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A parallel interface provides a multiline data channel in which bits are sent across multiple conductors simultaneously. The bits must stay in synchronization as they cross the wires, so the parallel interfaces are limited in distance. Parallel interfaces are usually associated with printer connections, but several technologies implement parallel interfaces, including

• HIPPI (High-Performance Parallel Interface)    HIPPI is a high-performance parallel interface that is used in data centers and supercomputer applications.


• SCSI (Small Computer System Interface)    SCSI is a parallel interface for disk storage devices that is characterized by 50-pin or 68-pin connectors.


• OFDM (Orthogonal Frequency Division Multiplexing)    OFDM is a multicarrier modulation (MCM) scheme in which many parallel data streams are transmitted at the same time over a channel with each transmitting only a small part of the total data rate.


• PCI bus    The PCI bus is the peripheral interface bus inside most desktop PCs and servers. Most systems implement the 32-bit PCI bus, but 64-bit PCI bus systems should be selected when high I/O speeds are required. PCI-X is a high-bandwidth version of the PCI bus that operates at 133 MHz with data rates over 1 GB/sec in 64-bit mode. See "Servers" and "Switch Fabrics and Bus Design" for more information.

Typically, printers are connected via parallel ports and communication devices are connected via serial ports. Traditional IBM PC parallel printer cables have a 25-pin D connector on the PC side and a Centronics connector on the printer side. This interface is called SPP (standard parallel port).

Today, parallel ports follow the IEEE 1284 standards defined in the early 1990s. It specifies the traditional 25-pin D connector and Centronics connector, as well as a new 36-conductor connector that is smaller than the Centronics connector. The connector has better electrical and physical properties.

The parallel interface has been expanded with EPP (enhanced parallel port) and ECP (extended capabilities port) while retaining backward compatibility with the original SPP:

• EPP has a bidirection mode that allows both devices to communicate. Intelligent printers, for example, use this mode to report status information back to the PC. EPP is also ten times faster than the original Centronics interface.


• ECP has all the features of EPP and also provides data compression via RLE (run length encoding). Repetitive scanner and printer data benefits from this compression. Under Windows, ECP uses DMA channels and buffers to move data.

EPP and ECP modes must be enabled in the BIOS setup of computers. Most printers today support these modes. The best choice in most cases is the one called "EPP/ECP." If a device does not support these modes, the older SPP mode is used.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.