Abstract:
A control system has a plurality of spatially distributed stations. At least some of the stations have input connectors for connecting sensors or output connectors for connecting actuators. The stations are connected from station to station to form a series of stations. The series has a first station, at least one second station and a last station. The first station and every second station have a successor in the series. Every second station and the last station have a predecessor in the series. In order to transmit data in this control system, the first station generates a number of separate data telegrams which is equal to the number of second stations plus the last station. Each data telegram is addressed to precisely one of the stations. The first station sends the data telegrams on by one at defined time intervals to its successor, wherein the data telegrams are addressed in reverse order of the stations. Every second station receives a data telegram from its respective predecessor and sends the received data telegram to its respective successor in the manner of a bucket brigade until it receives a data telegram which is addressed itself. The second stations and the last station each generate an individual reply telegram which is directed to the first station, and send the individual reply telegrams to their respective predecessors in the manner of a bucket brigade.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of international patent application PCT/EP2009/008261 filed on Nov. 20, 2009 designating the U.S., which international patent application has been published in German language and claims priority from German patent application DE 10 2008 060 007.5 filed on Nov. 25, 2008. The entire contents of these priority applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for transmitting data in an automated control system having a plurality of spatially distributed stations which communicate via a communication medium, and to a corresponding control system and station. In particular, the invention relates to a method which allows a control unit to communicate with remote I/O station of an automated control system in a simple and inexpensive manner. 
     The use of what is called field buses for data communication among a plurality of stations of a control system which are remote from one another is sufficiently known in control and automation technology. One example of a control system comprising a field bus is described in DE 199 28 517 A1. In said document, the field bus operates according to the Interbus protocol. A so-called bus master, which is arranged in the control unit, generates a data frame having a defined starting word (referred to as a loop back word) and a number of data fields which exactly corresponds to the number of stations which are connected to the field bus in addition to the control unit. The bus master and the further stations are connected in an annular structure in which the stations form a type of shift register. The data frame generated by the bus master is shifted step by step through the stations until the bus master receives back the starting word from the last station of the annular structure. By means of a control signal, the bus master then signals to all the stations that they are to read the data fields located in the station at this moment and they can assign their own new transmission data thereto. Subsequently, the bus master begins a new data cycle in which the data frame including the starting word is shifted around again in the annular shift register. Therefore, the transmission data of the stations arrive at the bus master in a serial manner. A characteristic of the Interbus protocol is therefore a single cyclically recirculating data frame, a part of which is respectively stored in each station at any time, wherein, in response to a control signal of the bus master, the individual stations read information out of assigned data fields and subsequently write their own information into these data fields. The functional principle can be presented as a circulating train which is pulled by a locomotive (in the form of the starting word), wherein the individual stations extract information from an individually assigned wagon and “load” the same wagon with information. An advantage of this principle is the determined timing behavior. On the basis of the length of the data frame and/or the number of stations it is possible to reliably predict how long it will take until the bus master has interrogated all the stations in one cycle. It is a disadvantage of the Interbus that every station has to read out and write to a location within the data frame which is assigned to said station, and this can only be implemented at an acceptable speed using special protocol chips. All the stations therefore require the special Interbus protocol chips. For applications in which only relatively small quantities of data are to be transmitted from and to a station, the Interbus concept is frequently too complex and too expensive. 
     Another field bus system in which communication between distributed stations is implemented via a cyclically circulating data frame is known from DE 10 2004 063 213 B4. According to exemplary embodiments proposed in this document, the circulating data frame is sent from one station to the next using Ethernet telegrams. In this case, the individual stations also have to read out and write to individually assigned data fields within the data frame, which also requires special protocol chips in order to reach acceptable speeds. In addition, the Ethernet telegrams require a relatively large protocol overhead and telegram overhead, and therefore the strengths of this known system are felt more in applications which require very powerful communication systems with relatively large information units. 
     DE 10 2006 040 709 A1 discloses a further method in which a plurality of stations are connected to form an annular structure. In this case again, a starting word followed by user data is passed on from one station to the next, wherein the individual stations append their own user data to the starting word. This known method is intended to operate without address data and, in particular, to transmit user data of short length. 
