Type of diagnostic test in which the transmitted signal is returned to the sending device after passing through a Data communications Link or network. A test typically runs on a four-wire circuit. The two transmit leads are joined to the two receive leads. A signal is then applied around the loop. This allows a technician or built-in diagnostic circuit to compare the returned signal with the transmitted signal and get some sense of what's wrong. Loopbacks are often done by excluding one piece of equipment after another.

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