Abstract: Apparatus is provided for automatic production testing of large digital circuit modules. A test station, under computer control, applies test bit patterns and clock pulses to the module under test, analyzes the resultant outputs, and isolates any fault found to one or several IC's. The test station contains power supplies and air cooling for the module, and a keyboard display and printer for use by the test operator. Test programs are developed off-line and are loaded from magnetic tape into a disk pack where they are available to the computer. The flip-flops on the module to be tested are used as an extension of the test station through the addition on the module of circuitry to allow the flip-flops to be reconfigured from their normal circuit configuration into a shift register connected at one end to one of the module connector pins.