In the above paper1, the partial Boolean difference is defined and an algorithm for deriving complete minimal test input sequences for single-fault detection is given. In this correspondence, it is shown that the presentation of the algorithm is inadequate with respect to the need for multiple path sensitization. It is further shown that the computational procedure used in footnote 1 does not guarantee minimality of the test set and does not permit the extension to fault location as claimed in the paper.

FAULT-TOLERANT computing has been defined as "the ability to execute specified algorithms correctly regardless of hardware failures, total system flows, or program fallacies" [1]. This definition of fault-tolerant computing leads directly to three major subdivisions [2]:

A new representation for faults in combinational digital circuits is presented. Faults that are inherently indistinguishable are identified and combined into classes that form a geometric structure that effectively subdivides the original circuit into fan-out-free segments. This fan-out-free characteristic allows a simplified analysis of multiple fault conditions. For certain circuits, including all two-level single-output circuits, it is shown that the detection of all single faults implies the detection of all multiple faults. The behavior of any circuit under fault conditions is represented in terms of the classes of indistinguishable faults. This results in a description of the faulty circuit by means of Boolean equations that are readily manipulated for the purpose of fault simulation or test generation. A connection graph interpretation of this fault representation is discussed. Heuristic methods for the selection of efficient tests without extensive computation are derived from these connection graphs.

This paper considers the design of diagnosable combinational networks. The diagnosability criterion used here requires that the single fault detection test set for the network also detects all multiple faults. Several recent studies in the area of multiple fault diagnosis are reviewed and the results which are pertinent to this investigation are summarized. It is shown that, under certain conditions, internal fan-out may be present without adversely affecting the detection of multiple faults. These results suggest design techniques which lead to readily diagnosable networks.