PuTTY is a free telnet/SSH client which can be downloaded from www.putty.org.
To perform a loopback test on a COM port, first plug in the loopback plug into the desired port on your Digi hardware. Loopback adapters ship with all Digi Terminal Server products. They are small RJ-45, DB-25 or DB-9 plugs with no cable. If a loopback needs to be made, see this Knowledge Base article: Creating a Loopback Adapter
Run PuTTY and fill in the appropriate serial port information:
Any data typed will echo onto your PuTTY screen:
If you can see the characters you typed in, the port is looping the data properly. If the data is not seen, it may be an indication of a hardware issue.