The Linux Serial HOWTO: What Are The Names Of The Serial Ports?

3. What Are The Names Of The Serial Ports?

An I/O port is a way to get data into and out of a computer. There are many types of I/O ports such as serial ports, parallel ports, disk drive controllers, ethernet boards, etc. We will be dealing with serial ports since modems and terminals are serial devices. Each serial port must have an I/O address, and an interrupt (IRQ). There are the four serial ports corresponding to COM1 - COM4:


ttyS0 (COM1) address 0x3f8 IRQ 4
ttyS1 (COM2) address 0x2f8 IRQ 3
ttyS2 (COM3) address 0x3e8 IRQ 4
ttyS3 (COM4) address 0x2e8 IRQ 3

If Linux does not detect any serial ports when it boots, then make sure that serial support is enabled and compiled into the kernel. In this document, I refer to COM1 as ttyS0, COM2 as ttyS1, COM3 as ttyS2, and COM4 as ttyS3. Notice that by default these devices have overlapping IRQs. You cannot use all of the ports in this default configuration, and you must reassign different IRQs. See section Can I Use More Than Two Serial Devices? on setting IRQs.

3.1 Devices: modem, mouse

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. I strongly discourage the use of these links. 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. Use them if you like, but be sure they point to the right device. However, if you change or remove this link, some applications (minicom for example) might need reconfiguration.

3.2 The cua Device

Each ttyS device has a corresponding cua device. There has been some talk about abolishing cua so perhaps it's best to use ttyS. The main difference between cua and ttyS is that for an ordinary "open" command (without certain flags) in an application program, cua will open the port even if the modem control signals (such as DCD) say not to. (stty must have been set to check modem control signals.) A ttyS port would refuse to open in such a case, but it can be forced to open by giving a certain flag to the "open" command.

Thus a ttyS port can do everything a cua port can. Except that a "forced" open results in read statements in the program behaving in a certain way but the programmer may prevent this by adding some other statements to the program. Eliminating cua would brings Linux more into compliance with the Posix standard and avoids certain problems with lock files.

3.3 Serial Port Devices and Numbers In `/dev`


/dev/ttyS0 major 4, minor 64    /dev/cua0 major 5, minor 64
/dev/ttyS1 major 4, minor 65    /dev/cua1 major 5, minor 65
/dev/ttyS2 major 4, minor 66    /dev/cua2 major 5, minor 66
/dev/ttyS3 major 4, minor 67    /dev/cua3 major 5, minor 67

Note that all distributions should come with these devices already made correctly (unless cua is abolished). You can verify this by typing:


linux% ls -l /dev/cua*
linux% ls -l /dev/ttyS*

Creating Devices In `/dev`

If you don't have a device, you will have to create it with the mknod command. Example, suppose you needed to create devices for ttyS0:


linux# mknod -m 666 /dev/cua0 c 5 64
linux# mknod -m 666 /dev/ttyS0 c 4 64

You can use the MAKEDEV script, which lives in /dev. This simplifies the making of devices. For example, if you needed to make the devices for ttyS0 you would type:


linux# cd /dev
linux# ./MAKEDEV ttyS0

This handles the devices creation for the incoming and outgoing devices, and should set the correct permissions.

3.4 Notes For Dumb Multiport Boards

The devices your multiport board uses depends on what kind of board you have. Some of these are listed in detail in rc.serial or in 0setserial which comes with the setserial package. I highly recommend getting the latest version of setserial if you are trying to use multiport boards. You will probably need to create these devices. Either use the mknod command, or the MAKEDEV script. Devices for multiport boards are made by adding ``64 + port number''. So, if you wanted to create devices for ttyS17, you would type:


linux# mknod -m 666 /dev/cua17 c 5 81
linux# mknod -m 666 /dev/ttyS17 c 4 81

Note that ``64 + 17 = 81''. Using the MAKEDEV script, you would type:


linux# cd /dev
linux# ./MAKEDEV ttyS17

Note: the SIIG manual for the IO1812 listing for COM5-COM8 is wrong. They should be COM5=0x250, COM6=0x258, COM7=0x260, and COM8=0x268.

Note: the Digi PC/8 Interrupt Status Register is at 0x140.

Note: for an AST Fourport, you might need to specify skip_test in rc.serial.

3.5 Notes For Intelligent Multiport Boards

Read the information that comes with the driver. These boards use special devices, and not the standard ones. This information varies depending on your hardware.