The interface circuit is intended to be connected between a module terminal and a module itself and provides means for stimulating and sensing signals between the module and its terminal. The interface circuit comprises a decoupling section which connects or disconnects the interface circuit from the module, a test data section for receiving test data from outside sources, an inside section which connects the test data section with the interior of the module and an outside section which connects it with the terminal. A monitor section is also included to allow a terminal to be monitored while it is connected to from its module. The control section requires only four connections and generates the various control signals necessary to control the operation of the interface circuit.