In theory, the Centronics port could transfer data as rapidly as 75,000 characters per second. When IBM implemented the parallel interface on the 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. For example, 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.
The printer side of the interface quickly became an industry de facto standard, but manufacturers used various connectors on the system side, so a variety of cables were required. 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. The host could also have the printer automatically start a new line by pulling the AUTOFEED line high, and keeping it there. Control characters in the data caused other actions, like the CR or EOF. The host could then send another character. The printer responded by pulling the BUSY line high, printing the character, and then returning BUSY to low again. When the data was ready, the host pulled the STROBE pin low, to 0 V. The host sent ASCII characters to the printer using seven of eight data pins, pulling them high to +5V to represent a 1. The Centronics Model 101 printer, featuring this connector, was released in 1970. The connector has become so closely associated with Centronics that it is now popularly known as the "Centronics connector".
The interface only required 21 of these pins, the rest were grounded or not connected. 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. In addition to the eight data pins, the system also needed various control pins as well as electrical grounds. A parallel port makes this simpler the entire ASCII value is presented on the pins in complete form. 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. This left the problem of sending the ASCII data to the printer. 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. 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. When power was applied to the solenoids, the pin was pushed forward to strike the paper and leave a dot. The printer used the dot matrix printing principle, with a print head consisting of a vertical row of seven metal pins connected to solenoids. Graphical printers, along with a host of other devices, have been designed to communicate with the system.Īn Wang, Robert Howard and Prentice Robinson began development of a low-cost printer at Centronics, a subsidiary of Wang Laboratories that produced specialty computer terminals.
It was primarily designed to operate printers that used IBM's eight-bit extended ASCII character set to print text, but could also be used to adapt other peripherals.
The parallel port interface was originally known as the Parallel Printer Adapter on IBM PC-compatible computers. Today, the parallel port interface is virtually non-existent because of the rise of Universal Serial Bus (USB) devices, along with network printing using Ethernet and Wi-Fi connected printers. It was an industry de facto standard for many years, and was finally standardized as IEEE 1284 in the late 1990s, which defined the Enhanced Parallel Port (EPP) and Extended Capability Port (ECP) bi-directional versions. There are many types of parallel ports, but the term has become most closely associated with the printer port or Centronics port found on most personal computers from the 1970s through the 2000s. To do this, parallel ports require multiple data lines in their cables and port connectors and tend to be larger than contemporary serial ports, which only require one data line. The name refers to the way the data is sent parallel ports send multiple bits of data at once ( parallel communication), as opposed to serial communication, in which bits are sent one at a time. In computing, a parallel port is a type of interface found on early computers ( personal and otherwise) for connecting peripherals. The Apple II Parallel Printer Port connected to the printer via a folded ribbon cable one end connected to the connector at the top of the card, and the other end had a 36-pin Centronics connector.
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