Chapter 23. Parallel Communications

While a serial interface communicates one bit at a time using one data line, a parallel interface communicates one byte at a time using eight data lines. This allows a parallel port to transfer data five or ten times faster than a traditional serial port, but the additional complexity of keeping a full byte synchronized during each cycle also means that a traditional parallel cable cannot be longer than six to ten feet, versus 50 feet or more for serial.

Nearly every PC has at least one parallel port, which may reside on the motherboard or on a video or I/O card. Many computers have two or three parallel ports installed, usually because expansion cards were added that just happened to have parallel ports on them. Fortunately, PCs are pretty smart about detecting parallel ports at boot time and avoiding conflicts.


The so-called “legacy-reduced” motherboards and systems that began coming to market in late 1999 may or may not provide parallel ports. “Legacy-free” systems and motherboards began shipping in volume in mid-2000, and do not provide parallel ports (or many other formerly standard connections, such as serial ports, PS/2 mouse and keyboard ports, a floppy diskette drive interface, etc.). These systems depend entirely on USB for external connectivity. If you need to connect a legacy parallel device such as a printer to such a system, there are two options. First, you can install a PCI parallel card, which typically also includes one or ...

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