PC Parallel Port

The parallel port was for many years a standard interface on personal computers, used mainly to connect printers. It is called parallel because it transmits several bits of data at the same time over separate wires, in contrast to a serial port, which sends bits one after another. The connector and signalling were based on an arrangement introduced by the printer maker Centronics, so the interface is often called a Centronics port.

Because early implementations varied, the interface was later formalised in the IEEE 1284 standard, which defined the connectors, signals and several modes of operation so that equipment from different makers would work together. The original, simplest behaviour is the Standard Parallel Port mode, or SPP, which sends data mainly in one direction toward the printer. This is adequate for basic printing but does not allow fast communication back from the device.

To improve speed and allow two-way communication, the standard added enhanced modes. The Enhanced Parallel Port, or EPP, supports faster bidirectional transfers and was useful for devices such as external drives and scanners. The Extended Capabilities Port, or ECP, adds features such as data buffering and compression aimed at printers and scanners. Over time, the parallel port was largely replaced by USB, which is faster, supports many device types and uses smaller connectors, so modern computers rarely include a parallel port.

Frequently asked questions

Why is it called a parallel port?

Because it sends several bits at the same time over separate wires, unlike a serial port that sends bits one after another.

What is the IEEE 1284 standard?

It is the standard that formally defined the parallel port’s connectors, signals and operating modes so equipment from different manufacturers would be compatible.

What is the difference between SPP, EPP and ECP?

SPP is the basic, mostly one-way mode, while EPP and ECP add faster bidirectional transfer and extra features such as buffering for devices like scanners.