Apparatus for the dynamic in-circuit testing of digital electronic devices employs a hardware testing circuit that is set by a microcomputer which takes no direct part in the test, so that the test hardware speed is not limited by the computer speed. The apparatus comprises a library of devices equivalent to the devices to be tested, the library including a ROM containing the information regarding the devices needed by the microcomputer for its purpose. An internal interface or router receives signals from the test device that are input signals to its terminals and routes them directly to the corresponding selected device in the library where it becomes an input to that device also. Signals from the test device that are output signals are routed instead to a comparison block where they are compared with the respective output signals from the library reference device. The signals at each corresponding pin of the two devices are compared and upon the presence of a fault the apparatus stops and identifies the pin or pins on which a fault has been detected. An external interface is provided to shift the signal levels as required between the test device and the transistor-transistor logic devices of the apparatus. The signals are sampled during timed periods to account for different propagation times through the apparatus, and different operating speeds of the devices. Provision is made for external or internal clocks, reset and ground connections.