A note on 3-state systems

D.M. Malon (ibid., vol. 38, p.275-6, Aug. 1989) points out a pitfall of certain reliability problems pertaining to systems of three-state devices. The inference to be drawn from his observation is that special care should be used in describing such systems and in specifying exactly what constitutes success and failure. The present authors (ibid., vol.37, p.388-94, Oct. 1988) have modeled systems that can be represented by a network with designated source and sink; the reliability problem they treated is described by the following two system failures: (1) the system fails (short) if there is a path of short-failed components joining the source and the sink; (2) the system fails (open) if every path joining the source and sink includes an open-failed component. The systems they modeled are therefore somewhat more general then the array structures treated by B.W. Jenney and D.J. Sherwin (ibid., vol.R-35, p.532-8, Dec. 1986). But since the two failure-modes are mutually exclusive, the pitfall that Malon describes does not affect the former. Jenney and Sherwin's difficulties do not arise because of the generality of the systems; rather, they appear in some of the more general reliability measures (such as j-out-of-m requirements) used on the sp-arrays and ps-arrays