Hack 76 Break the Rules with LPT Ports

Get back the use of IRQ lines by changing your LPT port settings.

Even though none of the early standard LPT port implementations ever used an IRQ line, Windows still insists that IRQ 7 or 5 is assigned to an LPT port (Windows performs no actual test for this—it's a programmed assumption), so any other use of these IRQ lines causes Windows to erroneously flag such use as a conflict. Some of us are finicky about conflicts, and some programs may not accept port connections if a conflict exists.

Trying to use the same IRQ as the parallel port is a potential conflict only if both of these are true:

• The LPT port is configured for Enhanced Parallel Port or Extended Capabilities Port (commonly used modes for pre-USB printers and scanners).

• The device attached to the LPT port is capable of signaling the PC for attention.

If the parallel port is used as a standard or even bidirectional port for simply dumping text to a printer, you can ignore this conflict because the chipset and simpler printing devices that may be attached to it have no need to use the IRQ line. Thus, you gain a free IRQ line to use for other devices despite the apparent conflict.

If your printer attaches to your PC through a USB port, or is connected as a network device or shared on another computer, you do not need the LPT port at all. In this case, you can disable the LPT port to ensure that the IRQ is never used by the LPT port, and is available for other uses in the BIOS [Hack#14] and in Windows through Device Manager with these steps: