LPT
From Wikipedia, the free encyclopedia
- For other uses, see LPT (disambiguation).
LPT is the original, yet still common, name of the parallel port interface on IBM PC-compatible computers. It was designed to operate a text printer that used IBM's 8-bit extended ASCII character set. The name derives from the fact that "line printer" was a common generic term at the time for any type of text printer. Graphical printers, along with a host of other devices, have been designed to communicate with the system. It was a de facto industry standard for many years, and was finally standardized as IEEE 1284 in the late 1990s. Today, the parallel port interface is seeing decreasing use because of the rise of Universal Serial Bus (USB) and FireWire (IEEE 1394) devices.
Most PC-compatible systems in the 1980s and 1990s had one or two ports, with communication interfaces defined like this:
Some systems also had an LPT3 port, but this was less-consistently defined. In reality, computers rarely had more than one LPT port.
A wide variety of devices were eventually designed to operate on a parallel port. Most were uni-directional (one-way) devices, only meant to respond to information sent from the PC. However, some devices such as Zip drives 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.
In MS-DOS and PC-DOS, the parallel ports could be accessed directly on the command line. For example, the command "type c:\autoexec.bat > LPT1" would direct the contents of the autoexec.bat file to the printer port. A PRN device was also available as an alias for LPT1. A special "print" command also existed to achieve the same effect. Microsoft Windows still refers to the ports in this manner in many cases, though this is often fairly hidden.
An LPT port has an 8-bit parallel databus, plus 4 pins for control output (Strobe, Linefeed, Initialize, and Select In), and 5 more for control input (ACK, Busy, Select, Error, and Paper Out). Its data transfer speed is at 12,000 kbit/s.
It is actually fairly unclear exactly what "LPT" stands for. Sources have variously mentioned Line Printing Terminal, Line PrinTer, and Local PorT. According to the Jargon File, a similar port naming convention was used on ITS, DEC systems, and CP/M.
