Editing Parallel port (section)

==History==

===Centronics===
[[An Wang]], Robert Howard and Prentice Robinson began development of a low-cost printer at [[Centronics]], a subsidiary of [[Wang Laboratories]] that produced specialty [[computer terminal]]s. The printer used the [[dot matrix printing]] principle, with a print head consisting of a vertical row of seven metal pins connected to [[solenoid]]s. When power was applied to the solenoids, the pin was pushed forward to strike the paper and leave a dot. To make a complete character [[glyph]], the print head would receive power to specified pins to create a single vertical pattern, then the print head would move to the right by a small amount, and the process repeated. On their original design, a typical glyph was printed as a matrix seven high and five wide, while the "A" models used a print head with 9 pins and formed glyphs that were 9 by 7.<ref name=centronics306>{{cite book |url=https://archive.org/stream/bitsavers_centronicschnicalManualMar76_13986426/37400040F_Model_306_Technical_Manual_Mar76_djvu.txt |title=Centronics model 306 Technical Manual |date=1976 |publisher=Centronics}}</ref>

This left the problem of sending the [[ASCII]] data to the printer. While a [[serial port]] does so with the minimum of pins and wires, it requires the device to buffer up the data as it arrives bit by bit and turn it back into multi-bit values. A parallel port makes this simpler; the entire ASCII value is presented on the pins in complete form. In addition to the eight data pins, the system also needed various control pins as well as electrical grounds. Wang happened to have a surplus stock of 20,000 [[Amphenol]] 36-pin micro ribbon connectors that were originally used for one of their early calculators. The interface only required 21 of these pins, the rest were grounded or not connected. The connector has become so closely associated with Centronics that it is now popularly known as the {{anchor|Centronics connector}} "'''Centronics connector'''".<ref name="webster"/>

The [[Centronics 101|Centronics Model 101]] printer, featuring this connector, was released in 1970.<ref name="webster">{{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}}</ref> The host sent ASCII characters to the printer using seven of eight data pins, pulling them high to +5V to represent a 1. When the data was ready, the host pulled the ''STROBE'' pin low, to 0 V. The printer responded by pulling the ''BUSY'' line high, printing the character, and then returning BUSY to low again. The host could then send another character. Control characters in the data caused other actions, like the <code>CR</code> or <code>EOF</code>. The host could also have the printer automatically start a new line by pulling the ''AUTOFEED'' line high, and keeping it there. The host had to carefully watch the BUSY line to ensure it did not feed data to the printer too rapidly, especially given variable-time operations like a paper feed.<ref name=centronics306/><ref name=centronics101/>

The printer side of the interface quickly became an industry [[de facto standard|''de facto'' standard]], but manufacturers used various connectors on the system side, so a variety of cables were required. For example, [[NCR Corporation|NCR]] used the 36-pin [[micro ribbon]] connector on both ends of the connection, early [[VAX]] systems used a [[DC-37]] connector, [[Texas Instruments]] used a 25-pin card [[edge connector]] and [[Data General]] used a 50-pin micro ribbon connector. When [[IBM]] implemented the parallel interface on the [[IBM Personal Computer|IBM PC]], they used the [[DB-25F]] connector at the PC-end of the interface, creating the now familiar parallel cable with a DB25M at one end and a 36-pin micro ribbon connector at the other.

In theory, the Centronics port could transfer data as rapidly as 75,000 characters per second. This was far faster than the printer, which averaged about 160 characters per second, meaning the port spent much of its time idle. The performance was defined by how rapidly the host could respond to the printer's BUSY signal asking for more data. To improve performance, printers began incorporating [[Data buffer|buffers]] so the host could send them data more rapidly, in bursts. This not only reduced (or eliminated) delays due to latency waiting for the next character to arrive from the host, but also freed the host to perform other operations without causing a loss of performance. Performance was further improved by using the buffer to store several lines and then printing in both directions, eliminating the delay while the print head returned to the left side of the page. Such changes more than doubled the performance of an otherwise unchanged printer, as was the case on Centronics models like the 102 and 308.<ref name=centronics101>{{cite book |url=http://chiclassiccomp.org/docs/content/computing/Centronics/101_101A_101AL_102A_306_SpecificationsInterfaceInformation.pdf |title=Centronics 101, 120A, 101AL, 102A, 306 Printers |url-status=live |archive-url=https://web.archive.org/web/20161003124447/http://chiclassiccomp.org/docs/content/computing/Centronics/101_101A_101AL_102A_306_SpecificationsInterfaceInformation.pdf |archive-date=2016-10-03 }}</ref>

