The software's Help file has a wealth of troubleshooting information and tests, so check there if you are having problems with your timer communications.
Version 10 and below: Support -> Troubleshooting -> Serial Port Troubleshooting
Version 11 and above: Troubleshooting -> Serial Port Troubleshooting
If it pertains to a USB to Serial Adapter, please also refer to http://www.derbytalk.com/viewtopic.php?f=5&t=2454.
See also our instructional videos on Serial Port Communications Troubleshooting.