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CMOS circuits present some unique testing problems. Certain physical failures are not adequately represented by the traditional stuck-at fault model. Opens in transistors or their connections, a 'stuck-open' fault, can require a sequence of tests. A number of test schemes employing exhaustive or pseudo-exhaustive input sequences have appeared in the literature. The authors examine the applicability of such a method to the testing of stuck-open faults in CMOS combinational circuits. It is shown that without careful planning an exhaustive test may not detect all stuck-open faults. A universal input sequence which will detect all stuck-open faults is proposed. This sequence corresponds to the Eulerian cycle in a directed hypercube. A circuit which generates such a sequence is outlined.