6 via a corresponding interface 13 interface card with bus driver 12 for representing information on a display 14 of the control panel 6 and for interrogating the control element 7 while the processing units 3 have separate arithmetic units since the processing units 3 can be exchanged with one another in all cases when one processing unit 3 fails it can be readily exchanged without it being necessary to keep a large number of different types in stock if no spare unit is available at a given time a processing unit 3 can be taken from a less important location and placed in the more important location in order to fulfil closed loop or open loop control tasks there additional processing units 3 can be used according to requirements and connected to the bus system 4 the processing units 3 expediently comprise separate diagnostic circuits which for the purpose of appropriate signaling of a partial failure supply signals ultimately to the user and in the case of a total failure no longer supply any message on interrogation which is likewise indicated to the user likewise the processing unit 3 expediently checks the failure of a sensor 1 39 and passes this information on so that this message reaches the operator the computer 5 and or the control panel 6 do not need to be arranged at a random location on.