===IBM===
[[IBM]] released the [[IBM Personal Computer]] in 1981 and included a variant of the Centronics interface— only IBM logo printers ([[Original equipment manufacturer|rebranded]] from [[Epson]]) could be used with the IBM PC.<ref name="durda">{{cite web |url=http://nemesis.lonestar.org/reference/computers/interfaces/centronics.html |title=Centronics and IBM Compatible Parallel Printer Interface Pin Assignment Reference |access-date=2007-10-05 |last=Durda IV |first=Frank |year=2004 |url-status=dead |archive-url=https://web.archive.org/web/20070913082745/http://nemesis.lonestar.org/reference/computers/interfaces/centronics.html |archive-date=2007-09-13 }}</ref>  IBM standardized the parallel cable with a [[D-subminiature|DB25F]] connector on the PC side and the 36-pin Centronics connector on the printer side. Vendors soon released printers compatible with both standard Centronics and the IBM implementation.

The original IBM parallel printer adapter for the IBM PC of 1981 was designed to support limited bidirectionality, with 8 lines of data output and 4 lines of data input.{{citation needed|date=March 2021}} This allowed the port to be used for other purposes, not just output to a printer. This was accomplished by allowing the data lines to be written to by devices on either end of the cable, which required the ports on the host to be bidirectional. This feature saw little use, and was removed in later revisions of the hardware. Years later, in 1987, IBM reintroduced the bidirectional interface with its [[IBM PS/2]] series, where it could be enabled or disabled for compatibility with applications hardwired not to expect a printer port to be bidirectional.

===Bi-Tronics===
As the printer market expanded, new types of printing mechanisms appeared. These often supported new features and error conditions that could not be represented on the existing port's relatively few status pins. While the IBM solution could support this, it was not trivial to implement and was not at that time being supported. This led to the Bi-Tronics system, introduced by [[Hewlett-Packard|HP]] on their [[HP LaserJet 4|LaserJet 4Si]] in April 1993.<ref>{{cite web |url=http://www.hp.com/hpinfo/newsroom/feature_stories/2004/printerstimeline.pdf |title=Twenty Years of Innovation: HP LaserJet and Inkjet Printers 1984–2004 |author= HP Corporate Archives |date=2004-05-24 |website=www.hp.com |publisher=HP |archive-url=https://web.archive.org/web/20071202142038/http://www.hp.com/hpinfo/newsroom/feature_stories/2004/printerstimeline.pdf |access-date=2021-11-05 |archive-date=2007-12-02 |quote=}}</ref> This used four existing status pins, ERROR, SELECT, PE and BUSY to represent a [[nibble]], using two transfers to send an 8-bit value. Bi-Tronics mode, now known as nibble mode, was indicated by the host pulling the SELECT line high, and data was transferred when the host toggles the AUTOFEED low.  Other changes in the handshaking protocols improved performance, reaching 400,000 cps to the printer, and about 50,000 cps back to the host.<ref>{{cite web |title= Nibble Mode |url= http://madang.ajou.ac.kr/~ydpark/archive/computer/ppi/fapo/nibble.htm |website= Department of Chemistry, Ajou University |url-status= dead |archive-url= https://web.archive.org/web/20170406082832/http://madang.ajou.ac.kr/~ydpark/archive/computer/ppi/fapo/nibble.htm |archive-date= 2017-04-06 |access-date= 2016-10-11 }}</ref> A major advantage of the Bi-Tronics system is that it can be driven entirely in software in the host, and uses otherwise unmodified hardware - all the pins used for data transfer back to the host were already printer-to-host lines.

