
|
<!-- high_speed.H begin In Serial and Modem HOWTOs but some of the speed
|
|
section is unique to each HOWTO.
|
|
-->
|
|
<p> You need to find out the highest speed supported by your hardware.
|
|
As of late 1998 most serial ports only supported speeds up to 115.2k
|
|
bps but as of 2001, most support higher speeds. Some 56k internal
|
|
modems support 230.4k bps (but it may be hard to find out which ones
|
|
do). Recent Linux kernels support high speeds (over 115.2k) but you
|
|
might have some problems using it because of one or both of the
|
|
following reasons:
|
|
|
|
<enum>
|
|
<item> It may require a special driver or a patch to the serial driver
|
|
<item> Older versions of an application program (or stty) may not
|
|
support the high speed.
|
|
<item> Setserial may show the wrong speed unless you use the baud_base
|
|
option. Even so, high speed will still work but the reported speed
|
|
will be wrong.
|
|
</enum>
|
|
|
|
To support the higher speeds you may get a shsmod (Super High Speed)
|
|
patch for the serial driver. There is also a module for the VIA
|
|
VT82C686 chip <url url="www.kati.fi/viahss/">.
|
|
|
|
<sect2> How speed is set in hardware: the divisor and baud_base <label
|
|
id="divisor_">
|
|
<p> Here's a list of commonly used divisors and their corresponding
|
|
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).
|
|
|
|
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 obviously sets the highest speed. If you happen to
|
|
have hardware with a maximum speed of say 230.4k, then specifying
|
|
115.2k will result in divisor 1 and 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.
|
|
|
|
<sect2> Work-arounds for setting speed
|
|
<p> 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. PROBLEM: stty and many
|
|
communication programs (as of mid 1999) still have 115.2k as their
|
|
maximum speed setting and will not let you set 230.4k, etc. So in
|
|
these cases one solution is not to change anything with <tt/setserial/
|
|
but mentally keep in mind that the actual speed is always double what
|
|
you set.
|
|
|
|
There's another work-around which is not much better. To use it you
|
|
set the baud_base (with setserial) to the maximal speed of your
|
|
hardware. This corrects the accounting so that if you set say 115.2k
|
|
you actually get 115.2k. Now you still have to figure out how to set
|
|
the highest speed if your communication program (or the like) will not
|
|
let you do it. Fortunately, setserial has a way to do this: use the
|
|
"spd_cust" parameter with "divisor 1". Then when you set the speed to
|
|
38400 in a communication program, the divisor will be set to 1 in the
|
|
port and it will operate at maximum speed. For example:<newline>
|
|
setserial /dev/ttyS2 spd_cust baud_base 230400 divisor 1<newline>
|
|
Don't try using "divisor" for any other purpose other than the special
|
|
use illustrated above (with spd_cust).
|
|
|
|
If there are two or more high speeds that you want to use that your
|
|
communication program can't set, then it's not quite as easy as above.
|
|
But the same principles apply. You could just keep the default
|
|
baud_base and understand that when you set a speed you are really only
|
|
setting a divisor. So your actual speed will always be your maximum
|
|
speed divided by whatever divisor is set by the serial driver. See
|
|
<ref id="divisor_" name="How speed is set in hardware: the divisor and
|
|
baud_base">
|
|
|
|
<sect2> Crystal frequency is not baud_base
|
|
<p> Note that the baud_base setting is usually much lower than the
|
|
frequency of the crystal oscillator in the hardware since the crystal
|
|
frequency is often divided by 16 in the hardware to get the actual top
|
|
speed. The reason the crystal frequency needs to be higher is so that
|
|
this high crystal speed can be used to take a number of samples of
|
|
each bit to determine if it's a 1 or a 0.
|
|
<!-- high_speed.H end -->
|
|
|
|
|
|
<sect1>Higher Serial Throughput <label id="higher_thruput">
|
|
<p>
|
|
If you are seeing slow throughput and serial port overruns on a
|
|
system with (E)IDE disk drives, you can get <tt>hdparm</tt>. 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.
