Open circuit detector for differential encoder feedback

An apparatus is provided for self-monitoring a communication circuit incorporating a differential driver for malfunctions with a malfunction detector located remote from the driver. The malfunction detector monitors the state of signals received at the remote location on a pair of lines coupled to the driver. Preferably, the detector includes a pair of back-to-back light emitting diodes, one of which will emit light so long as the received signals are in opposite states, and neither of which will emit light when the received signals are not in opposite states as will occur during a malfunction. The dection further preferably includes a phototransistor responsive to the diodes to provide a malfunction signal when neither diode emits light. Alternatively, the detector may comprise an exclusive-OR logic gate.