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Once upon a time the names of the serial ports were simple. Except
for some multiport serial cards they were named /dev/ttyS0,
/dev/ttyS1, etc. Then around the year 2000 came the USB bus with
names like /dev/ttyUSB0 and /dev/ttyACM1 (for the ACM modem on the USB
bus).
A little later with kernel 2.4 came the "device file system" (devfs)
with a whole new set of names for everything. The use of the device
file system was optional and many still continued using the old
system. Then in 2003-4, it was claimed that devfs had unsolvable
problems and will be replaced with what's called "udev". Although
udev doesn't provide all the functionality of devfs it does handle hot
plugging. With all this confusion, most distributions use neither
devfs nor udev.
If you use devfs or udev, ttyS1 becomes tts/1, ttyUSB1 becomes
/usb/tts/1, and ttyACM1 is /usb/acm/1. Note that the number 1
above is just an example. It could be replaced by 0, 2, 3, 4, etc.
One may use devfs but have the conventional names linked (via symlinks)
to the new names. So they use the new system with the old names but
may also use some of the new names for some devices. It's even
possible ?? to use the new names for the old (non-devfs) system.
Since DOS provided for 4 serial ports on the old ISA bus:
COM1-COM4, ttyS0-ttyS3 (tts/0-tts/3) most serial ports on the newer
PCI bus use higher numbers such as ttyS4 (tts/4) or ttyS14 (tts/14)
for kernel 2.6. This permits one to have both ISA serial ports and
PCI serial ports on the same PC with no name conflicts. 0-3 are
reserved for the old ISA bus and 4-upward (or 14-upward) are used for
PCI. It's not required to be this way but it often is. On-board
serial ports on motherboards which have both PCI and ISA slots are
likely to still be ISA ports. Even for all-PCI-slot motherboards, the
serial ports are often not PCI. They are either ISA, on an internal
ISA bus or on a LPC bus which is intended for slow legacy I/O devices:
serial/parallel ports and floppy drives.
In kernel 2.4 the devfs was created only to be obsoleted in favor
of udev in kernel 2.6. devfs created a new system of device naming
which was continued with udev. The naming system makes it easier to
deal with a huge number of devices. But there's also a popular option
to continue using the old names. However, a new device may not have
an old-style name so then one must use the new name. For a detailed
description of devfs see:
http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html Also see
the kernel documentation tree: filesystems/devfs.
Some more examples of devfs names: ttyS2 becomes tts/2 (Serial
port), tty3 becomes vc/3 (Virtual Console), ptyp1 becomes pty/m1 (PTY
master), ttyp2 becomes pty/s2 (PTY slave). "tts" looks like a
directory which contains devices "files": 0, 1, 2, etc. All of these
new names should still be in the /dev directory although optionally
one may put them elsewhere.
For devfs, device names in the /dev directory are created
automatically by the corresponding driver. Thus, if serial support
comes from a module and that module isn't loaded yet, there will not
be any serial devices in the /dev directory. This can be confusing:
you physically have serial ports but don't see them in the /dev
directory. However, if a device name is told to a communication
program and the serial module isn't loaded, the kernel is supposed to
try to find a driver for it and create a name for it in the /dev
directory.
This works OK if it finds a driver. But suppose there is no driver
found for it. For example, if you try to use "setserial" to configure
a port that the driver failed to detect, it claims there is no such
port. How does one create a devfs port in this case?
Before the device file system, devices in Linux had major and
minor numbers. The serial port ttySx (x=0,1,2, etc.) was major number
4. You could see this (and the minor numbers too) by typing: "ls -l
ttyS*" in the /dev directory. To find the old device names for various
devices, see the "devices" file in the kernel documentation.
There formerly was a "cua" name for each serial port and it behaved
just a little differently. For example, ttyS2 would correspond to
cua2. It was mainly used for modems. The cua major number was 5 and
minor numbers started at 64. You may still have the cua devices in
your /dev directory but they are now deprecated. For details see
Modem-HOWTO, section: cua Device Obsolete.
For creating the old devices in the device directory see:
Creating Devices In the /dev directory
Dos/Windows use the COM name while the messages from the serial driver
use ttyS00, ttyS01, etc. Older serial drivers (2001 ?) used just
tty00, tty01, etc.
The tables below shows some examples of serial device names. The
IO addresses are the default addresses for the old ISA bus (not for
the newer PCI and USB buses). The major/minor numbers aren't needed
for the devfs, but they often exist anyway just in case the devfs
method of locating drivers can't be used.
dos devfs common IO
name name name major minor address
COM1 /dev/tts/0 /dev/ttyS0 4, 64; 3F8
COM2 /dev/tts/1 /dev/ttyS1 4, 65; 2F8
COM3 /dev/tts/2 /dev/ttyS2 4, 66; 3E8
COM4 /dev/tts/3 /dev/ttyS3 4, 67; 2E8
- /dev/tts/4 /dev/ttyS4 4, 68; various
DEVICES-ON-THE-USB-BUS (acm is a certain type of modem)
devfs common name devfs common name
/dev/usb/tts/0 /dev/ttyUSB0 | /dev/usb/acm/0 /dev/ttyACM0
/dev/usb/tts/1 /dev/ttyUSB1 | /dev/usb/acm/1 /dev/ttyACM1
/dev/usb/tts/2 /dev/ttyUSB2 | /dev/usb/acm/2 /dev/ttyACM2
/dev/usb/tts/3 /dev/ttyUSB3 | /dev/usb/acm/3 /dev/ttyACM3
For more info see the usb subdirectory in the kernel documentation
directory for files: usb-serial, acm, etc.