|
|
<p>
|
|
Also have a look at a utility called <tt>irqtune</tt> 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 <tt/irqtune/ FAQ is at <url url="http://www.best.com/~cae/irqtune"
|
|
name="http://www.best.com/~cae/irqtune">
|
|
|
|
<sect> Locking Out Others
|
|
<sect1> Introduction
|
|
<p> 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:
|
|
<enum>
|
|
<item> create lock-files
|
|
<item> modify the permissions and/or owners of devices such as /dev/ttyS2
|
|
</enum>
|
|
|
|
<sect1>Lock-Files <label id="lockfiles_">
|
|
<p>
|
|
A lock-file is simply a file created to mean that a particular device is
|
|
in use. They are kept in <tt>/var/lock</tt>. Formerly they were in
|
|
<tt>/usr/spool/uucp</tt>. Linux lock-files are usually named
|
|
<tt/LCK../<EM/name/, where <EM/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.
|
|
|
|
When there were only lockfiles with device names, the following
|
|
problem could arise: If the same device has two different
|
|
names then two different processes could each use a differnet 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.
|
|
|
|
<enum>
|
|
<item> The lock checking software was made aware of ttyS vs. cua.
|
|
<item> The device cua was deprecated
|
|
<item> Additional locks were created which use unique device numbers
|
|
instead of names.
|
|
</enum>
|
|
|
|
Using alternate names such as /dev/modem for /dev/ttyS2 may cause
|
|
problems with older versions. 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.
|
|
|
|
<sect1> Change Owners, Groups, and/or Permissions of Device Files
|
|
<p> 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.
|
|
|
|
<sect>Communications Programs And Utilities<label id="comms">
|
|
<sect1> List of Software
|
|
<p>
|
|
Here is a list of some communication software you can choose from,
|
|
available via FTP, if they didn't come with your distribution.
|
|
|
|
<itemize>
|
|
<item><tt/ecu/ - a communications program
|
|
<item><url url="http://www.columbia.edu/kermit/" name="C-Kermit"> -
|
|
portable, scriptable, serial and TCP/IP communications including file
|
|
transfer, character-set translation, and zmodem support
|
|
<item><tt>gkermit</tt> Tiny GPLed kermit run only from the command line.
|
|
Can't connect to another computer
|
|
<item><tt/minicom/ - telix-like communications program
|
|
<item><tt/seyon/ - X based communication program
|
|
<item><tt/xc/ - xcomm communication package
|
|
|
|
<item><tt/term/ and <tt/SLiRP/ offer TCP/IP functionality using a
|
|
shell account.
|
|
|
|
<item><tt/screen/ is another multi-session program. This one behaves
|
|
like the virtual consoles.
|
|
|
|
<item><tt/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).
|
|
|
|
<item><tt/mgetty+fax/ handles FAX stuff, and provides an alternate
|
|
<tt/ps_getty/.
|
|
|
|
<item><tt/ZyXEL/ is a control program for ZyXEL U-1496 modems. It
|
|
handles dialin, dialout, dial back security, FAXing, and voice
|
|
mailbox functions.
|
|
|
|
|
|
<item>SLIP and PPP software can be found at
|
|
<tt> <htmlurl url="ftp://metalab.unc.edu/pub/Linux/system/network/serial"
|
|
name="ftp://metalab.unc.edu/pub/Linux/system/network/serial"></tt>.
|
|
</itemize>
|
|
|
|
<sect1>kermit and zmodem
|
|
<p> 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 <tt/.kermrc/ file:
|
|
<tscreen><verb>
|
|
define rz !rz < /dev/ttyS3 > /dev/ttyS3
|
|
define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3
|
|
</verb></tscreen>
|
|
Be sure to put in the correct port your modem is on. Then, to use it,
|
|
just type <tt/rz/ or <tt>sz <filename></tt> at the <tt/kermit/
|
|
prompt.
|
|
|
|
<sect>Serial Tips And Miscellany
|
|
|
|
<sect1> Serial Module <label id="ser_module">
|
|
<p> Often the serial driver is provided as a module. Parameters may
|
|
be supplied to certain modules in /etc/modules.conf. Since kernel 2.2
|
|
you don't edit this 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 here 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.
|
|
|
|
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 (revised in 1999 by Vern Hoxie but not at
|
|
LDP. Get it from <url url="scicom.alphacdc.com/pub/linux">)
|
|
|
|
<sect1> Serial Console (console on the serial port)
|
|
<p> See the kernel documentation in: Documentation/serial-console.txt.
