184 lines
9.1 KiB
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184 lines
9.1 KiB
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<TITLE> Modem-HOWTO: Serial Port Devices /dev/ttyS4, (or /dev/ttys/4) etc. </TITLE>
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<A HREF="Modem-HOWTO-10.html">Next</A>
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<A HREF="Modem-HOWTO.html#toc9">Contents</A>
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<H2><A NAME="ttySN_"></A> <A NAME="s9">9.</A> <A HREF="Modem-HOWTO.html#toc9">Serial Port Devices /dev/ttyS4, (or /dev/ttys/4) etc. </A></H2>
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<H2><A NAME="ss9.1">9.1</A> <A HREF="Modem-HOWTO.html#toc9.1">Serial Port Names: ttyS4, etc</A>
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</H2>
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<P>Once upon a time the names of the serial ports were simple. Except
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for some multiport serial cards they were named /dev/ttyS0,
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/dev/ttyS1, etc. Then around the year 2000 came the USB bus with
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names like /dev/ttyUSB0 and /dev/ttyACM1 (for the ACM modem on the USB
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bus).</P>
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<H2><A NAME="ss9.2">9.2</A> <A HREF="Modem-HOWTO.html#toc9.2">The PCI Bus</A>
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</H2>
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<P>Since DOS provided for 4 serial ports on the old ISA bus:
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COM1-COM4, ttyS0-ttyS3 most serial ports on the newer PCI bus used
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higher numbers such as ttyS4 or ttyS14 (prior to kernel 2.6.13). But
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since most PCs only came with one or two serial ports, ttyS0 and
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possibly ttyS1 (for the second port) the PCI bus now may use ttyS2
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(kernel 2.6.15 on). All this permits one to have both ISA serial ports
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and PCI serial ports on the same PC with no name conflicts. 0-1 (or
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0-3) are reserved for the old ISA bus (or the newer LPC bus) and
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2-upward (or 4-upward or 14-upward) are used for PCI. It's not
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required to be this way but it often is. On-board serial ports on
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motherboards which have both PCI and ISA slots are likely to still be
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ISA ports. Even for all-PCI-slot motherboards, the serial ports are
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often not PCI. Instead, they are either ISA, on an internal ISA bus
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or on a LPC bus which is intended for slow legacy I/O devices:
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serial/parallel ports and floppy drives.</P>
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<H2><A NAME="ss9.3">9.3</A> <A HREF="Modem-HOWTO.html#toc9.3">Serial Port Device Names & Numbers </A>
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</H2>
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<P>Devices in Linux have major and minor numbers. The serial port
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ttySx (x=0,1,2, etc.) is major number 4. You can see this (and the
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minor numbers too) by typing: "ls -l ttyS*" in the /dev directory. To
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find the device names for various devices, see the "devices" file in
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the kernel documentation.</P>
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<P>There formerly was a "cua" name for each serial port and it behaved
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just a little differently. For example, ttyS2 would correspond to
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cua2. It was mainly used for modems. The cua major number was 5 and
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minor numbers started at 64. You may still have the cua devices in
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your /dev directory but they are now deprecated. For details see
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Modem-HOWTO, section: cua Device Obsolete.</P>
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<P>For creating the old devices in the device directory see:
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the Serial-HOWTO: "Creating Devices In the /dev directory". </P>
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<H2><A NAME="dev_nos"></A> <A NAME="ss9.4">9.4</A> <A HREF="Modem-HOWTO.html#toc9.4">More on Serial Port Names </A>
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</H2>
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<P>Dos/Windows use the COM name while the messages from the serial driver
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use ttyS00, ttyS01, etc. Older serial drivers (2001 ?) used just
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tty00, tty01, etc. </P>
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<P>The tables below shows some examples of serial device names. The
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IO addresses are the default addresses for the old ISA bus (not for
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the newer PCI and USB buses).</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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dos common IO USB-BUS ( ACM => acm modem )
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name name major minor address || common name common name
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COM1 /dev/ttyS0 4, 64; 3F8 || /dev/ttyUSB0 | /dev/ttyACM0
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COM2 /dev/ttyS1 4, 65; 2F8 || /dev/ttyUSB1 | /dev/ttyACM1
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COM3 /dev/ttyS2 4, 66; 3E8 || /dev/ttyUSB2 | /dev/ttyACM2
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COM4 /dev/ttyS3 4, 67; 2E8 || /dev/ttyUSB3 | /dev/ttyACM3
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- /dev/ttyS4 4, 68; various
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss9.5">9.5</A> <A HREF="Modem-HOWTO.html#toc9.5">USB (Universal Serial Bus) Serial Ports</A>
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</H2>
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<P> For more info see the usb subdirectory in the kernel documentation
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directory for files: usb-serial, acm, etc.</P>
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<H2><A NAME="ss9.6">9.6</A> <A HREF="Modem-HOWTO.html#toc9.6">Link ttySN to /dev/modem</A>
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</H2>
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<P> On some installations, two extra devices will be created,
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<CODE>/dev/modem</CODE> for your modem and <CODE>/dev/mouse</CODE> for a
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mouse. Both of these are symbolic links to the appropriate
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device in <CODE>/dev</CODE>.</P>
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<P>Historical note: Formerly (in the 1990s) the use of
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<CODE>/dev/modem</CODE> (as a link to the modem's serial port) was
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discouraged since lock files might not realize that it was really say
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<CODE>/dev/ttyS2</CODE>. The newer lock file system doesn't fall into
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this trap so it's now OK to use such links.</P>
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<H2><A NAME="ss9.7">9.7</A> <A HREF="Modem-HOWTO.html#toc9.7">Devfs (The Improved but Obsolete Device File System)</A>
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</H2>
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<P>Kernel 2.4 introduced the now obsolete optional "device file system"
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(devfs) with a whole new set of names for everything. But in 2003-4,
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it was claimed that devfs had unsolvable problems and starting with
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kernel 2.6.12 it was replaced with "udev" (kernels prior to 2.6.12
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also could use udev but with some problems). Although udev doesn't
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provide all the functionality of devfs, it does handle hot plugging.
