components comprising the fieldbus master 2 the safe control unit 8 and the non safe control unit 10 advantageously allows more efficient execution and a higher level of diagnosis the safe signal unit 6 a further safe signal unit 32 and a non safe signal unit 34 are connected to the fieldbus 4 in series the signal units 6 32 and 34 are also linked to processes 40 42 and 44 which are controlled and monitored by the control units 8 and 10 via the fieldbus 4 and the corresponding signal units 6 32 and 34 in this context the processes 40 and 42 are safety critical processes and the process 44 is a non safety critical process the safe signal units 6 32 are by way of example safe slave modules sdio 4 4 which respectively have a safety related device 46 48 for handling the safe control of the safety critical processes 40 42 a safety critical process may be an emergency off function or the monitoring of a safety cage for example it goes without saying that further signal units and processes may be connected to the fieldbus again with reference to the interface module 16 the latter is provided with the following mechanisms starting up the fieldbus 4 and the safe controller 8 involves a reciprocal check being performed according to the following sequence the safe control unit 8 performs a self test after powerup and during this time keeps the bus master 2 in the reset condition using the communication 18 taking place by the interface 12 e g using a reset line only when the safe control unit 8 has been tested successfully.