|
|
Kernel 2.4+ has better documentation. See also "Serial Console" in
|
|
Text-Terminal-HOWTO.
|
|
|
|
<sect1> Line Drivers
|
|
<p> For a text terminal, the EIA-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.
|
|
|
|
<sect1> Stopping the Data Flow when Printing, etc.
|
|
<p> Normally flow control and/or application programs stop the flow of
|
|
bytes when its needed. But sometimes they don't. One example is
|
|
printing to printer on the serial port. If you want to instantly stop
|
|
printing you may try turning off the printer. With older versions of
|
|
the serial driver, the printer would attempt to resume printing if you
|
|
turned the printer back on again (before the time specified by
|
|
closing_wait of setserial had expired). The attempt to resume would
|
|
happen even if you used a command to stop the printing. The problem
|
|
was that once the printer software sent bytes to the large serial
|
|
buffer to be printed, these bytes were not removed from this buffer
|
|
when the print job was canceled. One way to remove them (for newer
|
|
serial drivers) is to simply turn off the printer. This will drop all
|
|
modem control signals from the printer and empty the buffer. Modern
|
|
printers have large buffers and often a button on the printer to empty
|
|
the buffer.
|
|
|
|
<sect1> Known Defective Hardware
|
|
<sect2> Avoiding IO Address Conflicts with Certain Video Boards <label
|
|
id="8514_">
|
|
<p> 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
|
|
<tt/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 <tt/ttyS3/ at this IO address. Another story is that Linux
|
|
will not detect your internal modem on <tt/ttyS3/ but that you can use
|
|
<tt>setserial</tt> to put <tt/ttyS3/ at this address and the modem
|
|
will work fine.
|
|
|
|
<sect2> Problem with AMD Elan SC400 CPU (PC-on-a-chip)
|
|
<p> 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?
|
|
|
|
<sect>Troubleshooting <label id="trouble_shoot">
|
|
<p> 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.
|
|
|
|
<sect1> Serial Electrical Test Equipment <label id="ser_elect_test">
|
|
<sect2> Breakout Gadgets, etc.
|
|
<p> 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 1998) a "RS-232 Troubleshooter" or "RS-232 Line
|
|
Tester" which 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" which permits connecting the pins anyway
|
|
you choose.
|
|
|
|
<sect2> Measuring Voltages
|
|
<p> 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.
|
|
|
|
<sect2> Taste Voltage
|
|
<p> 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.
|
|
|
|
<sect1> Serial Monitoring/Diagnostics
|
|
<p> A few Linux programs will monitor the modem control lines and
|
|
indicate if they are positive (1) or negative (0). See section <ref
|
|
id="serial_mon" name="Serial Monitoring/Diagnostics">
|
|
|
|
<!-- currently in <sect>Troubleshooting -->
|
|
<!-- troubleshooting.H begin (in Modem/Serial HOWTOs)
|
|
Change Log:
|
|
Apr. '00: 2 ports on same address
|
|
May '00: address conflict
|
|
Nov. '00: which connector is ttyS1, etc. Input/output error, overrun
|
|
error link
|
|
Dec. '00: /proc/tty/driver/serial shows info, I/O error+, pid 161 in
|
|
example n.g.
|
|
-->
|
|
<sect1>(The following subsections are in both the Serial and Modem HOWTOs)
|
|
|
|
<sect1> My Serial Port is Physically There but Can't be Found
|
|
<label id="cant_find_port">
|
|
<p> If a physical device (such as a modem) doesn't work at all it may
|
|
mean that the device is not at the I/O address that setserial
|
|
thinks it's at. It could also mean (for a PnP card) that is doesn't
|
|
yet have an address. Thus it can't be found.
|
|
|
|
Check the BIOS menus and BIOS messages. For the PCI bus use lspci or
|
|
scanpci. If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
|
|
and/or see Plug-and-Play-HOWTO. Using "scanport" will scan all ISA
|
|
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. You may try
|
|
probing with setserial. See <ref id="probing_ss" name="Probing">. If
|
|
nothing seems to get thru the port it may be accessible but have a bad
|
|
interrupt. See <ref id="slow_" name="Extremely Slow: Text appears on
|
|
the screen slowly after long delays">. Use <tt>setserial -g</tt> to
|
|
see what the serial driver thinks and check for IRQ and I0 address
|
|
conflicts. Even if you see no conflicts the driver may have incorrect
|
|
information (view it by "setserial" and conflicts may still exist.
