To Determine Whether an integrated digital circuit is working properly, one may apply to the circuit a set of well-devised test sequences and compare the resultant outputs with the corresponding correct outputs. Any discrepancies indicate the presence of a fault. The main task here is to find a set of test sequences which can detect the presence of any prescribed fault in the circuit. This test generation problem will become formidable for large scale integrated arrays, since large number of logic circuits may be contained in an array with a limited number of exterior terminals.