12. Serial Tips And Miscellany

12.1 Serial Module

Often the serial driver is provided as a module. Parameters may be supplied to certain modules in /etc/modules.conf. Since kernel 2.2 you don't edit this file but use the program update-modules to change it. The info that is used to update modules.conf is put in /etc/modutils/. The Debian/GNU Linux has a file here named /etc/modutils/setserial which runs the serial script in /etc/init.d/ every time the serial module is loaded or unloaded. When the serial module is unloaded this script will save the state of the module in /var/run/setserial.conf. Then if the module loads again this saved state is restored. When the serial module first loads at boot-time, there's nothing in /var/run/setserial.conf so the state is obtained from /etc/serial.conf. So there are two files that save the state. Other distributions may do something similar.

One may modify the serial driver by editing the source code. Much of the serial driver is found in the file serial.c. For info regarding writing of programs for the serial port see Serial-Programming-HOWTO (revised in 1999 by Vern Hoxie but not at LDP. Get it from scicom.alphacdc.com/pub/linux)

12.2 Serial Console (console on the serial port)

See the kernel documentation in: Documentation/serial-console.txt. See also "Serial Console" in Text-Terminal-HOWTO.

12.3 Line Drivers

For a text terminal, the EIA-232 speeds are fast enough but the usable cable length is often too short. Balanced technology could fix this. The common method of obtaining balanced communication with a text terminal is to install 2@ line drivers in the serial line to convert unbalanced to balanced (and conversely). They are a specialty item and are expensive if purchased new.

12.4 Known Defective Hardware

Avoiding IO Address Conflicts with Certain Video Boards

The IO address of the IBM 8514 video board (and others) is allegedly 0x?2e8 where ? is 2, 4, 8, or 9. This may conflict (but shouldn't if the serial port is well designed) with the IO address of ttyS3 at 0x02e8 if the serial port ignores the leading 0 hex digit when it decodes the address (many do). That is bad news if you try to use ttyS3 at this IO address. Another story is that Linux will not detect your internal modem on ttyS3 but that you can use setserial to put ttyS3 at this address and the modem will work fine.

Problem with AMD Elan SC400 CPU (PC-on-a-chip)

This has a race condition between an interrupt and a status register of the UART. An interrupt is issued when the UART transmitter finishes the transmission of a byte and the UART transmit buffer becomes empty (waiting for the next byte). But a status register of the UART doesn't get updated fast enough to reflect this. As a result, the interrupt service routine rapidly checks and determines (erroneously) that nothing has happened. Thus no byte is sent to the port to be transmitted and the UART transmitter waits in vain for a byte that never arrives. If the interrupt service routine had waited just a bit longer before checking the status register, then it would have been updated to reflect the true state and all would be OK.

There is a proposal to fix this by patching the serial driver. But Should linux be patched to accommodate defective hardware, especially if this patch may impair performance of good hardware?