     Moreover, there is a plurality of field bus systems in which the stations can communicate with one another via data telegrams of their own. The stations are typically each connected to a common line via so-called T branches. If a station sends a data telegram, all the other stations can listen to it. Collisions or transmission conflicts which occur when two transmitting stations send simultaneously must be solved by the allocation of priorities or other arbitration methods. Examples of such field buses are known to the skilled persons as a CAN-bus, Profibus, SafetyBus p, DeviceNet or ASI. These bus systems use a common bus line with the disadvantage that a short circuit or line break can only be determined by means of physical measuring methods, not from the data communication on the bus. In addition, such a bus system is without function over the entire line length if a short circuit or line break occurs at a location. In the case of long lines and high data transmission rates, a terminating impedance is necessary, and each station must receive an individual address which has to be re-assigned when a station is replaced. If the bus system also transmits safety-related data, such as for example information about the activation of an emergency-off button or a switch-off instruction for an actuator resulting therefrom, the address assignment is safety-related and therefore complex. 
     DE 103 20 522 A1 discloses a method and a device for controlling a safety-critical process, wherein remote I/O units send safety-related process data via a single-channel line to a control unit using a diverse multiple transmission, by virtue of the fact that the remote units transmit the useful data to be transmitted once “in plain text” and a second time in encoded form. The encoded data are generated here with a variable code. 
     SUMMARY OF THE INVENTION 
     Against this background, it is an object of the present invention to provide a method for allowing data communication of a distributed control system in an inexpensive and convenient manner. 
     It is also an object to provide a method for allowing simple and cost-effective data communication between a control unit and remote I/O units connecting various sensors and actuators to a central control unit. 
     It is yet another object to provide a method for data communication between a control unit and remote I/O units across a field bus having an efficient bus utilization with inexpensive hardware and low configuration complexity. 
     Furthermore, it is an object to provide a method and the control system which allow safety-related information to be transmitted over a field bus. 
     According to an aspect of the invention, there is provided a method for transmitting data in an automated control system having a plurality of spatially distributed stations which are connected from station to station to form a series of stations, wherein the series has a first station, at least one second station and a last station, wherein the first station and every second station each have a successor in the series, and wherein every second station and the last station each have a predecessor in the series, the method comprising the following steps: the first station generates a number of separate data telegrams, which number is equal to the number of second stations plus the last station, wherein each data telegram is addressed to one of the second and the last stations; the first station sends the data telegrams one by one at defined time intervals to its successor, wherein the data telegrams are addressed in reverse order of the stations; every second station receives a data telegram from its respective predecessor and sends the received data telegram to its respective successor in a manner of a bucket brigade, until it receives a data telegram which is addressed itself; and the second stations and the last station each generate an individual reply telegram which is directed to the first station, and they send the individual reply telegrams to their respective predecessor in a manner of a bucket brigade. 
     According to another aspect, there is provided a control system having a plurality of spatially distributed stations, at least some of which have input connectors for connecting sensors or output connectors for connecting actuators, wherein the stations are connected from station to station to form a series of stations, wherein the series of stations has a first station, at least one second station and a last station, wherein the first station and every second station each have a successor in the series, and wherein every second station and the last station each have a predecessor in the series, and wherein the first station is designed to generate a number of separate data telegrams, which number is equal to the number of second stations plus the last station, with every data telegram being addressed to one of the second and the last stations; the first station is further designed to send the data telegrams one by one at defined time intervals to its successor, wherein the data telegrams are addressed in reverse order of the stations; every second station is designed to receive a data telegram from its respective predecessor and to send the received data telegram to its respective successor in the manner of a bucket brigade until it receives a data telegram which is addressed to itself; and the second stations and the last station each are designed to generate an individual reply telegram directed to the first station, and to send the individual reply telegrams to their respective predecessors in the manner of a bucket brigade. 
     According to yet another aspect, there is provided a peripheral station for use in a control system having a plurality of spatially distributed stations, said stations being connected to one another from station to station in order to form a series of stations, wherein the series of stations defines a first station, at least one second station and a last station, wherein the first station and every second station each have a successor in the series, and wherein every second station and the last station each have a predecessor in the series, said peripheral station comprising a number of input connectors for connecting sensors and a number of output connectors for connection actuators; a first bus connector configured to be connected to a predecessor in the series of stations and a separate second bus connector configured to be connected to a successor in the series of stations; an address memory for storing a station address; and a communication module designed to receive a data telegram from the predecessor via the first bus connector, designed to detect whether the data telegram contains the station address, designed to forward said data telegram to said successor via said second bus connector if said station address is not contained in said data telegram, designed to generate a reply telegram in response to said data telegram if said station address is contained in said data telegram, designed to receive further reply telegrams from said successor via said second bus connector, and designed to forward the reply telegram and the further reply telegrams to said predecessor via said first bus connector. 
