Detection of coupling faults in RAMs

This article presents results fundamental to the problem of detecting coupling faults in random access memories (RAMs). Good and faulty memories are represented as Mealy automata using the formal framework for sequential machine testing developed by Brzozowski and Jürgensen. A precise description of the coupling fault is used to define two fault models: “general coupling,” which is the set of all possible multiple coupling faults, and “general toggling,” which is a subset of general coupling. A lower bound of 2n2 + n is derived on the length of any test that detects general toggling in an n cell memory; a test by Marinescu is thereby shown to be optimal for this fault model. A lower bound of 2n2 + 3n is derived on the length of any test that detects general coupling, and a corresponding test of length 2n2 + 4n is described. Abadir and Reghbati's improved version of the traditional test GALPAT, of length 4n2 + 4n, is shown to detect general toggling but not general coupling.