Method for automatic open-circuit detection

DETAILED DESCRIPTION We have developed a method of fault diagnosis for open circuits that requires no special probing--i.
e.
, no probing other than that routinely provided by a bed-of-nails fixture in an in-circuit test--but can diagnose many multiple faults.
It achieves this in a comparatively simple fashion, i.
e.
, without the simulation and storage burden that a fault-dictionary approach would impose.
Specifically, when a board device fails its in-circuit test--i.
e.
, when its response to the test vector sequence differs from the nominal response expected of a good device--the tester applies a modified vector sequence that differs from the test sequence only in the component in each vector that corresponds to a given input node.
That component remains at the signal level that the input pin assumes when an open circuit occurs in its connection to the board.
If the given device pin is properly connected to the board, the resultant response will differ from the observed, faulty response to the original test vector sequence.
If the response to the modified vector sequence is the same as the response to the test vector sequence, on the other hand, then there is an open circuit between the input node and its associated device input pin.
As will be described in more detail below, most practical implementations of this method require certain additional steps, which somewhat complicate the procedure just described.
Even with such complications, however, the resultant diagnostic procedure is fairly simple compared with most, yet it diagnoses many multiple faults.