Abstract: An arrangement for reliably detecting process states within freely couplable units each controlled by a computer. Each one of n couplable units forms its own internal signal current loop including a controllable current source and at least two current detection members and series connected process contacts actuated by the process. If a plurality of such units are coupled together, the internal signal current loops of these units are electrically separated and common signal current loops are formed for identical processes so as to pass through all units. Each current source belonging to a unit and its current detection members can be bridged by a subloop which can be activated for test purposes by means of a switch, with the position of the switch being monitored with respect to safety by predetermined tests employing additional, position specific current loops.