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Also, the use of udev isn't required to run Linux so some people don't
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use it. But many distributions install it by default.</P>
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<P>Devfs was a good idea and was claimed to be more efficient than udev.
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But unfortunately, the author of devfs didn't maintain it for long and
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it allegedly became not too well maintained. So for better or worse
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we now have udev instead although the debate of devfs vs. udev still
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continues. For a detailed description of devfs see:
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<A HREF="http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html">http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html</A> Also see
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the kernel documentation tree: filesystems/devfs.</P>
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<P>The names of devices for the devfs can be used in udev, but usually
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are not and may not be simple to activate. Here's the devfs names for
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serial devices: ttyS1 becomes tts/1, ttyUSB1 becomes /usb/tts/1, and
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ttyACM1 is /usb/acm/1. Note that the number 1 above is just an
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example. It could be replaced by 0, 2, 3, 4, etc. Some more examples
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of udev names: ttyS2 becomes tts/2 (Serial port), tty3 becomes vc/3
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(Virtual Console), ptyp1 becomes pty/m1 (PTY master), ttyp2 becomes
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pty/s2 (PTY slave). "tts" looks like a directory which contains
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devices "files": 0, 1, 2, etc. All of these new names should still be
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in the /dev directory although optionally one may put them elsewhere.</P>
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<P>For devfs device names in the /dev directory are created automatically
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by the corresponding driver. Thus, if serial support comes from a
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module and that module isn't loaded yet, there will not be any serial
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devices in the /dev directory. This can be confusing: you physically
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have serial ports but don't see them in the /dev directory. However,
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if a device name is told to a communication program and the serial
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module isn't loaded, the kernel is supposed to try to find a driver
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for it and create a name for it in the /dev directory.</P>
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<P>This works OK if it finds a driver. But suppose there is no driver
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found for it. For example, if you try to use "setserial" to configure
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a port that the driver failed to detect, it claims there is no such
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port. How does one create a devfs port in this case?</P>
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<P>For multiport devices for example, /dev/ttyF9 becomes /dev/ttf/9, or
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in a later version /dev/tts/F9. Substitute for F (or f) whatever
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letter(s) your multiport board uses for this purpose. A multiport
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driver is supposed to create a devfs name similar to the above and put
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it into the /dev directory</P>
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<H2><A NAME="cua_dev"></A> <A NAME="ss9.8">9.8</A> <A HREF="Modem-HOWTO.html#toc9.8">cua Device Obsolete </A>
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</H2>
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<P> Each ttyS device has a corresponding cua device. But the cua
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device is deprecated so it's best to use ttyS (unless cua is
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required). There is a difference between cua and ttyS but a savvy
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programmer can make a ttyS port behave just like a cua port so there
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is no real need for the cua anymore. Except that some older programs
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may need to use the cua.</P>
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<P>What's the difference? The main difference between cua and ttyS has
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to do with what happens in a C-program when an ordinary "open" command
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tries to open the port. If a cua port has been set to check modem
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control signals, the port can be opened even if the CD modem control
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signal says not to. Astute programming (by adding additional lines to
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the program) can force a ttyS port to behave this way also. But a cua
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port can be more easily programmed to open for dialing out on a modem
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even when the modem fails to raise CD (since no one has called into
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it and there's no carrier). That's why cua was once used for dial-out
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and ttyS used for dial-in.</P>
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<P>Starting with Linux kernel 2.2, a warning message is put in the
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kernel log when one uses cua. This is an omen that cua is defunct and
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should be avoided if possible.</P>
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<HR>
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