     The approach presented here differs from previously known methods and systems in various aspects, depending on which known method or system is used for the comparison. On the one hand, the stations of the new method and the new system are connected from station to station to form a series of stations. The new method and the new system therefore use a line topology instead of an annular structure. The data telegrams are sent one by one and in a serial manner from the first station to the adjacent second station and from there (and equally from all the further stations) to the respective adjacent station. Consequently, only a single data telegram does exist on each line part of the system. Due to the point-to-point connection between the individual stations in the series, collisions between data telegrams for various stations are avoided. The expenditure for arbitration is low. Preferably, the second stations and the last station each send their respective individual reply telegrams only in response to a data telegram addressed to them, so that a collision between data telegrams and reply telegrams is also avoided on the line parts of the system. In this case, arbitration can be dispensed with virtually completely. 
     On the other hand, the new method and the new system use individual data telegrams which are individually addressed to the stations, and corresponding individual reply telegrams. Each station receives and sends a telegram “of its own”. No station has to be able to completely read and/or understand the telegrams of the other stations, and there is either no need for a uniform data frame for all the stations. As a result, it is possible, on the one hand, to dispense with expensive protocol chips, such as are required in the case of an Interbus and similar methods, in order to permit rapid reading and writing to individually assigned data fields of a data frame which is uniform on a system-wide basis. In principle, it is possible that every station receives data telegrams and sends reply telegrams which are incompatible with the respective telegrams for the other stations, provided that it is ensured that the telegrams can be passed through all the stations. For example, it is possible that the address data for a second station is at a different location on the data telegram than the address data for the last station. The type and the structure of the address data can also differ from station to station. All that is required is that the addressed station can detect and read its address within a received data telegram. The length of the individual data telegrams and of reply telegrams and the internal protocol structure can also vary from station to station, provided that the physical level is uniform in accordance with the OSI reference model to the extent that the data telegrams and reply telegrams can be passed onto another station. 
     If the line between two stations is interrupted, or malfunctioning in some other way, communication between stations which lie upstream and/or downstream of the interruption can generally continue. Consequently, a line fault does not necessarily disable the entire system and can be localized by virtue of the telegram traffic. 
     With the new method, it is also possible to achieve a high bus utilization rate on the first line part between the first station and its successor. Here, bus utilization can be adapted in an optimum manner to the operating speed of the first station. The further the other stations are away from the first station, the greater is the drop in the bus utilization rate of the corresponding line parts. However, this does not have any overall adverse effect on the efficiency of the control system as long as the first station can communicate with its successor at an optimum clock rate over the first part. 
     Finally, the new method and the new system permit, by virtue of the uniformly defined communication according to the “first in/last out” principle (from the point of view of the first station), very efficient automatic assignment of addresses to the individual stations, as will be described further below on the basis of a preferred exemplary embodiment. As a result, a large part of the configuration complexity which is necessary in other address-based systems is dispensed with. This is particularly advantageous if the data telegrams and reply telegrams contain at least to a certain extent safety-related data because the assignment of addresses to stations which generate or process such safety-related data is a safety-related step. Due to the clearly defined communication structure, safety-related stations can also be assigned an address in a failsafe fashion in terms of Categories 3 or 4 of the European Standard EN 954-1 (or similar requirements according to ISO 13849 or IEC/EN 61 508). 
     In summary, the new method and the new control system can thus be implemented with relatively simple and cost-effective hardware components, a low degree of complexity in terms of management and configuration, and a high bus utilization rate. 
     In a preferred refinement, the stations send all the data telegrams largely simultaneously to their respective successor. 
     This refinement makes an advantageous contribution to optimizing the overall bus utilization rate. However, it is not necessary for the stations to send the data telegrams precisely at the same time since they continue to be separate data telegrams. 
     In a further refinement, the second stations and the last station process the received data telegrams largely simultaneously with one another in order to generate the individual reply telegrams largely simultaneously. 
     This refinement also makes an advantageous contribution to optimizing the bus utilization rate and to speeding up the data communication between the individual stations. Furthermore, this refinement is particularly suitable for an automated control system in which a large number of steps often have to be carried out synchronously or at least with coordinated timing with respect to one another. 
