Apparatus for testing electrical circuit units such as printed circuit cards

Claim 1

Modular electrical testing apparatus for circuit units having a plurality of connector terminals, comprising: (A) connector means for engaging terminals of either a reference circuit unit known to be without faults or a production circuit unit to be tested for faults; (B) means enabling said electrical testing apparatus to be operated in accordance with a Generating Mode and a Test Mode; (C) reference pattern generating means effective during said Generation Mode for generating a digital coded reference pattern from said reference circuit unit, said reference pattern generation means comprising: (i) application means for sequentially applying a series of preselected digital coded test stimuli to a selected terminal of a reference circuit card engaged in said connector means; (ii) means for scanning all of said terminals engaged in said connector means for each said application of said test stimuli to each of said terminals of said reference circuit unit; (iii) means for sensing a change in logic state of each of said scanned reference unit terminals and for identifying said terminals having a changed logic state as a result of said application of said test stimuli; and (iv) means for designating a part of reference code corresponding to said reference circuit unit (D) means for recording said test stimuli applied to each of said reference circuit unit terminals, said identified terminals having changed logic states, said logic state of each of said identified terminals, and said part reference code of said reference circuit unit; and (E) testing means effective during said Test Mode for testing said production circuit unit, said testing means comprising: (i) means for inputting a desired part reference code of said production circuit unit; (ii) means for selecting said recorded reference pattern corresponding to said desired part reference code; (iii) means for sequentially applying said test stimuli associated with said selected reference pattern to said terminals of said production circuit unit; (iv) a plurality of paired first and second flip-flop devices, each pair of said flip-flop devices being associated with a terminal of said production circuit unit and wherein said first flip-flop of said pair is connected to a terminal of said production circuit unit being tested and said second flip-flop of said pair is connected to said application means, and wherein said first flip-flop is effective to store an indication of a logic state change of its corresponding production unit terminal in response to said application of said test stimuli, and wherein said second flip-flop is effective to store an indication of an expected logic change of said terminal associated with said flip-flop pair as a result of said application of said recorded test stimuli, and wherein said plurality of paired flip-flop devices are disposed on one or more modular circuit cards for insertion into said electrical testing apparatus; (v) means for comparing said stored logic states of said plurality of flip-flop pairs; and (vi) means for indicating a fault in said production circuit unit being tested when a mismatch between any of said compared flip-flop logic states is present.