Circuit test method

A method of testing a circuit comprising nodes connected by circuit modules which impose constraints on signal passing between the nodes, each module comprising an electrical component or group of electric components, the method comprising the steps of

• (i) selecting a set of stimuli, including supply signals,
• (ii) applying the set of stimuli at first nodes of said circuits,
• (iii) taking a set of test measurements from second nodes of said circuit,
• (iv) determining, in an analysis cycle, at least two differently derived signal values at each of a number of nodes from the set of stimuli, the set of test measurements, and the constraints on signal values imposed by the modules,
• (v) detecting a fault if the differently derived signal values are incompatible, and
• (vi) if a fault is detected, repeating one or more times, an analysis cycle which has led to an incompatibility, each time with the constraints on signal values imposed by a different selected module being suspended, to determine that a module may be faulty if suspension of the constraints on a selected module that may be faulty leads to a removal of the incompatibility, and
• (vii) taking a further test measurement from at least one further node selected to reduce the number of selected modules which may be faulty in a further analysis cycle said at least one further node being the first node or nodes of a set of previously unselected nodes ranked in order of decreasing level of discrimination between modules which may be faulty.