This invention relates to a signal processing unit typically in the form of a status and control unit forming part of a supervisory control system.The unit monitors status inputs and transmits messages to a central station indicating the status of these inputs and also has control outputs which may be activated on a command from a central station to provide a control function. In response to a predetermined stimulus such as a change of state at a status input the unit generates a new message for a neighboring status and control unit by incrementing or decrementing its own address to obtain a new address for transmission. In this way messages may be transmitted between units as well as between the central station and the units thereby allowing communication to units which lie outside the range of the central station itself.The invention has particular application in supervisory control systems.