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2.2. Select a serial port
2.2.1. Serial port names
Linux names its serial ports in the UNIX tradition. The first serial port has the file name /dev/ttyS0 , the second serial port has the file name /dev/ttyS1 , and so on.
This differs from the IBM PC tradition. The first serial port is named COM1: , the second serial port is named COM2: , and so on. Up to four serial ports can be present on a IBM PC/AT computer and its successors.
Most boot loaders have yet another naming scheme. The first serial port is numbered 0 , the second serial port is numbered 1 , and so on.
If your distribution of Linux uses the devfs device manager then the serial ports have yet another name. The first serial port is /dev/TT> , the second serial port is /dev/tts/1 , and so on.
The result is that the first serial port is labeled COM1: on the chassis of the IBM PC ; is known as /dev/ttyS0 to Linux ; is known as /dev/tts/0 to Linux when configured with devfs ; and is known as port 0 to many boot loaders.
The examples in this HOWTO use this first serial port, as that is the serial port which most readers will wish to use.
Table 2-1. Many names for the same serial port
| IBM PC | Linux kernel | Linux kernel with devfs | Most boot loaders |
|---|---|---|---|
| COM1: | /dev/ttyS0 | /dev/tts/0 | 0 |
| COM2: | /dev/ttyS1 | /dev/tts/1 | 1 |
| COM3: | /dev/ttyS2 | /dev/tts/2 | 2 |
| COM4: | /dev/ttyS3 | /dev/tts/3 | 3 |
2.2.2. Cannot share interrupt used for console's serial port
When used for a console the serial port cannot share an interrupt with another device. The IBM PC devices are usually installed as shown in Table 2-2 . If you use the serial port /dev/ttyS0 for the console then you should avoid sharing interrupt 4 by not installing a serial port /dev/ttyS2 in your PC . If /dev/ttyS2 cannot be physically removed then disable it using the setserial command, as shown in Figure 2-1 .
Table 2-2. Interrupts used for IBM PC/AT RS-232 ports
| Device | Interrupt | Port |
|---|---|---|
| /dev/ttyS0 | 4 | 0x3f8 |
| /dev/ttyS1 | 3 | 0x2f8 |
| /dev/ttyS2 | 4 | 0x3e8 |
| /dev/ttyS3 | 3 | 0x2e8 |
Figure 2-1. Using the setserial command in /etc/rc.serial to disable the serial port /dev/ttyS2
# Disable /dev/ttyS2 so interrupt 4 is not shared, # then /dev/ttyS0 can be used as a serial console. setserial /dev/ttyS2 uart none port 0x0 irq 0 |
Reading the source code suggests that the interrupt-sharing constraint applies to all computer architectures, not just Intel Architecture-32.
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