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.