     In a further refinement, the first station generates a plurality of address allocation telegrams and sends them to its successor before the data telegrams are sent for the first time, wherein the second stations and the last station each are assigned a unique station address on the basis of the address allocation telegrams. 
     The address allocation telegrams are advantageously generated and sent in an initialization mode which can be started by an operator and/or is started automatically whenever re-activation occurs or voltage is restored. Alternatively or additionally, it is possible in other refinement that the individual stations have an interface and/or one or more switches which can be used to individually assign an address to the respective station. The preferred refinement does not require such interfaces and switches even though it is possible to permit the preferred automatic address assignment in addition to manual address assignment. The preferred refinement with automatic address assignment permits simpler and faster configuration of the system. Furthermore, faults resulting from manual inputs are avoided. Moreover, the individual stations can be more easily and cost-effectively integrated into housings of the protective type IP 67, which facilitates use in areas at risk of spray water or splash water. The use of address allocation telegrams from the first station also provides the advantage that the first station is given control over all the other stations, and address collisions can easily be avoided. 
     In a further refinement, all the second stations which have received a unique station address send the address allocation telegrams to their respective successor. 
     In this refinement, automatic address assignment occurs from front to rear within the series of stations, i.e. from the successor of the first station to the last station. In particularly preferred variants of this refinement, each station sends an address confirmation telegram to the first station once it has received an address assigned to it. This permits failsafe address assignment to be implemented very quickly and easily. 
     In a further refinement, every second station has a first and a separate second bus connector, wherein every second station is connected via the first bus connector to its predecessor and via the second bus connector to its successor. 
     In this refinement, at least every second station (preferably also the last station) has two independent interfaces. The two bus connectors are preferably DC isolated from one another. The second stations can therefore receive and send data telegrams and reply telegrams simultaneously and independently of one another. This permits very rapid and flexible data communication and is particularly advantageous if the data telegrams and reply telegrams of the individual stations can differ in length. In one particularly preferred variant of this refinement, every bus connector comprises a separate controller module which generates the transmission signal. Every station therefore functions as a repeater, with the result that relatively large line lengths can be spanned between the stations. 
     In a further refinement, the bus connectors are CAN-bus compatible. 
     In this refinement, CAN bus hardware can be combined with a communication structure in the manner of a bucket brigade. This refinement is advantageous because CAN-bus compatible modules are very widespread and are accordingly available at low cost. Furthermore, CAN bus modules have integrated and proven fault recovery and correction mechanisms at a low level of the OSI model which can be very easily used for the present invention. However, a characteristic of this refinement is that the arbitration procedures of the CAN bus are not used as a result of the new communication structure. 
     In a further refinement, the first station sends data telegrams at cyclically recurring time intervals, wherein each data telegram comprises a code word which varies from time interval to time interval, and wherein the second stations and the last station generate the respective individual reply telegrams as a function of the code word which varies over time. 
     This refinement is very advantageous if safety-related data are exchanged between the first station and the further stations. Safety-related data in this sense are, for example, information about the state of an emergency-off button (activated, non-activated, undefined) or a switch-off instruction to an actuator resulting therefrom. This refinement makes it possible to transmit even such safety-related data on a single data line. By using the varying code word it is possible to check the plausibility of the transmitted safety-related data even if the data do not change over many time intervals, which is, by way of example, often the case with an emergency-off button. 
     In a further refinement, the first station generates and sends an individual data telegram for every second station and for the last station, wherein the individual data telegrams for at least two different stations are structurally different. 
     Structurally different data telegrams are, in particular, those whose structure and composition are different. For example, the address data in the one data telegram could be located relatively far to the front while the address data in a data telegram to another station could be located relatively far to the rear or at any other location. The structure and/or length of the address data, the number and use of protocol data, such as for example a checksum (CRC), or diverse multiple transmissions of data using varying code words can form the basis of structural differences. This refinement uses the advantageous possibilities of the new communication structure which result from the fact that the stations do not have to share common data frames but communicate via separate telegrams instead. 
