A processing system has a facility to handle instructions, signals and switching operations over a bus (1) as well as handling supply voltage coupled to the bus is central unit (2) and a signal/instruction unit (3). In normal operation a power supply (7) in the central unit is coupled to the bus. During a specific time window the bus is isolated from the supply voltage and a current supply (5) switched in and data signals (15,16,6) are transmitted over the bus. If a fault occurs the unit (3) is isolated by a fuse (17) and the central station signalled.
