DIAGNOSTIC CIRCUIT FOR STEPPING MOTOR

PROBLEM TO BE SOLVED: To diagnose the output accurately by performing diagnosis while avoiding the time when the level at the output terminal is inverted through induction between windings in a stepping motor. SOLUTION: Pulses are outputted sequentially from the output ports 10-40 of a microcomputer 5 at a predetermined period. Based on the pulses, a drive circuit 3 turns the phases A-/B of a stepping motor 2 on/off. A diagnostic circuit 4 provides the microcomputer 5 with signals 1I-4I corresponding to the terminal level of the phases A-/B. When the output from the microcomputer 5 is Lo level, corresponding output terminal level of the stepping motor is equal to the battery voltage and a decision is made that the stepping motor is normal it the input level to the microcomputer 5 is Hi otherwise the stepping motor is abnormal. Since no current flow through phase A when diagnosis is made immediately before outputting a drive pule, phase A can be diagnosed accurately.