Parallel port hardware may be of five types, described below in the order of their appearance in PCs. A computer may contain any of these port types, and may include ports of more than one type. Earlier ports are limited in functionality and performance. Later ports provide increased functionality and performance, and may often be configured to emulate earlier port types when necessary to support older peripherals.
unidirectional 4-bit parallel port, also
standard parallel port (SPP), is based
on the defacto Centronics standard, and was the
type of parallel port supplied with the original IBM PC and its
clones. These ports are misnamed, as they are not unidirectional and
are not limited to 4-bit transfers. An SPP does 8-bit output and can
accept 4-bit (nibble) input.
In theory, these ports are limited to using a two-meter (about six-foot) cable, but this distance can be extended to three to five meters (10 to 16 feet) by using a high-grade parallel cable. Unidirectional 4-bit parallel ports are commonly found in older desktop and laptop systems, and are still supplied on some low-end I/O cards. These ports provide native throughput of 40 to 60 KB/s, although certain design tricks can push this to the 150 KB/s range.
When IBM introduced the PS/2 line in 1987, all but the two
lowest-cost models (the Models 25 and 30) included a
bidirectional 8-bit parallel port. Initially, these were ...