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 two high-speed serial ports. Otherwise, you can buy a USB-to-parallel converter, various models of which are now widely available from Belkin and others.

Probably because it is so commonplace and so unlikely to cause problems, nearly everyone takes the lowly parallel port for granted. Unlike the serial port, the parallel port is so completely standardized that you can use it without thinking about it. In fact, a good argument could be made for the parallel port as the predecessor of Plug-N-Play. You can mix and match new and old ports, cables, and printers in any combination, and everything usually just works. Therein lies the problem.

The parallel port is a victim of its own standardization and backward compatibility. Parallel ports, peripherals, and even cables have undergone significant improvements over the years, but those improvements have been so low-key as to be almost invisible. Most people aren't even aware that they've taken place. So, although any old combination of parallel components will probably work, it may be doing so at a least-common-denominator level, crippling performance and functionality.

Something as simple as using an old printer cable to connect a new PC to a new printer may choke throughput. Even worse, you may use all new hardware and still get low performance or limited functionality just because you didn't know that you needed to make a minor change in the BIOS Setup for the parallel port. The following sections describe what you need to know to use parallel ports efficiently.