The system (310) has safety control device and signal unit that are respectively provided with safety-related devices. A superimposed control device (320) is separated from safety control device, and signal unit is connected to field bus. One safety-related device is adapted to provide two safety-related data, and another safety-related data is transmitted to superimposed control device. A superimposed control unit is adapted to evaluate another safety-related data, and to change and to transmit another safety-related data to signal unit or safety control device. The system has a safety control device that controls safety-critical process. A signal unit is associated with I/O channels. Communication on field bus is controlled by superimposed control device.
