A parallel interface refers to a multiline channel, each line capable of transmitting several bits of data simultaneously. Most commonly, personal computers (PCs) have at least one parallel interface for connecting a printer using a parallel port.
In contrast, a "serial interface" uses a serial port, a single line capable of only transmitting one bit of data at a time; a computer mouse connection is a good example.
The first parallel interface was the Centronics parallel interface developed and used in the Centronics 101 model printer in 1970. This became the standard; but a variety of cables were required. Dataproducts and other manufacturers created up to 50-pin connectors. By 1981 IBM introduced their personal computers with printer connections using a cable with a DB25F 25-pin connector on the PC end and a 36-pin Centronics connector on the printer end. In 1987 IBM introduced a bidirectional parallel interface; and by 1992 Hewlett-Packard introduced their version, called “Bitronics,” with its LaserJet 4. These were both superseded by the IEEE 1284 parallel interface standard in 1994.
The IEEE 1284 standard specified five modes of operations, each specifying a direction of data flow, i.e. toward or away from the computer or bi-directional. These are:
Compatibility Mode: This is the original Centronics parallel interface.
Nibble Mode: This allowed data transfer back to the computer.
Byte Mode: This allows data to be sent back to the computer at the same speed that data is sent from the computer to the printer or other device.
ECP Mode: This stands for “enhanced capability port” and allows bidirectional data flow for printers and scanners.
EPP Mode: This uses data cycles to quickly transfer data in both directions at speeds of 500 kilobytes to 2 megabytes per second.
The mode used is determined by a sequence of events known as “negotiation” and is dependent upon the modes each connected device can handle.
One of the latest parallel interface technologies is known as “high-performance parallel interface” or HIPPI. It is used for transferring billions of bits of data per second over short distances on local area networks (LANs). By interconnecting computers and network storage devices, this technology has been described as functioning as a supercomputer; one company used the term “SuperLAN.” The fastest data transfer rates are 6.4 Gbps (gigabytes per second) for distances up to 1 kilometer.