22.5 Troubleshooting Serial Port Problems
So many things can go wrong with a
serial connection that whole books have been written on the topic.
Without test equipment, you're limited to using
common-sense solutions, but they will serve to resolve nearly any
problem you are likely to encounter. Use the following guidelines to
resolve serial port problems:
If a newly installed serial connection doesn't work,
it's almost always because the serial port is
disabled or misconfigured, communications parameters are set wrong,
or you're using the wrong cable. To resolve these
problems, take the steps described in the preceding sections to
verify that the port hardware is installed and configured properly,
that the BIOS and Windows recognize the port, and that communications
parameters are set properly. If the preceding steps don't solve the problem, try
connecting the cable to a different serial port. If the device works
when connected to another port, the new port may be defective. If the
device doesn't work on the known-good port, suspect
a problem with the cable or the device itself. If a working serial connection stops working, it's
usually because someone or something changed port settings or
communications parameters. Verify these first. If they appear
correct, it's possible that Windows has munged its
own configuration. Uninstall and reinstall the affected device (e.g.,
a modem) from within Windows. If that doesn't solve
the problem, use the Device Manager to remove the port and restart
the system. Windows will detect the
"new" port and configure it
automatically. If uninstalling and reinstalling doesn't solve the
problem, the next likely cause is that the serial port, cable, or
peripheral is bad. Determine which by swapping cables and/or
connecting the peripheral to a different serial port. Note that some serial ports are very sensitive to voltage spikes and
may be damaged during lightning storms by surges induced on
inadequately protected phone lines without any other apparent damage
to the PC. If a serial port suddenly stops working after an
electrical storm, suspect physical port damage.
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