|
|
|
|
If two ports have the same IO address then probing it will erroneously
|
|
indicate only one port. Plug-and-play detection will find both ports
|
|
so this should only be a problem if at least one port is not
|
|
plug-and-play. All sorts of errors may be reported/observed for
|
|
devices illegally "sharing" a port but the fact that there are two
|
|
devices on the same a port doesn't seem to get detected (except
|
|
hopefully by you). In the above case, if the IRQs are different then
|
|
probing for IRQs with setserial might "detect" this situation by
|
|
failing to detect any IRQ. See <ref id="probing_ss" name="Probing">.
|
|
|
|
<sect1> Extremely Slow: Text appears on the screen slowly after long delays
|
|
<label id="slow_">
|
|
<p> 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
|
|
|
|
<ref id="irq_prob_details" name="Interrupt Problem Details"> and/or <ref
|
|
id="irq_conflict" name="Interrupt Conflicts"> and/or <ref id="irq_ng"
|
|
name="Mis-set Interrupts">.
|
|
If it involves Plug-and-Play devices, see also 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
|
|
<ref id="busy_err" name="/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.
|
|
|
|
<sect1> Somewhat Slow: I expected it to be a few times faster
|
|
<p> One 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
|
|
|
|
<ref id="uart_" name="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 <ref
|
|
id="set_serial" name="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.
|
|
|
|
<sect1>The Startup Screen Show Wrong IRQs for the Serial Ports.
|
|
<label id="irqs_shown_wrong">
|
|
<p> 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 <tt/ttyS2/ set at IRQ 5, I still see
|
|
<tscreen><verb>
|
|
ttyS02 at 0x03e8 (irq = 4) is a 16550A
|
|
</verb></tscreen>
|
|
at first when Linux boots. (Older kernels may show "ttyS02" as
|
|
"tty02" which is the same as ttyS2). You may need to use
|
|
<tt/setserial/ to tell Linux the IRQ you are using.
|
|
|
|
<sect1> "Cannot open /dev/ttyS?: Permission denied"
|
|
<p> 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
|
|
should show 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 ways to get access like belonging to a "group" that
|
|
has group permission.
|
|
|
|
<sect1> "Operation not supported by device" for ttyS?
|
|
<p> 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 <ref
|
|
id="io-irq_in_hdw" name="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.
|
|
|
|
<sect1> "Cannot create lockfile. Sorry"
|
|
<p> 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: usually rwx for everyone (repeated 3 times). If
|
|
it's wrong, use "chmod" to fix it. Of course, if there is no "lock"
|
|
directory no lockfile can be created there. For more info on
|
|
lockfiles see <ref id="lockfiles_" name="What
|
|
Are Lock Files">
|
|
|
|
<sect1> "Device /dev/ttyS? is locked."
|
|
<p> 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).
|
|
|
|
<sect1> "/dev/tty? Device or resource busy" <label id="busy_err">
|
|
<p> 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 (the resource is "busy").
|
|
This message is easy to understand if it only means that the device is
|
|
busy (in use). But it often means that a resource is in use. What
|
|
makes it even more confusing is that in some cases neither the device
|
|
not the resources that it needs are actually "busy".
|
|
|
|
The ``resource busy'' part often means (example for <tt/ttyS2/) ``You
|
|
can't use <tt/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 <tt/ttyS2/ since the setserial data (and kernel data) indicates
|
|
that another device is using <tt/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:
|
|
<enum>
|
|
<item> There may be a real resource conflict that is being avoided.
|
|
<item> Setserial has it wrong and the only reason <tt/ttyS2/ can't be
|
|
used is that setserial erroneously predicts a conflict.
|
|
</enum>
|
|
|
|
What you need to do is to find the interrupt setserial thinks
|
|
<tt/ttyS2/ is using. Look at /proc/tty/driver/serial (if you have
|
|
it). You should also be able to find it with the "setserial" command
|
|
for <tt/ttyS2/. But due to a bug (reported by me in Nov. 2000) you
|
|
get the same "... busy" error message when you try this with
|
|
"setserial".