     Of course, the features specified above and the features to be explained below can be used not only in the respectively specified combination but also in other combinations or alone, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawing and will be explained in more detail in the following description in which: 
         FIG. 1  shows a simplified illustration of a preferred exemplary embodiment of the new control system, 
         FIG. 2  shows a schematic illustration explaining the cyclical data traffic in the control system according to  FIG. 1 , and 
         FIG. 3  shows the control system from  FIG. 1  at a later instance of time during the data communication according to the new method. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , an exemplary embodiment of the new control system is denoted by the reference number  10  in its entirety. The control system  10  serves here to safeguard a robot  12  whose movements can pose a risk for persons owing to the speed and mass of the robot  12 . Such robots are typically safeguarded using fixed protective fences and movable protective doors, light curtains and similar access protections as well as using emergency-off buttons. For the sake of simplicity,  FIG. 1  merely illustrates one emergency-off button  14 . A contact breaker is respectively illustrated by the reference numbers  16  and  18 . The working contacts of the contact breakers  16 ,  18  are arranged in series with one another in the power supply path for the drives of the robot  12 . When the emergency-off button  14  is activated, when a protective door opens or when a light curtain is traversed, the robot  12  must be shut down in a failsafe manner using the contact breakers  16 ,  18 . “Failsafe” means in this context that the safety function is not lost even if a fault occurs within the control system  10 . It may, for example, be the case that the working contacts of the contact breaker  16  are fused owing to a previous switching process and can no longer be opened. However, this fault does not lead to a loss of safety since the power supply can still be interrupted by the working contacts of the contact breaker  18 . Similarly, the required single fault safety for other components of the control system  10  can be achieved by redundancy and/or integrated test functions. 
       FIG. 1  shows, for the sake of simplicity, only the control system  10  for safeguarding the robot  12  but not a control system for controlling the working operation of the robot  12 . In principle, both control systems can be implemented in one control unit, However, in many cases a separate control unit, represented here by reference number  20 , has been used for the safety functions to date. The control unit  20  in the preferred embodiments is established in a failsafe fashion, which is illustrated here in a simplified form on the basis of two redundant processors  22   a ,  22   b  which carry out all the operations redundantly and compare them with one another. The latter is symbolized by arrow  24 . 
     What is referred to as a bus module is denoted by reference number  26  and can be integrated in the control unit  20 , or is connected as a separate assembly to the control unit  20 . In the illustrated exemplary embodiment, the bus module  26  is an assembly in a separate housing which can be attached to the housing of the control unit  20 . In this exemplary embodiment, the bus module  26  is a first station in terms of the invention. 
     Reference numbers  28 ,  30 ,  32  and  34  denote further stations, wherein the station  34  is here a last station in terms of the invention. Stations  28  to  34  each have two bus connectors  36 ,  38  which are DC isolated and can be used independently of one another. The bus module  26  and the stations  28  to  34  are connected from station to station to form a series  41  of stations by means of line parts  40 . For example, a first line part  40   a  connects the bus connector (here single connection)  36  of the bus module  26  to the first bus connector  36  of the station  28 . A second line part  40   b  connects the second bus connector  38  of the station  28  to the first bus connector  36  of the following station  30 . All the stations including the bus module  26  are therefore connected to at least one neighbor and at maximum two neighbors via point-to-point connectors. Station  28  is the successor of bus module  26  and the predecessor of station  30 . No station is connected to more than two other stations here. 
     In this case, stations  28  to  34  are remote I/O units for connecting sensors including manually activated signaling devices such as, for example, the emergency-off button  14 , and for connecting actuators such as, for example, the contact breakers  16 ,  18 . In preferred exemplary embodiments, stations  28  to  34  are accommodated in a housing of the protection type IP 67 and therefore have spray water and even splash water protection. In other exemplary embodiments, stations  28  to  34  can be plug-type modules which are plugged onto a module carrier, or they can be accommodated in other housing designs. 
     Each station  28  to  34  has here an interface module  42  which is connected to both bus connectors  36 ,  38 . In preferred exemplary embodiments, the interface module  42  is a microcontroller or an ASIC which is capable of executing defined signal processing steps or data processing steps which serve to condition sensor data for sending to the control unit  20  and to process actuator data from the control unit  20  in order to actuate, as a function thereof, actuators such as, for example, the contact breakers  16 ,  18 . 
     Each station  28  to  34  has a number of connection sockets  44  to which sensors and/or actuators can be connected. In the exemplary embodiment according to  FIG. 1 , the emergency-off button  14  is connected via a forward line  46  and a return line  48  to a connection socket  44 . A clock signal  50 , which is fed back via the return line  48 , is transmitted via the forward line  46  to the emergency-off button  14 . In this way, station  32  can determine the activation state of emergency-off button  14  and report to the control unit  20  using a data telegram. 
