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:
- 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:
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
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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.
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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: Launch Windows Device Manager (right-click on My Computer, click
Properties, select the Hardware tab, and click Device Manager). Locate the Ports (COM & LPT) listing and expand it. Open the Properties dialog and select the Resources tab for each COM
port in turn. 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. 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.
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