SCO Unix ® FAQ section

Serial Communications and UUCP

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

What do the terms UART, 8250, 16450 and 16550 mean?

UART means Universal Asynchronous Receiver/Transmitter. This is a chip which receives and transmits data serially; each serial port you have will use one, though it is possible that several may be integrated into one chip.

8250, 16450 and 16550 are all common types of UARTs. The 8250 is an old chip which cannot run at high speed. The 16450 is similar to the 8250 except that it supports data communications at higher speeds. Both of these chips generate an interrupt for every character that is sent or received, which basically tells the CPU either "Here is some data for you" or "Feed me!" This is all very well, except that at high speed, the number of interrupts (nearly 4000 per port per second at 38 400 bps) can overwhelm a CPU, bringing system performance way down. Also, if the CPU is busy servicing another interrupt at the time, the serial port's interrupt may not be serviced in time, which will cause a character to be lost.

The 16550 is pin-compatible with the 16450 and, by default, runs in 16450 mode. This makes it compatible with software which is not 16550-aware. If your software is 16550-aware, it can turn on a special mode in which the 16550 buffers all data with 16-byte internal buffers. This not only allows the CPU to deal with far more bytes at a time, increasing efficiency, but also means that if the CPU can't service the interrupt before the next character comes in, there's still space in the buffer for it.

16550 support was introduced in Xenix 2.3.4, ODT 1.1 and Unix 3.2.2. If you have these, or later, versions, your operating system will automatically detect 16550-equipped ports and will enable their buffering. A third-party serial driver called FAS includes 16550 awareness in its feature set; you may wish to investigate this as well. FAS can be found at ftp://ftp.fu-berlin.de/pub/unix/driver/fas/.

Note that the above is not really applicable to intelligent multiport serial cards. While these cards may well use 16550s, it is the processor on the serial card which is responsible for dealing with the serial ports it controls, and the main CPU has nothing to do with the UARTs.

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