PC Hardware in a Nutshell, Second Edition

Serial Ports

PC serial ports, also known as communications ports, comm ports, asynchronous ports, or async ports, connect slow bit-oriented peripherals like modems, mice, printers, and plotters to the fast, byte-oriented system bus. Serial ports may reside on motherboards or expansion cards, or be embedded on devices like internal modems. PC serial ports haven’t changed much over the years, although they’re faster now and have larger buffers.

Serial ports were formerly used to connect almost anything to a PC—modems, mice, printers, plotters, etc. Nowadays, serial ports are used mostly to connect modems and other peripherals (like the Palm cradle) that do not require high- speed communications. Although serial ports are obsolescent and will eventually be replaced by USB ports, nearly any motherboard or PC you buy today will have one or two serial ports, although one or both may exist only as a group of header pins on the motherboard, rather than as visible ports on the rear panel connector. For now at least, the serial port is sometimes the best (or only) way to get the job done. The following sections describe what you need to know to use serial ports effectively.

Tip

The so-called “legacy-reduced” motherboards and systems that began coming to market in late 1999 may or may not provide serial ports. “Legacy-free” systems and motherboards began shipping in volume in mid-2000, and do not provide serial ports (or many other formerly standard connections, such as parallel ports, PS/2 ...