Abstract: In this paper,an expert diagnostic system(EDS)for detecting electrical system faults in a briquette-boxingmanipulator is introduced.The expert diagnostic system works in synchronism with the manipulator controlsystem by the handshaking signal line,which can,in real time and on-line,detect and diagnose the steppermotor drive circuit.The 8031 single-chip computer is the center of the expert diagnostic system in whichknowledge base is set up by production rules,backward reasoning is made by the breadth-first search and thesearching speed is increased by using the parallel search.The executing results of the prototype shows thatthe expert diagnostic system meets all design requirements.