In the early days of personal computing a parallel printer cable could refer to any of a wide range of cables that connected a computer and a printer. Over time, standards were established or adopted, and a parallel printer cable now consists of a 36-pin plug at one end (Centronics) and 25-pin plug at the other end (DB-25). Cables that connect a parallel printer to a USB plug are also available.
The Centronics Plug
In 1970, Centronics Data Corporation introduced the Model 101 dot matrix printer. Among its innovations was a set of electrical standards for sending parallel data to the printer. Parallel referred to a method of sending data over eight wires simultaneously, while using additional wires for control data. The physical plug was a 36-pin connector common on telecommunications equipment at the time. Centronics was the dominant printer company in the 1970s, and other printer companies adopted the parallel standard, including the plug. Within the computer industry, the electrical standards became known as the Centronics interface, and the plug and cable end became known as the Centronics connector. Though most printers and personal computers used the Centronics interface, the connector used at the computer end of the cable depended on the make of the computer.
The DB-25 Plug
When IBM introduced their Personal Computer in 1981, they used a twenty-five pin printer plug known by its manufacturer’s part number as DB-25. IBM modified the Centronics interface such that their PC could print only to their own printers, which were IBM branded Epson printers. However, the printers still used the common Centronics plug and cable. The popular and widely imitated IBM PC led to the DB-25 plug being adopted as the standard parallel printer plug on personal computers. Other printer manufacturers modified their printers to be able to operate in IBM or Centronics mode.
In 1987, IBM modified their standard to allow bidirectional communication on the existing plug and cables — and by 1988, most computer manufacturers were using the IBM interface — though the Centronics connector name did not change. A smaller version of the connector, known as micro-Centronics, became common during the 1980s. An independent standard for parallel ports and cables was released as IEEE-1284 in 1994. The standard established that printer cables should not exceed 32 feet. With the widespread adoption of USB connections starting in the 1990s, this is now a legacy standard.
Cables Available Today
Some printers for specialized and industrial applications still use a parallel connection. A variety of cables are available to connect these and older printers using either the standard or micro-Centronics plugs at the printer end and the DB-25 plug at the computer end. Extension cables and adapters are available, as well as cables designed to connect multiple parallel printers to a printer switch box. USB adapter cables are also available, making it possible to connect a parallel printer with a Centronics plug to a computer’s USB plug.