A brief history of parallel ports

When computers started to conquer small offices and homes in the early 1980's, they were equipped with a port that would let you connect to either a printer or plotter. This "Centronics" port is what we now call a unidirectional parallel port or a Standard Parallel Port (SPP).

It didn't take long for people to see that a parallel port could transfer data faster than a serial port: theoretically 5 times faster.

But there was a bottle-neck caused by the design of the port. The SPP provided only 4 wires with which to transfer data. Parallel port file transfer was faster than serial transfer but the extra speed didn't justify the extra cost.

Later, when IBM introduced the PS/2 line of computers, they also introduced a new standard called "bi-directional". The new port had 8 data lines with which to transfer data and transfer speed increased to 150 Kbytes/sec and more.

Many companies redesigned their products to take advantage of the parallel port's speed. Such devices as hard disks, scanners, CD-ROM drives, CCD cameras and even networks appeared in rapid succession.

Three companies (Intel, Zenith and Xircom) created a new standard called EPP (Enhanced Parallel Port), which increased the transfer speed of the port to 1000 Kbytes/sec.

Finally, Microsoft introduced another standard called ECP (Enhanced Capability Port) and the transfer rate jumped to 2000 Kbytes/sec. Unfortunately all these different protocols are only partially compatible with each other. It's a matter of "plug and pray", i.e., try it and hope that it works for you and your hardware.