Abstract: An automatic printed circuit board tester to test printed circuit boards and diagnose faults in digital and analog electrical circuit networks is disclosed. The system is comprised of control, stimuli, and interconnection networks in combination with measuring instrumentation. For automated fault isolation, a moving probe assembly driven by stepping motors controlled by a minicomputer is utilized. The fault isolation system establishes a fault path from an output pin to a terminal point within the network. The location of the terminal point together with a directed graph is utilized to select branches that could cause the failure.