The occurrence of a second fault may uncover a previously undetectable fault. To be complete, a vector test set must include tests for this case. Faults may be permanent, i.e., considered hard or solid, in which case they may be detected if they are not masked, i.e., are observable. Multiple faults occur when more than one fault occurs at one time. The probability of multiple faults occurring in a circuit is relatively less than the probability of a single fault, but is increasing with the increase in circuit density.
Single faults remain the most likely event.
Multiple faults can occur in such a manner that they can be degraded to an equivalent single fault. In this case, the input vectors that test for the occurrence of the single fault also test for the occurrence of the multiple fault condition. There are several equivalencies that exist which are useful in fault detection and which make fault location potentially more difficult. Table 9-2 Fault Equivalencies One or more inputs to an OR gate stuck at 1 is equivalent to the output of the OR gate stuck at 1. One or more inputs to an AND gate stuck at 0 is equivalent to the output of the AND gate stuck at 0.