Parallel port

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 also several extra wires on the port that are used for control and status signals to indicate when data is ready to be sent or received, initiate a reset, indicate an error condition (such as paper out), and so forth. On many modern (2005) computers, the parallel port is omitted for cost savings, and is considered to be a legacy port.

Connectors

Parallel port connectors usually have at least 25 pins, most of which are used, resulting in thick cables. These cables are also limited in length to a maximum of 3-8 meters, depending on the specific port and cable characteristics. Although several standards for parallel ports exist today, they are not always followed (especially on older devices), which sometimes makes finding the proper cable and software driver difficult.

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Parallel ports have four types of pins:

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• Data pins, usually 8, sometimes 16, and sometimes with an extra pin for a parity bit. They can be either unidirectional (e.g., from a computer to a printer) or bidirectional.
• Control pins, used to send control signals such as STROBE to indicate that the data on the data pins is ready and R/W to specify whether bidirectional ports are reading or writing data.
• Status pins, used to send status signals such as BUSY to indicate the device is not ready to receive data and ACK to acknowldge successful receipt of the symbol.
• Ground pins, to complete the circuits from the other pins.

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 device (such as printers) to a computer.

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Some examples of parallel ports:

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