PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

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 10 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 10 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. Some older 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-and-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. This chapter describes what
you need to know to use parallel ports efficiently.