INPUT CIRCUIT

PURPOSE:To obtain the input circuit with high reliability in its system without malfunction by allowing one comparator inputted with signals whose phases differ by 180 deg. to detect input line discontinuity or a short circuit in the other comparator so as to implement recovery. CONSTITUTION:The input circuit is provided with a 2nd comparator 8 whose noninverting input connects to a noninverting input of a 1st comparator 1 and whose inverting input connected to a reference potential, a 3rd comparator 9 whose noninverting input connected to a reference potential and whose inverting input connected to an inverting input of the comparator 1, line discontinuity/short circuit detection circuit 20 to which an output of the comparator 1 and an exclusive NOR output between outputs of the comparators 8, 9 and a recovery logic section receiving the outputs of the comparators 8, 9, 1 and outputting a recovery signal in addition to the 1st comparator 1 receiving two signals whose phases differ by 180 deg.. When one input is lost or short- circuited, the line discontinuity state is detected to extract other input signal for the recovery.