Hardware Loopback Connectors
Hardware loopback connectors are simple devices that redirect outgoing transmissions from a system directly back into it. Hardware loopback connectors are used in conjunction with diagnostic software for diagnosing transmission problems. Loopback connectors are available for a number of ports, including RJ-45, serial, and parallel ports.
Specifically, a hardware loopback connector loops the outgoing data signal wires back into the system on the incoming data signal line. In effect, it tricks the system into thinking that the PC is sending and receiving data on the network, when in fact the data being sent is just being rerouted back in. Note that in some cases, a hardware loopback connector is referred to as an adapter or a plug.