|
|
|
|
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 <tt/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 conflcit. It's not a good idea to
|
|
leave it permanently set at 0 since it will make the CPU work too
|
|
hard.
|
|
|
|
<sect1> "Input/output error" from setserial or stty
|
|
<p> You may have typed "ttys" instead of "ttyS". You will see this
|
|
error message if you try to use the setserial command for any device
|
|
that is not a serial port. 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 could also mean that there isn't any
|
|
serial port at the IO address that setserial thinks your port is at.
|
|
|
|
<sect1> Overrun errors on serial port
|
|
<p> This is an overrun of the hardware FIFO buffer and you can't
|
|
increase its size. See
|
|
|
|
|
|
|
|
<sect1> Port get characters only sporadically
|
|
<p> There could be some other program running on the port. Use "top"
|
|
(provided you've set it to display the port number) or "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.
|
|
|
|
<sect1> Troubleshooting Tools
|
|
<p> These are some of the programs you might want to use in
|
|
troubleshooting:
|
|
<itemize>
|
|
<item> "lsof /dev/ttyS*" will list serial ports which are open.
|
|
<item> "setserial" shows and sets the low-level hardware configuration
|
|
of a port (what the driver thinks it is). See <ref id="set_serial"
|
|
name="What is Setserial">
|
|
<item> "stty" shows and sets the configuration of a port (except for
|
|
that handled by "setserial").
|
|
See the section <ref id="stty_" name="Stty"><item> "modemstat" or "statserial" will show the current state of
|
|
various modem signal lines (such as DTR, CTS, etc.)
|
|
<item> "irqtune" will give serial port interrupts higher
|
|
priority to improve performance.
|
|
<item> "hdparm" for hard-disk tuning may help some more.
|
|
<item> "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.
|
|
<item> "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
|
|
PnP devices on the ISA bus.
|
|
<item> Some "files" in the /proc tree (such as ioports, interrupts,
|
|
and tty/driver/serial).
|
|
</itemize>
|
|
|
|
<!-- troubleshooting.H end -->
|
|
|
|
|
|
<sect> Interrupt Problem Details <label id="irq_prob_details">
|
|
<p> While the section <ref id="trouble_shoot" name="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.
|
|
|
|
<sect1> Types of interrupt problems
|
|
<p> 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.
|
|
|
|
<sect1> Symptoms of Mis-set or Conflicting Interrupts
|
|
<p> 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 :-)
|
|
|
|
<sect1> Mis-set Interrupts <label id="irq_ng">
|
|
<p> If you don't understand what an interrupt does see <ref
|
|
id="interrupt_" name="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 a 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 many seconds later.
|
|
|
|
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 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.
|
|
|
|
<sect1> Interrupt Conflicts <label id="irq_conflict">
|
|
<p> 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 <ref id="irq_ng" name="Mis-set
|
|
Interrupts"> for more details.
|
|
|
|
<sect1> Resolving Interrupt Problems
|
|
<p> 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 <ref id="what_is_io_irq" name="What is the current IO
|
|
address and IRQ of my Serial Port ?">
|
|
|
|
<sect>What Are UARTs? How Do They Affect Performance? <label id="uart_">
|
|
<sect1> Introduction to UARTS
|
|
<p> UARTs (<BF/U/niversal <BF/A/synchronous <BF/R/eceiver
|
|
<BF/T/ransmitter) 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.
|
|
|
|
The UART's purpose is 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 is 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 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 rebuilds the (parallel) byte and puts it
|
|
in a buffer.
|
|
|
|
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.
|
|
|
|
<sect1> Two Types of UARTs
|
|
<p> 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, 16c552, late 16650, 16750, and 16C950).
|
|
|
|
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.
|
|
|
|
<sect1> FIFOs <label id="fifo_">
|
|
<p> 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.
|
|
|
|
<sect1> Why FIFO Buffers are Small
|
|
|
|
<p>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.
