For creating devices in the device directory see the Serial-HOWTO: "Creating Devices In the /dev directory".
Devices in Linux have major and minor numbers. Each serial port may have 2 possible names in the /dev directory: ttyS and cua. Their drivers behave slightly differently. The cua device is deprecated and will not be used in the future. See The cua Device.
Dos/Windows use the COM name while the 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).
IO
dos major minor major minor address
COM1 /dev/ttyS0 4, 64; /dev/cua0 5, 64 3F8
COM2 /dev/ttyS1 4, 65; /dev/cua1 5, 65 2F8
COM3 /dev/ttyS2 4, 66; /dev/cua2 5, 66 3E8
COM4 /dev/ttyS3 4, 67; /dev/cua3 5, 67 2E8
linux% ls -l /dev/cua*
linux% ls -l /dev/ttyS*
On some installations, two extra devices will be created,
/dev/modem for your modem and /dev/mouse for your
mouse. Both of these are symbolic links to the appropriate
device in /dev which you specified during the
installation (unless you have a bus mouse, then /dev/mouse
will point to the bus mouse device).
There has been some discussion on the merits of /dev/mouse
and /dev/modem. The use of these links is discouraged. In
particular, if you are planning on using your modem for dialin you may
run into problems because the lock files may not work correctly if you
use /dev/modem. However, if you change or remove this
link, some applications might need reconfiguration.
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 DCD 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 DCD (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.