Abstract: A device for detecting the existence and location of failures on circuit boards is disclosed. The device is specially adapted for use with boards having a number of serially connected digital loops. Digital circuitry is provided which simultaneously compares a number of checkpoints on the faulty board with corresponding points on a known good board, and the first point to fail is isolated. Inhibit circuitry prevents failure indications at checkpoints if those failures occur subsequent to the first failure.

Abstract: A failing component connected to a node on a circuit board is located by applying voltage probes to the printed circuit lands connected to the node to determine the direction and magnitude of current flow. By determining the direction and magnitude of current flow to or from each of the components, a reliable determination can be made of the failing circuit component. The direction and magnitude of current flow is determined by use of a polarity indicator which includes a differential chopper amplifier, a high Q, high gain, active filter and digital gating circuitry.