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Encyclopedia > Parallel communications

In computing, a parallel port is an interface from a computer system where data is transferred in or out in parallel, that is, on more than one wire. A parallel port carries one bit on each wire thus multiplying the transfer rate obtainable over a single cable (contrast serial port). There are usually several extra wires on the port that are used for control signals to indicate when data is ready to be sent or received.

This type of port is most often used by a microprocessor to communicate with peripherals. The most common kind of parallel port is a printer port, e.g. a Centronics port which transfers eight bits at a time. Disks are also connected via special parallel ports, e.g. SCSI, ATA.

Before USB connections became widespread on mass-market computers, many external devices, such as portable disk drives, for Windows systems used a rather awkward pass-through connector so the device could share a parallel port with a printer. This was done because on mass-market Windows boxes of the era lacked any equivalent of the SCSI connections then common on some other platforms; the only convenient connection was usually the single printer port.

The parallel port of an IBM-PC compatible is the only standard computer peripheral that brings standard computer logic voltages directly out to a set of pins. It is much beloved by experimenters and engineers who often use it for inexpensive computer controlled projects. Standard logic voltages are virtually harmless: five volts (roughly the same as two run-down flashlight batteries), and ground (zero volts).

The connector is rather bulky and has 25 pins, the wire is also pretty thick.

Diagram of a 25-way Female D-Type Connector


Lately, the Universal Serial Bus (USB) port has grown in popularity and started displacing parallel ports because USB makes it simple to add more than one printer to a computer.

Some examples of parallel ports:


Port addresses

Traditionally IBM PC systems have allocated their first two parallel ports according to the configuration in the table below.

PORT NAME Interrupt # Starting I/O Ending I/O
LPT1 IRQ 7 0x378 0x37f
LPT2 IRQ 5 0x278 0x27f

Suggested Reading

Interfacing to the IBM-PC Parallel Printer Port (http://www.doc.ic.ac.uk/~ih/doc/par/)

See also


This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

http://www.fapo.com/porthist.htm - Warp Nine Engineering: history of the parallel port

  Results from FactBites:
Parallel Communications Explained (674 words)
Parallel communications are used over very short distances; typically inside the computer itself and to printers.
This is why parallel communications are generally restricted to short distances unless boosters are used.
As you can send all the bits at once rather than one after the other, parallel communications are fast and accurate, because it's easy to identify which bit is which by knowing what wire it came in on, and when.
Parallel communications - Wikipedia, the free encyclopedia (420 words)
The distinction between a parallel and a serial communication channel is the number of distinct wires or strands at the physical layer used for simultaneous transmission from a device.
Speed: Superficially, the speed of a parallel data link is equal to the number of bits sent at one time times the bit rate of each individual path; doubling the number of bits sent at once doubles the data rate (see Parallel transmission).
Creating a parallel port in a computer system is relatively simple, requiring only a latch to copy data onto a data bus.
  More results at FactBites »



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