On some installations, two extra devices will be created,
/dev/modem for your modem and /dev/mouse for a
mouse. Both of these are symbolic links to the appropriate serial
device in /dev which you specified during the installation.
Except if you have a bus mouse, then /dev/mouse will point to
the bus mouse device).
Historical note: Formerly (in the 1990s) the use of
/dev/modem was discouraged since lock files might not realize
that it was really say /dev/ttyS2 . The newer lock file
system doesn't fall into this trap so it's now OK to use such links.
Inspect the connectors
Inspecting the connectors may give some clues but is often not
definitive. The serial connectors on the back side of a PC are
usually DB connectors with male pins. 9-pin is the most common but
some are 25-pin (especially older PCs like 486s). There may be one
9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??). For two
9-pin ones the top one might be ttyS0.
If you only have one serial port connector on the back of your PC,
this may be easy. If you also have an internal modem, a program like
wvdial may be able to tell you what port it's on (unless it's a PnP
that hasn't been enabled yet). A report from setserial (at
boot-time or run by you from the command line) should help you
identify the non-modem ports.
If you have two serial ports it may be more difficult. You could have
only one serial connector but actually have 2 ports, one of which
isn't used (but it's still there electronically). First check manuals
(if any) for your computer. Look at the connectors for meaningful
labels. You might even want to take off the PC's cover and see if
there are any meaningful labels on the card where the internal ribbon
serial cables plug in. Labels (if any) are likely to say something like
"serial 1", "serial 2" or A, B. Which com port it actually is will
depend on jumper or PnP settings (sometimes shown in a BIOS setup
menu). But 1 or A are more likely to be ttyS0 with 2 or B ttyS1.
Send bytes to the port
Labels are not apt to be definitive so here's another method. If
the serial ports have been configured correctly per setserial, then
you may send some bytes out a port and try to detect which connector
(if any) they are coming out of. One way to send such a signal is to
copy a long text file to the port using a command like: cp
my_file_name /dev/ttyS1. A voltmeter connected to the DTR pin (see
Serial-HOWTO for Pinout) will display a positive voltage as soon as
you give the copy command.
The transmit pin should go from several volts negative to a voltage
fluctuating around zero after you start sending the bytes. If it doesn't
(but the DTR went positive) then you've got the right port but it's
blocked from sending. This may be due to a wrong IRQ, -clocal being
set, etc. The command "stty -F /dev/ttyS1 -a " should show
clocal (and not -clocal). If not, change it to clocal.
Another test is to jumper the transmit and receive pins (pins 2 and 3
of either the 25-pin or 9-pin connector) of a test serial port. Then
send something to each port (from the PCs keyboard) and see if it gets
sent back. If it does it's likely the port with the jumper on it.
Then remove the jumper and verify that nothing gets sent back. Note
that if "echo" is set (per stty) then a jumper creates an infinite
loop. Bytes that pass thru the jumper go into the port and come right
back out of the other pin back to the jumper. Then they go back in
and out again and again. Whatever you send to the port repeats itself
forever (until you interrupt it by removing the jumper, etc.). This
may be a good way to test it as the repeating test messages halt when
the jumper is removed.
As a jumper you could use a mini (or micro) jumper cable (sold in some
electronic parts stores) with mini alligator clips. A small scrap of
paper may be used to prevent the mini clips from making electrical
contact where it shouldn't. Metal paper clips can sometimes be bent
to use as jumpers. Whatever you use as a jumper take care not to bend
or excessively scratch the pins. To receive something from a port,
you can go to a virtual terminal (for example Alt-F2 and login) and
type something like "cp /dev/ttyS2 /dev/tty". Then at another virtual
terminal you may send something to ttyS2 (or whatever) by "echo
test_message > /dev/ttyS2". Then go back to the receive virtual
terminal and look for the test_message. See
Serial Electrical Test Equipment for more info.
Connect a device to the connector
Another way to try to identify a serial port is to connect some
physical serial device to it and see if it works. But a problem here
is that it might not work because it's not configured right. A serial
mouse might get detected at boot-time if connected.
You may put a device, such as a serial mouse (use 1200 baud), on a port
and then use minicom to communicate with that port. Then by clicking
on the mouse, or otherwise sending characters with the device, see if
minicom displays them. It not reconfigure minicom to use the other
serial port, but keep the device connected to the same port.
Missing connectors
If the software shows that you have more serial ports than you
have connectors for (including an internal modem which counts as a
serial port) then you may have a serial port that has no connector.
Some motherboards come with a serial port with no cable or external
serial DB connector. Someone may build a PC from this and decide not
to use this serial port. There may be a "serial" connector and label
on the motherboard but no ribbon cable connects to its pins. To use
this port you must get a ribbon cable and connector. I've seen
different wiring arrangements for such ribbon cables so beware.
If you don't use devfs (which automatically creates such devices) and
don't have a device "file" that you need, you will have to create it.
Use the mknod command or with the MAKEDEV shell script.
Example, suppose you needed to create ttyS0 :
linux# mknod -m 666 /dev/ttyS0 c 4 64
The MAKEDEV script is easier to use.
See the man page for it. For example, if you needed to make the
device for ttyS0 you would just type:
linux# MAKEDEV ttyS0
If the above command doesn't work (and you are the root user), look
for the MAKEDEV script in the /dev directory and run it.
This handles the devices creation and should set the correct permissions.
For making multiport devices see
Making multiport devices in the /dev directory.
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