Parallel Port Types
Parallel port hardware may be of five types, described next 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
The unidirectional 4-bit parallel port, also called a Standard Parallel Port (SPP), is based on the de facto 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 2-meter (about 6 foot) cable, but this distance can be extended to 3 to 5 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.
- Bidirectional 8-bit parallel port
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