7. Serial Port Devices /dev/ttyS2, etc.

For creating devices in the device directory see

Creating Devices In the /dev directory

7.1 Serial Port Device Names & Numbers

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 Modem-HOWTO section: "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*

7.2 Link ttySN to /dev/modem ?

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.

7.3 Which Connector on the Back of my PC is ttyS1, etc?

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 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.

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) 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 "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 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 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 if connected.

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 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.

7.4 Creating Devices In the /dev directory

If you don't have a device "file" that you need, you will have to create it with 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

linux# cd /dev
linux# ./MAKEDEV ttyS0