UPGRADING REPAIRING PCs

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Chapter 6—Serial Ports and Modems
1. Right-click the My Computer icon and select Properties. Or,
you can open Control Panel and left-click the System icon
twice.
Then, click the Device Manager tab, Ports entry, and select a
specific port (such as COM 1).
2. Click the Properties button and then click the Resources tab to
display the current resource settings (IRQ, I/O) for that port.
3. Check the Conflicting Devices List to see whether the port is
using resources that conflict with other devices. If the port is
in conflict with other devices, click the Change Setting button
and then select a configuration that does not cause
resource conflicts. You might need to experiment with these
settings until you find the right one.
4. If the resource settings cannot be changed, they most likely
must be changed via the BIOS Setup. Shut down and restart
the system, enter the BIOS setup, and change the port configurations
there.
In addition to the COM 1/COM 3 and COM 2/COM 4 IRQ conflicts
noted earlier, some video adapters have an automatic address
conflict with COM 4’s I/O port address (not IRQ).
You can also use the Modems Diagnostic tab (discussed earlier in
this chapter) to test a serial port, whether or not a modem is actually
present.
Advanced Diagnostics Using Loopback Testing
One of the most useful types of diagnostic test is the loopback test,
which can be used to ensure the correct function of the serial port
and any attached cables. Loopback tests are basically internal (digital)
or external (analog). You can run internal tests by unplugging
any cables from the port and executing the test via a diagnostics
program.
The external loopback test is more effective. This test requires that
a special loopback connector or wrap plug be attached to the port
in question. When the test is run, the port is used to send data out
to the loopback plug, which routes the data back into the port’s
receive pins so that the port is transmitting and receiving at the
same time. A loopback or wrap plug is nothing more than a cable
that is doubled back on itself.
Following is a list of the wiring necessary to construct your own
loopback or wrap plugs. Check with the vendor of your testing
software to determine which loopback plug design you need to use,
or purchase pre-built ones from the vendor.