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.
Uses
Parallel ports are most often used by microprocessors 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).
Connectors
Parallel port connectors usually have at least 25 pins, most of which are used. So the cables are pretty thick, and are 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), so finding the proper cable and software driver can sometimes be difficult.
Parallel ports have four types of pins:
- 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 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.
| PORT NAME | Interrupt # | Starting I/O | Ending I/O |
|---|---|---|---|
LPT1 |
IRQ 7 |
0x378 |
0x37f
|
LPT2 |
IRQ 5 |
0x278 |
0x27f
|
Suggested Reading
See also
References
This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later. http://www.fapo.com/porthist.htm - Warp Nine Engineering: history of the parallel port