Comtrol Hub DeviceMaster - Manual (Page 167)

DeviceMaster Installation and Configuration Guide: 2000594 Rev. A Troubleshooting and Technical Support - 167
Troubleshooting and Technical Support
15. Remove the loopback plug and type abc. No characters should display because
the return path is open.
16. Re-attach the loopback plug, type abc, and the characters should appear.
17. If you want to test additional ports, simply repeat this procedure on that port
or ports.
18. Remove the loopback plug from the serial port and attach your serial device.
You may need to set the serial parameters as necessary to match your
attached equipment.