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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0111:  Peripheral Devices
      9  III  Peripheral Device Interfaces

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Parallel

If serial communications are like a one lane road, then parallel communications are like a freeway. A standard parallel connection on a PC transmits data eight bits at a time, with the total bandwidth of 12 Mb per second. Originally designed for use by printers, the parallel port has seen many devices that take advantage of this almost universal interface on PCs. Common uses on the parallel port range from external hard drives and CD-ROMs and two special security blocks used by a high-end software packages called dongles. Before the advent of inexpensive networking hardware, there were even some software packages that could link to PCs with a special cable that connected to the parallel ports of the computers.

A parallel port on a PC uses a DB-25 female connector. When connecting a printer through a parallel port, a special cable called an IEEE 1284 cable is used. This cable has a DB-25 male connector on one end and a Centronics-36 connector on the other end. The length of this cable should not exceed 15 ft. Parallel ports can be configured to use three separate modes that allow for varying degrees of functionality with the port and any device attached to it. The first mode is Standard Parallel Port mode that we have discussed thus far.

The second mode is EPP, or Enhanced Parallel Port mode. This mode allows for synchronous bi-directional communication between the port and any device attached to it. However, in order to make the most use of this mode, the device and the operating system must support EPP mode.

The third parallel port mode is known as the ECP, or Enhanced Capabilities Port mode. ECP mode allows for high data transfer rates that are two to fifteen times that of standard parallel port mode. Like EPP mode, ECP allows for synchronous bi-directional communication between the port and any device attached to it. The performance gains come from fact that ECP mode uses a DMA channel that is set in the BIOS, typically DMA 1 or DMA 3. Of course, you would probably want to choose DMA 3, as sound cards typically use DMA 1. As is true with EPP mode, the device and the operating system must support ECP mode.


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