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
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 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 creased 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
This is 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:
the Serial-HOWTO: "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
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.
Each ttyS device has a corresponding cua device. But the cua
device is deprecated so it's best to use ttyS (unless cua is
required). There is a difference between cua and ttyS but a savvy
programmer can make a ttyS port behave just like a cua port so there
is no real need for the cua anymore. Except that some older programs
may need to use the cua.
What's the difference? The main difference between cua and ttyS has
to do with what happens in a C-program when an ordinary "open" command
tries to open the port. If a cua port has been set to check modem
control signals, the port can be opened even if the CD modem control
signal says not to. Astute programming (by adding additional lines to
the program) can force a ttyS port to behave this way also. But a cua
port can be more easily programmed to open for dialing out on a modem
even when the modem fails to assert CD (since no one has called into
it and there's no carrier). That's why cua was once used for dial-out
and ttyS used for dial-in.
Starting with Linux kernel 2.2, a warning message is put in the
kernel log when one uses cua. This is an omen that cua is defunct and
should be avoided if possible.