     According to the new method, the bus module  26  generates a number of separate data telegrams  52  which are passed on from one station to the next in the manner of a bucket brigade. In  FIG. 1 , a first data telegram  52   a  has been sent from the bus module  26  to the station  28  via the first line part  48 . Station  28  has passed on the same data telegram  52   a  to station  30 . Station  30  sends the same data telegram  52   a  without modification to the following station  32 . At the same time, i.e. in the same time interval, station  28  sends a second data telegram  52   b  to station  30 . Likewise at the same time, bus module  26  sends a third data telegram  52   c  to station  28 . Overall, bus module  26  generates four separate data telegrams in this exemplary embodiment (the fourth data telegram is not illustrated in  FIG. 1 ), and it sends them to the series  41  of stations at four successive time intervals. Each data telegram  52  comprises an address field  54  which includes an address (not illustrated separately here) which is correlated with the station address  56 . In the exemplary embodiment according to  FIG. 1 , station  28  has the address 001, station  30  has the address 010, station  32  has the address 011, and station  34  has the address 100. In preferred exemplary embodiments, the assignment of addresses to the stations  28  to  34  is carried out using addresses allocation telegrams  58  which are represented using only dashed lines in  FIG. 1  because the address allocation takes place before the sending of data telegrams  52 , shown in  FIG. 1 , takes place. 
     In a particularly preferred exemplary embodiment, each data telegram  52  also comprises a code word  60  which varies from time interval to time interval. In one exemplary embodiment, the code word  60  is changed in each case after a complete communication cycle is terminated, which is explained in more detail below with reference to  FIG. 2 . In another exemplary embodiment, the code word is changed by the control unit at longer or even shorter time intervals. 
       FIG. 2  shows a circle  64  which symbolizes a complete communication cycle. The communication cycle  64  starts with a first time interval  66   a  in which the bus module  26  sends the first data telegram  52   a  to its successor, i.e. to station  28 . The other stations  30  to  34  do not send any data telegrams  52  in this time interval  66   a  unless direct cross communication is to take place between individual stations in such “unused” time intervals, which is perfectly possible in exemplary embodiments of the invention. However, in a preferred exemplary embodiment no such cross communication between the individual stations takes place. 
     In the second time interval  66   b , station  28  passes on the received data telegram  52   a  to the station  30 . Furthermore, said station  28  receives a second data telegram  52   b  from bus module  26  within the same time interval  66   b . In the third time interval  66   c , three data telegrams  52   a ,  52   b ,  52   c  are en route on the various line parts between the stations  26 ,  28 ,  30  and  32 . In the fourth time interval  66   d , bus module  26  sends a fourth data telegram to station  28 , while the last station  34  receives the first data telegram  52   a . After a number of time intervals  66   a - 66   d , corresponding to the number of second stations plus the last station, every station has therefore received a data telegram  52  which is addressed to it. In the preferred exemplary embodiment according to  FIG. 1 , the individual stations  28  to  34  each detect, on the basis of the address fields  54  in the data telegrams  52 , whether a data telegram is addressed to said stations. Alternatively to this, it is possible in other exemplary embodiments that the stations detect their respective position within the series of stations and also count the number of passed-on data telegrams  52  in order, in this way, to detect whether a data telegram is addressed to said stations. In such an exemplary embodiment, it is conceivable to dispense with an explicit address field  54  in the data telegrams  52  because the address arises implicitly from the number of data telegrams. 
     After each station has received a data telegram addressed to it, stations  28  to  34  process the received data telegrams largely simultaneously. The time required for the data processing is not illustrated in  FIG. 2  for reasons of clarity. Subsequently, each station  28  to  34  sends to its predecessor a reply telegram  70  which is addressed to the first station  26 . This situation is illustrated in  FIG. 3 . In the first reply interval  66   e , the last station  34  sends its reply telegram  70   a  to its predecessor  32 . In the same interval, station  28  sends its reply telegram  70   d  to the bus module  26  and therefore to the control unit  20 . In the next time interval  66   f , bus module  26  receives the reply telegram  70   c  from station  30 . In the next time interval  66   g , bus module  26  receives the reply telegram  70   b  from station  32 . In the last time interval  66   h  of the communication cycle  64 , bus module  26  receives the reply telegram  70   a  of the last station  34 . A new communication cycle  64  can then start, and is represented by arrow  72  in  FIG. 2 . 
