Abstract: An interface circuit, having a single-chip microprocessor with program instructions in internal memory, includes switching devices between A-D (Analog to Digital) ports of the microprocessor and ports of the interface circuit for selectively connecting and disconnecting the microprocessor and interface ports. The interface circuit includes a connector through which address latch units and external memory having program instructions can be connected to the single-chip microprocessor. The connector provides for switch over of operation of the microprocessor from internal memory to external memory, and for operation of the switching devices. The connector provides for connection of an I/O (Input/Output) unit to provide input and output to ports of the interface circuit.