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Parallel I/O Cards | Tech Info Home Page |


Parallel cards are add-on cards that plug into the motherboard and have one or more connectors sticking out of the back of the computer. Devices such as Printers, Parallel-scanners, and external Zip drives are connected to the plugs on the back of the cards.

If you are looking for parallel cards, or drivers go to the "Parallel Cards Hardware" page.

The connectors are normally 25-pin Female (DB-25), and have small posts on each side so the cable can be screwed on to the connector.

The connectors are referred to as "parallel connectors", "LPT ports", "printer connectors" or "parallel port adapters". The newer type of parallel ports follow the IEEE1284 standard which allows for faster speeds and gives devices a path to communicate with the host as well as much larger cable distances.

Parallel ports also have 4 modes which can normally be set at the BIOS/CMOS area. They are the "Standard", "bi-directional", "EPP", and "ECP" modes. The EPP and ECP modes normally require a special cable which runs about $35 and is usually a much heavier cable and quiet often is labeled as an "IEEE 1284" cable. A lower quality cable is also used for "bi-directional" communications.

Your printer or parallel device will determine which type of cable you will need. It is not enough to just buy a IEEE-1284 cable and expect the device to operate using that mode. The I/O parallel card and the device must also support the mode you are using.

Most computers have a parallel port on the motherboard, and a few have an add-on parallel card. Add-on cards can be of the type where there is one or more functions included such as "com port", "parallel port", and "hard/floppy drive controllers".

When installing parallel cards you must be careful. An IRQ setting must be set (usually with DIP switches) so that it does not interfere with another device that uses that same port. They are usually referred to as LPT port settings. The IRQ setting is normally IRQ7 for the LPT1 (parallel port 1). On the negative side, most computers have so many components installed that there might not be an IRQ line available to use, so you have to check in advance to see what IRQ lines are used before deciding to add a second printer port. Also a unique address must be set for each port. The typicall Parallel port addresses are 3BCh, 378h, 278h (378h is the default LPT1 address).

An alternative to using a second parallel port is installing a switch box. Many technicians recommend using electronic switch boxes, but many people have used standard mechanical switch boxes such as the Curtis Data transfer switch model DS3 (sold at major retailers) which has 2 DB-25 female and one DB-25 male connectors. A male-to-male cable connects the center connector to the computer's printer plug, and one of each of the other female plugs connect to each printer using a DB-25 male/Centronics cable (standard printer cable).

One final note, the IRQ settings that a card provides are normally in the IRQ3 to IRQ7 range, which might not be usable. If all those IRQ lines have been used, you will need to buy a card that allows IRQ settings higher than IRQ7.

As of late 1998, PCI type of parallel cards are now available that automatically allow the card to share the IRQ with other cards. Highly recommended.




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