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.

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:

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.

Power down the computer, unplug it, and remove the card.
Locate the circuit traces that lead from pins B24 and B25 and go into circuitry on the card�most likely to a switch or jumper block. You can decide to simply leave the respective switches or jumpers in their open condition rather than cutting the copper foil traces, but you do run the risk of inadvertently closing those connections and shorting two IRQ lines together.
Cut the foil traces using the pointed end of a small hobby knife, making sure the cut is clean and all the way into the circuit board material to ensure you completely open the foil trace.
Prepare two short pieces of wire (26-gauge wire-wrap wire is the traditional circuit board mending wire of choice) as long as needed to reach from the connector pins to the switch or jumper block you traced the cut IRQ 3 and 4 lines to. Strip about 1/8" of insulation off each end and tin (wet or soak with hot molten solder) the exposed wire ends with solder as necessary.
Locate and tin the very top edge of pins B21 and B23.
Attach one end of each wire to each connector pin.

Attach the other end of each wire to the point on the switch or jumper block that was connected to the copper traces for IRQs 3 and 4.

Which end goes to which former IRQ line? Because COM1 uses IRQ4 and COM2 uses IRQ3, I stuck with convention and attached the wire for IRQ7 to where IRQ4 used to be connected and used it for COM3 (and configured the card accordingly) and used IRQ5 for COM4.

Make sure the connections are secure: add a dab of glue or tape to hold them in place.
Install the card in your system.
Boot up your system and configure Windows or your communications software to use the new settings. Your system BIOS and Windows should recognize the new COM ports, but Windows needs a little help in its configuration because it assumes COM3 and COM4 use IRQ4 and IRQ3, respectively, which needs to be changed to reflect reality. Windows allows you to manually change the COM port configurations in Device Manager. To do this:
1. Launch Windows Device Manager (right-click on My Computer, click Properties, select the Hardware tab, and click Device Manager).
2. Locate the Ports (COM & LPT) listing and expand it.
3. Open the Properties dialog and select the Resources tab for each COM port in turn.
4. Disable the "Use automatic settings" checkbox (clear the checkmark), then choose a new configuration setting in the "Settings based on" drop-down menu. As you change the selection, notice the I/O Range and IRQ values change in the "Resource settings:" box. When these values change to I/O Range = 3E8-3EF and IRQ = 7 for the COM3 port (to match the changes you made on the I/O card), stop and click OK.
5. Repeat this process for COM4 until the resources indicate I/O Range = 2E8-2EF and IRQ = 5 (to match the changes you made on the I/O card), then click OK. Close Device Manager, and your new ports and Windows settings are ready for testing and use.

Hack 78 Rewire Your COM Ports

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