N-Fail-Safe Sequential Machines

Abstract

Let S be a sequential machine with binary (i. e., 0 or 1) input and binary output. Suppose that the third value N, different from 0 and 1, represents the faulty input or output value. An N-fail-safe (NFS) machine S of S is then defined as a sequential machine with ternary (i.e., 0, 1, or N) input and ternary output such that: 1) S is a submachine of S, and 2) possible output failures of S are 0→N or 1→ N for any input failures 0→N or 1→N. This machine may be considered fail-safe since no failure such as 0→1 or 1→0 occurs in the output.