|
|
|
|
<sect1> UART Model Numbers
|
|
<p> Here's a list of UARTs. <em/TL/ is <em/T/rigger <em/L/evel
|
|
<itemize>
|
|
<item> 8250, 16450, early 16550: Obsolete with 1-byte buffers
|
|
<item> 16550, 16550A, 16c552: 16-byte buffers, TL=1,4,8,14
|
|
<item> 16650: 32-byte buffers. Speed up to 460.8 kbps
|
|
<item> 16750: 64-byte buffer for send, 56-byte for receive. Speed up
|
|
to 921.6 kbps
|
|
<item> Hayes ESP: 1k-byte buffers.
|
|
</itemize>
|
|
|
|
The obsolete ones are only good for modems no higher than 14.4k (DTE
|
|
speeds up to 38400 bps). For modern modems you need at least a 16550
|
|
(and not an early 16550). 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 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 ...
|
|
|
|
<sect> Pinout and Signals <label id="pinout_">
|
|
<sect1> Pinout
|
|
<p>
|
|
<tscreen><verb>
|
|
PINOUT of the SERIAL PORT (--> direction is out of PC)
|
|
(Note DCD is sometimes labeled CD)
|
|
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
|
|
</verb></tscreen>
|
|
|
|
<sect1> Signals May Have No Fixed Meaning
|
|
<p> 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).
|
|
|
|
<sect1> Cabling Between Serial Ports <label id="cabling_">
|
|
<p> 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 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.
|
|
|
|
<sect1> RTS/CTS and DTR/DSR Flow Control <label id="rts_cts">
|
|
<p> This is "hardware" flow control. Flow control was previously
|
|
explained in the <ref id="flow_control" name="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:<newline>
|
|
stty crtscts < /dev/ttyS2 (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 <ref id="cabling_" name="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).
|
|
|
|
<sect2> The DTR and DSR Pins
|
|
<p> 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 <ref id="2_term_interfaces" name="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?
|
|
|
|
<sect1> Preventing a Port From Opening
|
|
<p> 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.
|
|
|
|
<sect> Voltage Waveshapes <label id="volt_shape">
|
|
|
|
<sect1> Voltage for a Bit
|
|
<p> At the EIA-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 port
|
|
on a Mac computer as an EIA-232 (requires a special cable) or EIA-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.
|
|
|
|
<sect1> Voltage Sequence for a Byte <label id="byte_seq">
|
|
<p> 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).
|
|
|
|
<sect1> Parity Explained <label id="parity_def">
|
|
<p> 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 <tt/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.
|
|
|
|
<sect1> Forming a Byte (Framing)
|
|
<p> In serial transmission of bytes via EIA-232 ports, the low-order
|
|
bit is always sent first. 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.
|
|
|
|
<sect1> How "Asynchronous" is Synchronized
|
|
|
|
<p> The EIA-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.
|
|
|
|
<sect> Other Serial Devices (not async EIA-232) <label id="non_rs232">
|
|
|
|
<sect1> Successors to EIA-232 <label id="non_232">
|
|
<p> A number of EIA standards have been established for higher speeds
|
|
and longer distances using twisted-pair (balanced) technology.
|
|
Balanced transmission can sometimes be a hundred times faster than
|
|
unbalanced EIA-232. For a given speed, the distance (maximum cable
|
|
length) may be many times longer with twisted pair. But PC-s keep
|
|
being made with the "obsolete" EIA-232 since it works OK with modems
|
|
and mice since the cable length is short. If this appears in the
|
|
latest version of this HOWTO, please let me know if any of the
|
|
non-EIA-232 listed below are supported by Linux.
|
|
|
|
<sect1> EIA-422-A (balanced) and EIA-423-A (unbalanced)
|
|
<p> EIA-423 is just like the unbalanced EIA-232 except that the
|
|
voltage is only 5 volts. Since this falls within EIA-232 specs it
|
|
can be connected to a EIA-232 port. Its specs call for somewhat
|
|
higher speeds than the EIA-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 EIA-232, it is not popular
|
|
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 EIA-232). Apple's Mac computer prior to mid-1998
|
|
with its EIA-232/EIA-422 Port uses it. The Mac used a small round
|
|
"mini-DIN-8" connector. It also provided conventional EIA-232 but at
|
|
only at 5 volts (which is still legal EIA-232). To make it work like
|
|
at EIA-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 <url url="http://www.modemshop.com/csm-comm-faq.html"
|
|
name="Macintosh Communications FAQ">. 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.
