A loop back connector can be used to echo RS232 data transmitted by a serial port back into its own RS232 receiver. In this way, the function of the serial port can be tested. Lantronix - Loopback cable - for SecureLinx SLC16, SLC32, SLC48, SLC8; SLC 8000
A loopback cable/plug is a basic troubleshooting tool used to test and isolate network problems. What does a CSU/DSU do? It acts as a translator between LAN data format and the WAN data format.