PC Hacks

Hack #78. Rewire Your COM Ports

Rewire an old COM port board to use available IRQs to run four ports simultaneously.

In this hack, we'll rewire the IRQ lines of an old 8-bit COM port card to configure COM3 and COM4 ports to use IRQs 5 and 7. Rewiring older I/O cards takes a bit of skill—specifically with hand tools and a soldering iron.

Figure 8-3 shows an obvious rewire of IRQ signals. Accomplishing this rewiring required opening the circuit traces from the IRQ 3 and 4 connection "fingers," then adding wires from the IRQ 5 and 7 "fingers" to the circuit traces that once connected to IRQs 3 and 4.

Figure 8-3. IRQ lines rewired on a COM port I/O card

If you decide to try this at home, you need to know the connector pin information to locate the respective IRQ circuit traces, which are:

Pin B21 = IRQ7

Pin B23 = IRQ5

Pin B24 = IRQ4

Pin B25 = IRQ3

The "B" designates the component side of the circuit board, as shown in Figure 8-3. Pins are counted from the bracket (off camera to the left) towards the end of the card. Pin B21 is the 21st pin from the left.

To perform this hack, you need the following tools:

• A 60-watt soldering iron or a temperature-controlled soldering iron with a small 700-800 degree tip

• Solder

• 2 to 3 inches of 22- to 26-gauge wire

• Wire cutters and strippers

• An X-ACTO or similar hobby knife

With the right tools at hand, follow these steps:

1. Power down the computer, unplug it, and ...