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:

• IEEE 1284 is the IEEE standard for the common PC printer port.
• 8255 for Intel microprocessors
• Z80PIO for Zilog microprocessors

Port addresses

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

Suggested Reading

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

See also

• Serial port
• Port

References

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