The problem considered is the diagnosis of failures of automata, specifically, failures that manifest themselves as logical malfunctions. A review of previous methods and results is first given. A method termed the “calculus of D-cubes” is then introduced, which allows one to describe and compute the behavior of failing acyclic automata, both internally and externally. An algorithm, called the D-algorithm, is then developed which utilizes this calculus to compute tests to detect failures. First a manual method is presented, by means of an example. Thence, the D-algorithm is precisely described by means of a program written in Iverson notation. Finally, it is shown for the acyclic case in which the automation is constructed from AND's, NAND's, OR's and NOR's that if a test exists, the D-algorithm will compute such a test.
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