===EPP and ECP===
The introduction of new devices like [[Image scanner|scanner]]s and [[multi-function printer]]s demanded much more performance than either the Bi-Tronics or IBM style backchannels could handle. Two other standards have become more popular for these purposes. The Enhanced Parallel Port (EPP), originally defined by [[Zenith Electronics]], is similar to IBM's byte mode in concept, but changes details of the handshaking to allow up to 2 MB/s.<ref>{{patent|EP|0640229|Buxton, C.L. / Kohtz, R.A. / Zenith Data Systems Corp.: ''Enhanced parallel port.'' filing date 15 May 1992}}</ref> The Extended Capability Port (ECP) is essentially an entirely new port in the same physical housing that also adds [[direct memory access]] based on [[Industry Standard Architecture|ISA]] and [[run-length encoding]] to compress the data, which is especially useful when transferring simple images like [[fax]]es or black-and-white scanned images. ECP offers performance up to 2.5 MB/s in both directions.<ref name=lava>{{cite tech report |url=http://www.nor-tech.com/solutions/dox/ieee1284_parallel_ports.pdf |title=IEEE 1284: Parallel Ports |publisher=Lava |date=2002 |access-date=2 November 2007 |archive-url=https://web.archive.org/web/20060523125620/http://www.nor-tech.com/solutions/dox/ieee1284_parallel_ports.pdf |archive-date=23 May 2006 |url-status=dead |df=dmy-all }}</ref>

All of these enhancements are collected as part of the [[IEEE 1284]] standard. The first release in 1994 included original Centronics mode ("compatibility mode"), nibble and byte modes, as well as a change to the handshaking that was already widely used; the original Centronics implementation called for the BUSY lead to toggle with each change on any line of data (busy-by-line), whereas IEEE 1284 calls for BUSY to toggle with each received character (busy-by-character). This reduces the number of BUSY toggles and the resulting interruptions on both sides. A 1997 update standardized the printer status codes. In 2000, the EPP and ECP modes were moved into the standard, as well as several connector and cable styles, and a method for [[Daisy chain (electrical engineering)|daisy chaining]] up to eight devices from a single port.<ref name=lava/>

Some host systems or print servers may use a strobe signal with a relatively low voltage output or a fast toggle.  Any of these issues might cause no or intermittent printing, missing or repeated characters or garbage printing. Some printer models may have a switch or setting to set busy by character; others may require a handshake adapter.{{citation needed|date=October 2016}}

===Dataproducts===
[[Dataproducts]] introduced a very different implementation of the parallel interface for their printers.  It used a [[D-subminiature|DC-37]] connector on the host side and a 50 pin connector on the printer side—either a [[D-subminiature|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.<ref>{{cite web |url=http://www.hardwarebook.info/Dataproducts_D-Sub_50_Parallel |title=Dataproducts D-Sub 50 Parallel |access-date=2008-01-25 |work=Hardware Book |url-status=live |archive-url=https://web.archive.org/web/20071214155050/http://www.hardwarebook.info/Dataproducts_D-Sub_50_Parallel |archive-date=2007-12-14 }}</ref><ref>{{cite web |url=http://www.hardwarebook.info/Dataproducts_M/50_Parallel |title=Dataproducts M/50 Parallel |access-date=2008-01-25 |work=Hardware Book |url-status=live |archive-url=https://web.archive.org/web/20071214105905/http://www.hardwarebook.info/Dataproducts_M/50_Parallel |archive-date=2007-12-14 }}</ref>  Dataproducts parallel was available in a short-line for connections up to {{convert|50|ft|m}} and a long-line version using [[differential signaling]] for connections 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.

A wide variety of devices were eventually designed to operate on a parallel port. Most devices were uni-directional (one-way) devices, only meant to respond to information sent from the PC. However, some devices such as [[Zip drive]]s were able to operate in bi-directional mode. Printers also eventually took up the bi-directional system, allowing various status report information to be sent.