the method according to the invention is particularly suitable for all ring bus systems , in which the exemplary embodiments which are described in the following text using the interbus standard . the schematic layout of an automation system 1 with a bus system 2 based on the interbus standard is illustrated in fig1 . topologically , interbus is a ring system , that is to say all of the bus subscribers 21 to 28 are actively included in a closed transmission path . each bus subscriber 21 to 28 regenerates the incoming signal , and passes it on . as a special feature in comparison to other ring systems , both the data forward line and the data return line in the interbus system are passed through all the bus subscribers within one cable . this results in the physical appearance of a line or tree structure . a main trunk originates from the control device 3 , which acts as the bus master , from said main trunk a subsystem for structuring the overall system is formed by means of a bus coupler 10 . the data are not assigned to the individual bus subscribers , as it is necessary in other systems , by the allocation of a bus address , but automatically by means of the physical position of the bus subscribers 21 to 28 in the system . interbus operates on the basis of the sum frame method with only one protocol frame for the messages for all of the bus subscribers 21 to 28 . the data for all the connected bus subscribers are included , combined to form a block , in the sum frame . in practice , this method may be regarded as a register , which is formed by the bus subscribers 21 to 28 that are connected to form a ring system . in the event of changes to the configuration of the bus system , all of the bus subscribers are identified again , and the sum frame is redefined , by carrying out identification cycles . as illustrated in fig2 , configuration changes may occur , for example , as a result of the connection of a further bus subscriber 29 to the bus system 2 . a further example of possible changes to the configuration of the bus system 2 is illustrated in fig3 . if the control device identifies a disturbance 30 in data transmission , identification cycles are carried out , with one further bus subscriber in each case being added to the bus system 2 . this allows the disturbance 30 to be located and , as in this example , allows continued operation of the bus system with the bus subscribers 23 to 25 being excluded . fig4 shows a part of an automation system according to the invention . the drive 43 for a conveyor belt 41 , on which conveyed goods 42 are transported , is controlled via an output 241 of a bus subscriber 24 . this hazardous process is shielded by a guard 51 . access is possible only via a guard door 52 . during normal operation , a sensor 54 which responds to the opening of the guard door 52 is read via the input 231 of the bus subscriber 23 , which is connected to the bus system 2 . if the guard door is opened , the control device 3 ( which is not illustrated here ) transmits a control command to the bus subscriber 24 to shut down the drive 43 of the conveyor belt 41 via the input 241 . this ensures safety when any one enters the hazardous area . the bus subscribers 23 and 24 are in this case safety - compliant bus subscribers . if the data communication between the control device 3 and the bus subscribers 23 and / or 24 is interrupted as a result of a switching process or a disturbance in the bus system 2 , then normal operation is no longer possible . in this case , the bus subscriber 24 is switched to a quasi - safe state . the response of the bus subscriber 24 is in this case governed by previous programming or parameterization by the control device 3 . in this exemplary embodiment , the conveyor belt 41 can still be driven in the quasi - safe state . the safety is ensured by an interlock 53 on the guard door 52 being activated via the output 242 of the bus subscriber 24 in such a way that it is not possible to enter the hazardous area . if the data communication between the control device 3 and the bus subscribers 23 and 24 is restored within a defined time period , then normal operation can be continued . this allows switching processes in the bus system to be bridged , thus increasing the availability of the overall system , while ensuring safety . if the data communication is not restored within the defined time period , then the bus subscriber 24 is switched to a safe final state . in this exemplary embodiment , the drive 43 for the conveyor belt 41 is shut down via the output 241 in the safe final state , and the interlock 53 for the guard door 52 is deactivated again , in the safe final state . fig5 shows a further exemplary embodiment , in which a process which is not classified as being hazardous is being controlled . during normal operation , a sensor 65 which identifies a conveyed item 42 that is being transported on the conveyor belt 41 is read via the input 261 of the bus subscriber 26 . the drive 62 of a slide 61 is controlled via an output 272 of the bus subscriber 27 . when the data communication via the bus system 2 is interrupted in this exemplary embodiment , the bus subscriber 27 switches to a transitional state . in this state , the slide can still be controlled since the bus subscriber 27 has a dedicated input 271 via which the signals from the sensor 65 are read . in this exemplary embodiment as well , the programming and / or parameterization of the bus subscriber 27 can provide for the transitional state to be maintained only for a maximum time period which can be defined . if the bus subscriber 27 is still not receiving any signals from the control device 3 once this time period has elapsed , then the bus subscriber 27 switches to a final state in which , for example , the slide 61 is moved to a rest position , and the drive 62 for the slide 61 is shut down . the method according to the invention is illustrated once again , in the form of a timing diagram , in fig6 . the illustration shows the time since the respective last data communication between a control device 3 and an appropriately programmed or configured safety - compliant bus subscriber 24 . during normal operation 100 , data communication takes place at regular time intervals t 0 . if no data communication is recorded for a time period t 1 , which can be defined , then the connection is assumed to be interrupted , and the bus subscriber 24 is switched to a quasi - safe state 101 . this quasi - safe state 101 is maintained for a time period t 2 which can be defined . if data communication takes place within the time period t 2 , the bus subscriber is switched back to normal operation 100 . otherwise , once the time period t 2 has elapsed , the bus subscriber is switched to a safe final state 102 . a corresponding procedure is used for non - safety - compliant bus subscribers 27 . it should be understood that various alternatives , combinations and modifications of the teachings described herein could be devised by those skilled in the art . the present invention is intended to embrace all such alternatives , modifications and variances that fall within the scope of the appended claims .