Abstract: 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.