|
|
|
|
<sect1> EIA-485
|
|
<p> 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). Since many
|
|
nodes share the same twisted pair the need to use the electrical
|
|
tri-state mode where besides the 0 and 1 binary states there is also
|
|
an open circuit state to permit other nodes to uses 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.
|
|
|
|
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 EIA-232 but
|
|
the slaves must still be EIA-485. See
|
|
<url url="http://www.hw.cz/english/docs/rs485/rs485.html"> for more
|
|
details.
|
|
|
|
<sect1> EIA-530
|
|
<p> EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
|
|
(balanced) was intended to be a replacement for EIA-232 but few have
|
|
been installed. It uses the same 25-pin connector as EIA-232.
|
|
|
|
<sect1> EIA-612/613
|
|
<p> 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.
|
|
|
|
<sect1> The Universal Serial Bus (USB)
|
|
<p> The Universal Serial Bus (USB) is being built into PCI chips.
|
|
Newer PC's have them. It is 12 Mbps (with 200 Mbps planned) 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 attached to it. 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 documentation, see the USB directory in /usr/share/doc/kernel ...
|
|
It would be nice to have a HOWTO on the USB. See also <url
|
|
url="http://www.linux-usb.org"> and/or <url
|
|
url="http://.www.qbik.ch/usb/">.
|
|
|
|
<sect1> Firewire
|
|
<p> 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.
|
|
|
|
<sect1> Synchronization & Synchronous <label id="sync">
|
|
<p> Beside the asynchronous EIA-232 (and others) there are a number of
|
|
synchronous serial port standards. In fact EIA-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.
|
|
|
|
<sect2> Defining Asynchronous vs Synchronous
|
|
|
|
<p> 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.
|
|
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.
|
|
|
|
There are certain situations that need to be classified as either
|
|
sync or async. The async serial port often sends out bytes in a
|
|
steady stream which would make this a synchronous case but since they
|
|
still have the start/stop bits (which makes it possible to send them
|
|
out erratically) it's called async. 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 must be transmitted synchronously.
|
|
|
|
<sect2> Synchronous Communication
|
|
<p> 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 on PC's. There
|
|
are pins for sync timing signals as well as for a sync reverse
|
|
channel. The EIA-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 USART chip or the equivalent where the "S" stands for
|
|
Synchronous. Since sync is a niche market, a sync serial port is
|
|
likely to be quite expensive.
|
|
|
|
Besides the sync part of the EIA-232, there are various other EIA
|
|
synchronous standards. For EIA-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.
|
|
|
|
<sect> Other Sources of Information
|
|
<sect1> Books
|
|
<p>
|
|
<enum>
|
|
<item> Axleson, Jan: Serial Port Complete, Lakeview Research, Madison,
|
|
WI, 1998.
|
|
<item> Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
|
|
Computer Society Press, Los Alamitos, CA, 1996.
|
|
<item> Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
|
|
<item> Campbell, Joe: C Programmer's Guide to Serial Communications,
|
|
2nd ed., Unknown Publisher, 1993.
|
|
<item> <url url="http://www.ora.com/catalog/posix/" name="Levine, Donald:
|
|
POSIX Programmer's Guide">, O'Reilly, 1991.
|
|
<item> Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
|
|
Hungry Minds, 2000.
|
|
<item> Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
|
|
<item> Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall,
|
|
1991.
|
|
<item> <url
|
|
url="http://heg-school.aw.com/cseng/authors/stevens/advanced/advanced.nclk"
|
|
name="Stevens, Richard W.: Advanced Programming in the UNIX Environment">,
|
|
(ISBN 0-201-56317-7; Addison-Wesley)
|
|
<item> Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
|
|
Chapter 7: Serial Ports
|
|
</enum>
|
|
|
|
Notes re books:
|
|
<enum>
|
|
<item>"... Complete" has hardware details (including register) but the
|
|
programming aspect is Window oriented.
|
|
<item>"Physical Layer ..." covers much more than just EIA-232.
