An X-Tree test apparatus for testing combinatorial logic circuits in a multiplexed digital system. The apparatus includes a plurality of contacts for electrical connection with pins on a device under test. A first driver drives a logic one signal, and a second driver drives a logic zero signal, both of which are used for testing the device. Multiplexing circuitry selectively connects the first driver and the second driver with each of the plurality of contacts and device pins. A receiver, connected with the device under test, measures a response from the logic circuit under test to the logic one and zero signals to determine if it functions as intended.