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.

Diagnosability without fault repair of a digital system containing at most t faults is considered. A system-level diagnostic model defined in an earlier paper [1] is employed. The model is to an extent independent of the means used to implement diagnostic procedures, i.e., whether the tests are accomplished via hardware, software, or combinations thereof. Two parameters, the masking and exposure indices, are defined. Conjoined with the previously defined closure index, the parameters fundamentally characterize the capability for executing valid tests in a multiple-fault environment. Necessary and sufficient conditions for a system to be t-fault diagnosable without repair are derived in terms of these parameters. Examples are presented to illustrate the application of the model for systems close to those encountered in actual practice.

Determination of the detectability and diagnosability of a digital system containing at most t faulty system components is considered. The model employed is to an extent independent of the means used to implement diagnostic procedures, i.e., whether the tests are accomplished via hardware, software, or combinations thereof. A parameter, called the closure index, is defined which characterizes the capability for executing valid tests in the presence of faults. The closure index can be thought of as the size of the smallest potentially undetectable multiple-fault in the system as modeled. On the basis of this parameter, results are presented which permit the determination of t-fault detectability and t-fault diagnosability with repair for the system. Examples are presented to illustrate the application of the model for systems close to those encountered in actual practice.

The set covering problem is considered and an efficient procedure for finding an irredundant cover is presented. For an m × n cover table, the execution time of the procedure is, in the worst case, proportional to mn . Methods are suggested for obtaining alternate irredundant covers based on an initially obtained irredundant cover. The basic cost-independent algorithm is heuristically extended to consider costs so that a reduced-cost irredundant cover can be obtained. A summary of some computational experience is presented, which indicates that the procedure is fast and applicable to large problems.

The problem of fault detection and diagnosis of irredundant multiple-output combinational networks is discussed from a purely structural viewpoint, i.e., with the functions of the network components essentially ignored. Two structural models for a network are given in the form of labeled directed graphs. Graph theoretic concepts are introduced, and theorems are developed which give necessary conditions and sufficient conditions for the detectability of faults at particular primary outputs based solely on the structure of the network. Procedures are then presented for obtaining the structural diagnostic resolution for both line and logic module faults. Finally, an application, test point placement for increased network maintainability, is considered.