TRANSMITTER

PURPOSE:To surely detect a fault by detecting the presence of an input transmission signal and an output transmission signal and discriminating it to be a fault of a transmitter when either the input transmission signal or the output transmission signal exist for a prescribed time or over. CONSTITUTION:An output of a 1st integration device 19 is compared with a 1st reference voltage 23 by a 1st comparator 22 and output of the comparator 22 goes to H when an input signal Pin exists. Similarly, when an output signal Pout exists, an output of a 2nd comparator 24 goes to H. The timing is slightly deviated due to difference from the time constant of integration devices 19,21. As a result, an H level is caused at an output of an exclusive OR 26 for a short time at the leading and the trailing of the signal Pin, a monostable multivibrator circuit 27 is triggered and outputs an L pulse for a prescribed time. Since the output of the AND circuit 28 always goes to an L output, a malfunction of lighting a light emitting element 30 when no fault takes place in a transmitter 16 is not caused by setting the L period longer than the H period of the OR 26.