|
|
</enum>
|
|
|
|
<sect1> Serial Software
|
|
<p> It's best to use the nearest mirror site, but here's the main
|
|
sites:<newline>
|
|
<url url="ftp://metalab.unc.edu/pub/Linux/system/serial/"
|
|
name="Serial Software"> for Linux software for the serial ports
|
|
including getty and port monitors.<newline>
|
|
<url url="ftp://metalab.unc.edu/pub/Linux/apps/serialcomm"
|
|
name="Serial Communications"> for communication programs.
|
|
|
|
<itemize>
|
|
<item> <tt/irqtune/ will give serial port interrupts higher
|
|
priority to improve performance. Using <tt/hdparm/ for hard-disk tuning
|
|
may help some more.
|
|
|
|
<item> <tt/modemstat/ and <tt/statserial/ show the current state of
|
|
various modem control lines. See <ref id="serial_mon" name="Serial
|
|
Monitoring/Diagnostics">
|
|
</itemize>
|
|
|
|
<sect1> Related Linux Documents
|
|
<p>
|
|
<itemize>
|
|
<item>man pages for: <tt>setserial</tt> and <tt/stty/
|
|
<item> <url url="www.gnu.org/manual/glibc/html_chapter/libc_12.html"
|
|
name="Low-Level Terminal Interface"> part of "GNU C Library Reference
|
|
manual" (in libc (or glibc) docs package). It covers the detailed
|
|
meaning of "stty" commands, etc.
|
|
<item>Modem-HOWTO: modems on the serial port
|
|
<item>PPP-HOWTO: help with PPP (using a modem on the serial port)
|
|
<item>Printing-HOWTO: for setting up a serial printer
|
|
<item>Serial-Programming-HOWTO: for some aspects of serial-port programming
|
|
<item>Text-Terminal-HOWTO: how they work and how to install and configure
|
|
<item>UPS-HOWTO: setting up UPS sensors connected to your serial port
|
|
<item>UUCP-HOWTO: for information on setting up UUCP
|
|
</itemize>
|
|
|
|
<sect1> Usenet newsgroups:
|
|
<p>
|
|
<itemize>
|
|
<item> comp.os.linux.answers
|
|
<item> comp.os.linux.hardware: Hardware compatibility with the Linux
|
|
operating system.
|
|
<item> comp.os.linux.networking: Networking and communications under Linux.
|
|
<item> comp.os.linux.setup: Linux installation and system administration.
|
|
</itemize>
|
|
|
|
<sect1> Serial Mailing List
|
|
<p>
|
|
The Linux serial mailing list. To join, send email to <tt><htmlurl
|
|
url="mailto:majordomo@vger.rutgers.edu"
|
|
name="majordomo@vger.rutgers.edu"></tt>, with ``<tt>subscribe
|
|
linux-serial</tt>'' in the message body. If you send ``<tt/help/'' in
|
|
the message body, you get a help message. The server also serves
|
|
many other Linux lists. Send the ``<tt/lists/'' command for a list
|
|
of mailing lists.
|
|
|
|
<sect1> Internet
|
|
<p>
|
|
<itemize>
|
|
<item> <url url="http://serial.sourceforge.net/" name="Linux Serial
|
|
Driver home page"> Includes info about PCI support.
|
|
<item> <label id="vern_"> <url
|
|
url="ftp://scicom.alphacdc.com/pub/linux" name="Serial Suite"> by
|
|
Vern Hoxie is a collection of blurbs about the care and feeding of
|
|
the Linux serial port plus some simple programs. He also has a
|
|
Serial-Programming-HOWTO (not yet available from the Linux
|
|
Documentation Project). Your browser should automatically log you in
|
|
but if you do it manually login as "anonymous" and use your full
|
|
e-mail address as the password.
|
|
|
|
<item> A white paper discussing serial communications and multiport
|
|
serial boards was available from Cyclades at <tt><htmlurl
|
|
url="http://www.cyclades.com" name="http://www.cyclades.com"></tt>.
|
|
</itemize>
|
|
|
|
<sect> Appendix: Obsolete Hardware (prior to 1990) Info
|
|
|
|
<sect1>Replacing obsolete UARTS
|
|
<p> 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 the chip may unplug so that you may be able to
|
|
upgrade by buying a 16550A chip and replacing your 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 is easy
|
|
(if you can 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 today) or find a used one.
|
|
|
|
<p> END OF Serial-HOWTO
|
|
</article>
|
|
|