     The address of the first station  26  can be implicitly contained in the reply telegrams  70 , since the address is already apparent from the transmission and/or reception direction of the reply telegrams  70 . However, the first station can also have an explicit address, such as for example the address 000. It is therefore in principle not necessary for the reply telegrams  70  to include an explicit address information item in order to address the first station  26 . 
     The first line part  40   a  (first bus segment) is utilized to an optimum degree without considerable waiting times in this method since data telegrams and reply telegrams are transmitted via this line part  40   a  during the entire communication cycle  64 . In other words, a data telegram or reply telegram is transmitted via the first line element  40   a  in each time interval  66 . The second line element  40   b  is utilized somewhat less because no data telegram or reply telegram is transmitted via this line element  40   b  in the first time interval  66   a  or in the last time interval  66   h . Telegrams are transmitted in all the other time intervals  66 . The further a line part or bus segment is from the first station  26 , the lower the bus utilization on the segment, which is, however, not a disadvantage since the first line part  40   a  can be utilized to an optimum degree and adapted to the processing speed of the bus module  26  or the control unit  20 . 
     The first station  26  therefore sends a data telegram  52   a  to the last station  34  in the first time interval  66   a . Station  26  subsequently sends further data telegrams, which are addressed to the second stations in reverse order with respect to the distance from the first station. In this case, all the stations  28  to  34  send the reply telegrams  70  largely simultaneously to the first station  26 , wherein the reply telegrams  70  arrive in different time intervals  66   e  to  66   h  at the first station  26  as a result of the different distances. In contrast to a common data frame, as it is used in Interbus or in similar methods, data communication takes place on the basis of separate data telegrams and reply telegrams which are independent of one another and can be addressed individually. It is basically possible that the first station  26  does not address every further station  28  to  34  in each communication cycle  64 , with the result that the stations  28  to  34  can be interrogated in different cycles. This makes it possible to adapt the bus utilization to the properties of the control system  10  on a more individual basis. 
     In the preferred exemplary embodiments, each station  28  to  34  is assigned a unique address  56 . This is preferably done using the address allocation telegrams  58  in an initialization mode which can either be started manually, or, when voltage is restored, can be started automatically, which is very advantageous. In the initialization mode, the first station  26  sends a first address allocation telegram  58  to its successor  28 . If the successor  28  has not had an address assigned to it, it adopts the address (here address 001) included in the address allocation telegram  58 , and confirms the address assignment by means of a confirmation telegram to the first station  26 . The first station  26  then sends a further address allocation telegram with a further (other) address to its successor  28 . Since the successor  28  has already had an address assigned to it, it passes on the address allocation telegram  58  to the subsequent station  30 . The latter adopts the address and confirms the address assignment. In this way, all the stations in the series receive a unique address, which address corresponds, in the simplest case, to the position of the respective station within the series. On the basis of the confirmation telegrams, the first station  26  is informed how many subsequent stations are connected. In the case of safety-related stations, an assigned address can advantageously be verified using further telegram traffic. 
     As has already been indicated above, the new method and the new device are flexible to the extent that the data telegrams  52   a ,  52   b  which are addressed to various stations  34 ,  32 , can differ from one another structurally, which is represented symbolically in  FIG. 1  by means of the two data telegrams  52   a    52   b.    
     In preferred exemplary embodiments, the stations  28  to  34  encode their transmission data to the first station  26  with a code word  60  which is sent with each data telegram  52  by the first station  26 . In preferred exemplary embodiments, the first station changes the code word with each new communication cycle  64 . In preferred exemplary embodiments, the stations  28  to  34  additionally send the uncoded information, that is to say as it were in “plain text”. In this way, the first station  26  receives the data from the other stations in duplicate and can perform plausibility checks on the basis of the different transmission (plain text and variable encoding). In particular, by using the variable code words it is possible to discover stuck-at faults, even if the data of the individual stations are constant over a large number of time intervals. In these exemplary embodiments, the control system  10  is particularly well suited for controlling safety-related processes. Further details for encoding the data on the basis of code words  60  which vary over time are described in DE 103 20 522 A1, which is incorporated by reference herewith in its entirety.