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IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices. It was originally developed in the 1970s by Centronics, and was widely known as the Centronics port, both before and after its IEEE standardization. ==History== In the 1970s, Centronics developed the now-familiar printer parallel port that soon became a ''de facto'' standard. Centronics had introduced the first successful low-cost seven-wire print head, which used a series of solenoids to pull the individual metal pins to strike a ribbon and the paper. Centronics lays claim to having developed the first dot matrix printer, although DEC introduced a similar design around the same time. A dot matrix print head consists of a series of metal pins arranged in a vertical row. Each pin is attached to some sort of actuator, a solenoid in the case of Centronics, which can pull the pin forward to strike a ribbon and the paper. The entire print head is moved horizontally in order to print a line of text, striking the paper several times to produce a matrix for each character. Character sets on early printers normally used 7 by 5 "pixels" to produce 80-column text. At any particular instant, the print head has to send instructions to all seven pins at the same time. The obvious solution to this problem is to use a parallel port of some sort, allowing drivers on the host computer to directly control the pins by setting bits in appropriate registers. At the time most computers featured a serial port and this could have been used for this purpose, but it would have required a string of bits sent one at a time (the meaning of "serial") to be buffered in the printer until the entire instruction was received. Using a parallel port greatly reduced the complexity of the electronics in the printer. This style of operation did require careful timing on the part of the host, to ensure the data was available when the print head reached the proper position. In early microcomputers this generally required the entire computer's resources to be dedicated to printing. This was fine in the DOS era when most operations were single-tasking. However, this also meant that the timing had to be emulated by printers that didn't use a pin-based print head, daisy wheel systems for instance. As printers grew in sophistication, and the cost of memory dropped, printers began adding increasing amounts of buffer memory, initially a line or two, but then whole pages and then documents. The original port design was send-only, allowing ''data'' to be sent from the host computer to the printer. Separate pins in the port allow status information to be sent back to the computer. This was a serious limitation as printers became "smarter" and a richer set of status codes were desired. This led to an early expansion of the system introduced by HP, the "Bitronics" implementation released in 1992. This used the status pins of the original port to form a 4-bit parallel port for sending arbitrary data back to the host. A further modification, "Bi-Directional", used the status pins to indicate the direction of data flow on the 8-bit main data bus; by indicating there was data to send to the host on one of the pins, all eight data pins became available for use. This proved adaptable, and led to the "Enhanced Parallel Port" standard, which worked like Bi-Directional mode but greatly increased the signalling speeds to 2 MByte/s, and later the "Extended Capability Port" version increased this to 2.5 MByte/s. In 1991 the Network Printing Alliance was formed to develop a new standard. In March 1994, the IEEE 1284 specification was released. 1284 included all of these modes, and allowed operation in any of them. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IEEE 1284」の詳細全文を読む スポンサード リンク
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