Abstract: An input/output circuit for performing tests prior to controlling an electrical load (2) which is decoupled by a transformer (5). An input (E) thereto acts on a first switching member (6) connected in series with a primary winding of the transformer in a power supply circuit for said primary winding. An output (S) provides a signal on the basis of the voltage present across the terminals of an output resistor (7) connected in series with the primary winding of the transformer in the power supply circuit of said winding. A second switching member (3) is inserted in series with the load in a power supply circuit for said load, and is controlled from the input to the input/output circuit via the transformer and a rectifier circuit (10) connected to the terminals of a secondary winding of the transformer. An impedance-switching circuit (13, 16) is connected to the terminals of the second winding in order to act on the input/output circuit by changing the voltage across the terminals of its output resistor.