Slide 47 of 173
Notes:
This example illustrates the basics of how PODEM orders the search space by applying values at the primary inputs. After an input assignment is selected, a simulation like process is performed to determine what values on circuit nodes are implied by the new input value. If, after implication, a test is no longer possible (fault set a s-a value, no propagation path, etc.) the opposite value for that input is tried. If that also precludes a test, the last input value tried is “popped off of the stack” and the alternate value is tried. This process of “backtracking” insures that PODEM is complete.
This example uses the simple heuristic of input/value selection of taking the inputs in order and always trying the "1" value first. PODEM in fact has some fairly complex heuristics and procedures that use circuit topology and current state information during the test generation process to select the next input and value to be tried.
More efficient heuristics for this process and earlier determination of the inconsistency of an input combination are focus of much of the follow-on work to PODEM.
