Editing Parallel port (section)

===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>