A wrap plug, also known as a loopback plug, is a special plug that can be inserted into a port on a communications device to perform a diagnostic test called a loopback test. There are numerous possible configurations, depending on the hardware and the nature of the test to be performed.
Wrap plugs are manufactured commercially for specific systems and tests. A wrap plug can also be "home-brewed" by taking a cable with the correct type of plug attached, cutting the cable, stripping the wires, and then twisting certain wires together to short out specific pins in the port. In some cases, attenuators, consisting of networks of resistors, are used in place of direct short-circuits. This simulates the path loss in a real-life communications circuit.
The effect of a wrap plug is to cause transmitted (output) data to be returned as received (input) data, simulating a complete communications circuit using a single computer. In any case, the manufacturer's instructions must be closely followed to be sure valid test results are obtained, and to avoid damage to the equipment under test.