Open circuit detector for differential encoder feedback

A position encoder feedback circuit with malfunction self-monitoring, the circuit comprising:

• (a) encoder means for producing digital feedback signals correlated to a position measured by an encoder;
• (b) a differential driver responsive to the digital feedback signals for producing first output signals and second output signals, the first and second output signals being simultaneously in opposite states;
• (c) a differential receiver responsive to the first and second output signals for producing digital output signals correlated to the digital feedback signals;
• (d) a pair of conductors for conducting the first and second output signals from the differential driver to the differential receiver;
• and
• (e) a malfunction detector connected to the pair of conductors adjacent to the differential receiver and including an optical coupler having a pair of back-to-back light emitting diodes adapted so that one or the other of said diodes emits light so long as the first and second output signals transmitted to the diodes are in opposite states, and having a photo transistor optically coupled to the pair of diodes and adapted to produce a malfunction signal in response to an absence of light from the pair of diodes indicative of a malfunction.