an apparatus for the automatic , in - circuit testing of the electrical properties of complex digital integrated circuit assemblies is disclosed . a programmed processor is provided to control a set of selectable switches , which connect selected nodes of a circuit under test to certain ones of a plurality of signal lines . one of the signal lines supplies a selected digital test signal from a set of selectable test signals to the selected node . the set of test signals including a gray code . another of the signal lines provides a response line connecting a selected node to a functional tester that performs one of a selectable number of intermediate functional tests . one of the functional tests is a signature analysis of the digital response signal in accordance with a cyclic redundancy check coding technique . each test performed by the apparatus is specified through processor routines which select and encode the proper test signals for the particular circuit or device under test and analyze the results of the intermediate functional tests to determine if the device has functioned properly .