A connector used for diagnosing transmission problems. Also called a "wrap plug," it plugs into an Ethernet or serial port and crosses over the transmit line to the receive line so that outgoing signals can be redirected back into the computer for testing. Resistors are often used to simulate path loss in a real circuit. The plugs are available as products but can also be made by stripping wires from any plug and wiring them in the proper order. See loopback address
This loopback plug is designed for a 25-pin serial port. It crosses the sending pin over to the receiving pin so that outgoing signals come right back in.