Parallel port

A parallel port is a type of interface found on computers (personal and otherwise) for connecting various peripherals. It is also known as a printer port or Centronics port. The IEEE 1284 standard defines the bi-directional version of the port.

History

The Centronics Model 101 printer was introduced in 1970 and included the first parallel interface for printers.cite book |author=Webster, Edward C. |title=Print Unchained: Fifty Years of Digital Printing: A Saga of Invention and Enterprise |publisher=DRA of Vermont |location=West Dover, VT |year=2000 |isbn=0-9702617-0-5] The interface was developed by Dr. An Wang, Robert Howard and Prentice Robinson at Wang Laboratories. The now-familiar connector was selected because Wang had a surplus stock of 20,000 Amphenol 36-pin micro ribbon connectors that were originally used for one of their early calculators. The Centronics parallel interface quickly became a de facto industry standard; manufacturers of the time tended to use various connectors on the system side, so a variety of cables were required. For example, early VAX systems used a DC-37 connector, NCR used the 36-pin micro ribbon connector, Texas Instruments used a 25-pin card edge connector and Data General used a 50-pin micro ribbon connector.

Dataproducts introduced a very different implementation of the parallel interface for their printers. It used a DC-37 connector on the host side and a 50 pin connector on the printer side—either a DD-50 (sometimes incorrectly referred to as a "DB50") or the block shaped M-50 connector; the M-50 was also referred to as Winchester. [cite web |url=http://www.hardwarebook.info/Dataproducts_D-Sub_50_Parallel |title=Dataproducts D-Sub 50 Parallel |accessdate=2008-01-25 |work=Hardware Book] [cite web |url=http://www.hardwarebook.info/Dataproducts_M/50_Parallel |title=Dataproducts M/50 Parallel |accessdate=2008-01-25 |work=Hardware Book] Dataproducts parallel was available in a short-line for connections up to convert|50|ft|m and a long-line version for connections from convert|50|ft|m to convert|500|ft|m. The Dataproducts interface was found on many mainframe systems up through the 1990s, and many printer manufacturers offered the Dataproducts interface as an option.

IBM released the IBM Personal Computer in 1981 and included a variant of the Centronics interface— only IBM logo printers (rebranded from Epson) could be used with the IBM PC. cite web|url=http://nemesis.lonestar.org/reference/computers/interfaces/centronics.html |title=Centronics and IBM Compatible Parallel Printer Interface Pin Assignment Reference |accessdate=2007-10-05 |last=Durda IV |first=Frank |date=2004 ] IBM standardized the parallel cable with a DB25F connector on the PC side and the Centronics connector on the printer side. Vendors soon released printers compatible with both standard Centronics and the IBM implementation.

IBM implemented an early form of bidirectional interface in 1987. HP introduced their version of bidirectional, known as "Bitronics", on the LaserJet 4 in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284 standard in 1994.

Uses

Before the advent of USB, the parallel interface was adapted to access a number of peripheral devices other than printers. Probably one of the earliest devices to use parallel were dongles used as a hardware key form of software copy protection. Zip drives and scanners were early implementations followed by external modems, sound cards, webcams, gamepads, joysticks and external hard disk drives and CD-ROM drives. Adapters were available to run SCSI devices via parallel. Other devices such as EPROM programmers and hardware controllers could be connected parallel.

Current use

At the consumer level, the USB interface—and in some cases Ethernet—has effectively replaced the parallel printer port. Many manufacturers of personal computers and laptops consider parallel to be a legacy port and no longer include the parallel interface. USB to parallel adapters are available to use parallel-only printers with USB-only systems. However, due to the simplicity of its implementation, it is often used for interfacing with custom-made peripherals, such as the RepRap 3d printer.

Implementation on IBM personal computers

Port addresses

Traditionally IBM PC systems have allocated their first three parallel ports according to the configuration in the table below.

Unidirectional parallel ports

In early parallel ports the data lines were unidirectional (data out only) so it was not easily possible to feed data in to the computer. However, a workaround was possible by using 4 of the 5 status lines. A circuit could be constructed to split each 8-bit byte into two 4-bit nibbles which were fed in sequentially through the status lines. Each pair of nibbles was then re-combined into an 8-bit byte.

ee also

* Parallel communications
* Parallel transmission
* IEEE 1284

References

* Axelson, Jan (2000). "Parallel Port Complete". [http://www.lvr.com Lakeview Research] . ISBN 0-9650819-1-5.
* [http://kernelbook.sourceforge.net/parportbook.pdf The (Linux) Parallel Port Subsystem by Tim Waugh]

External links

* [http://www.beyondlogic.org/spp/parallel.htm Interfacing to the Standard Parallel Port]
* [http://lcdinterfacing.googlepages.com/parallelport Video on basics of standard parallel port]
* [http://www.globu.net/pp/english/pp/ Parallel Port programming and interfacing]