Abstract: An input card (7) is put in communication with apparatuses (16) the operation of which is to be monitored in order to initiate certain operations. A terminal-box (13) is interposed between the card (7), which possesses detector circuits (17), and the members (16).The two ends of each circuit (17) are associated with a respective inner terminal (12) of the terminal-box (13), and the two ends of each member (16) are associated with a respective outer terminal (14) of the terminal-box (13). Shunts (19,21) mounted in the terminal-box and connected to supply terminals (14a,14b) put one of the ends of each circuit (17) or member (16) at a common potential. The other terminal of each member (16) is connected to the other terminal of a circuit (17) by means of a connection (18).The invention is used to avoid the need for auxiliary terminal-boxes and cards with a common potential point.