Abstract:
A method for cyclical communication between communications stations controls or surveys a technical process, through a bus. Communications relations, which are provided for the communication stations are executed during each bus cycle of predetermined duration and if a communications relation is disturbed, its repetition is scheduled for a succeeding bus cycle and the disturbed communications relation is acknowledged with a special acknowledgement code.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to PCT Application No. PCT/EP01/08820 filed on 30 Jul. 2001 and European Application No. 001 17 177.6 filed on 10 Aug. 2000, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for cyclic communication between communication stations provided for controlling or monitoring a technical process, via a bus, in which communication sessions planned for the communication stations are executed during in each case one bus cycle. 
     Such a communication method is known from the standard EN 50 170 or PROFIBUS standard. The PROFIBUS is a so-called field bus which is used for communicatively linking communication stations provided for automating a technical process. A communication station at this or a similar bus is, e.g., a so-called stored-program control (SPC). A further communication station at the bus is, e.g., a so-called decentralized peripheral device to which external sensors or actuators can be connected for controlling or monitoring the technical process. 
     Controlling a technical process frequently also includes closed-loop control tasks. In this context, closed-loop control comprises picking up a measurement value from the technical process and outputting control information to the technical process. Both the picking-up of the measurement value and the outputting of the control information is usually cyclic. Since the measurement value is thus available not continuously but only in each case at the time when it is picked up, i.e. at the sampling time, this is a sampled-data control system, the quality or stability of which is primarily dependent on the equidistance of the sampling times. 
     Frequently, the measurement value is picked up from the process by a first communication station and processed by a second communication station. Using the measurement value, this second communication station also generates the control information to be output. The control information is then output to the process by a third communication station. The distance between two sampling times, and, therefore, the sampling frequency is thus determined by the duration of the communication between the respective communication stations. 
     To ensure equidistance of the sampling times, a constant bus cycle time is provided, e.g. in the PROFIBUS. The bus cycle time is the time interval within which all cyclic communication sessions planned for the communication stations connected to the bus are executed exactly once. A communication method with constant bus cycle time is known, e.g., from German patent application 199 39 182 (date of application 20, Aug. 1999). 
     A communication comprises the transfer of a message via the bus from the transmitting communication station to the receiving communication station. The time required for transferring a message is essentially determined by the volume of data transferred. The volume of data of cyclic communications, however, is essentially constant. Thus, an approximate equidistance between the individual communications is obtained with a constant bus cycle time. The approximate equidistance of the individual communications is accompanied by an approximate equidistance of the sampling times because the measurement value picked up from the process is a data item of a communication or, respectively, a message. 
     To ensure actual equidistance, the bus cycle time is longer than the time which would be required for executing all planned communications. The additional time is available as reserve for message retransmissions and so-called acyclic messages. If no message retransmissions are required in a bus cycle or there are no acyclic messages ready for transfer, the system still waits until the predetermined bus cycle time (including the spare time) has elapsed before it begins the next bus cycle. This results in a fixed timing pattern for the planned communications, by which the equidistance of the samples can be ensured. 
     The disadvantageous factor in this known communication method is, however, that this equidistance is no longer guaranteed in the case of a disturbed communication session. According to EN 50 170, in the case of a disturbed communication session, this session is repeated between once (1 time) and fifteen times (15 times) in the same bus cycle. This leads to the duration of the bus cycle being extended by the duration resulting from the repetition of the disturbed communication session. Equidistance of individual communication sessions over a number of bus cycles—as is required, in particular, for critical sampled-data control systems—cannot be guaranteed with the known communication method in the case of communication and/or transmission disturbances. 
     The reason for this is that each communication session which is executed after the disturbed communication session in time in the bus cycle is offset in time in comparison with a “normal” bus cycle, i.e. a bus cycle without disturbed communication session. This also lowers the quality of a sampled-data control system. In the extreme case, even the stability of the sampled-data control system can be put in question. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is based on the object, therefore, of specifying a communication method by which equidistant sampling of a measurement value of the technical process is also still possible in the case of transmission disturbances. 
     For this purpose, it is provided in a method for cyclic communication between communication stations provided for controlling or monitoring a technical process, via a bus, in which communication sessions planned for the communication stations are executed during in each case one bus cycle of predeterminable duration, that, in the case of a disturbed communication session, its retransmission is planned for a subsequent bus cycle and that the disturbed communication session is acknowledged with a special acknowledgement code. According to an alternative, no retransmission of the disturbed communication session takes place after a disturbed communication session which is acknowledged with a special acknowledgement code. This can be tolerated since, in the case of a cyclic communication, a retransmission is replaced by the communication session of the next cycle. In this arrangement, it is possible to adjust whether no retransmission at all, one, two etc. retransmissions of the disturbed communication session are to take place. 
     The advantages achieved by the method relate, in particular, to the duration of the bus cycle with the disturbed communication session remains unaffected by retransmission and/or correction measures. This is achieved by displacing a retransmission of a disturbed communication session into a subsequent bus cycle. In the case of an immediate retransmission of a disturbed communication session in the same bus cycle, the starting time of all communication sessions following the disturbed communication session in the bus cycle is displaced with reference to the bus cycle. Equidistance of these displaced communication sessions over a number of bus cycles would no longer be guaranteed. 
     If values which are included in a sampled-data control system are transmitted by the displaced communication sessions, the sampling frequency of the sampled-data control systems concerned is no longer constant which is accompanied by a deterioration in the quality of the sampled-data control system and possibly even instability of the sampled-data control system. By displacing the retransmission of a disturbed communication session into a subsequent bus cycle, the starting times of the subsequent communications essentially remain unaffected. Thus, the equidistance of the individual communication sessions is also guaranteed over a number of bus cycles so that the communication method is also suitable for critical sampled-data control systems. 
     So that no retransmission and/or correction measures take place in the bus cycle with the disturbed communication sessions, the disturbed communication session is acknowledged with a special acknowledgement code. This leads to the omission of, in particular, transmission and/or correction measures so that the remaining communication sessions planned for the bus cycle can be executed. 
     If further disturbed communication sessions occur in the same bus cycle, the remaining disturbed communication sessions are also dealt with like the first disturbed communication session. i.e. its retransmission, too, is displaced into a subsequent bus cycle. 
     So that the retransmission of a disturbed communication session in a subsequent bus cycle does not impair the equidistance of the communication sessions planned for this subsequent bus cycle, the retransmission of the disturbed communication session takes place following the communication sessions planned for this bus cycle. Due to the fact that the retransmission of the disturbed communication session takes place following the communication sessions planned for this bus cycle, their starting times, based on the bus cycle, remain unaffected. The equidistance of the communication sessions over a number of bus cycles is thus guaranteed even during the retransmission of the disturbed communication session. 
     The retransmission of the disturbed communication session is advantageously planned for a bus cycle immediately following the bus cycle with the disturbed communication session. As a result, the time difference between the disturbed communication session, the time of the planned execution and the retransmission, the time of the actual execution, remains as small as possible. 
     If the disturbed communication session is acknowledged with the special acknowledgement code like a faultless communication session, a delay in the execution of the communication sessions following in the bus cycle is avoided. It is only in the case of a communication session which has been executed faultlessly or acknowledged as faultless that the execution of the communication sessions following in the bus cycle can be continued. Due to the acknowledgement of the disturbed communication session with the special acknowledgement code, this session cannot be distinguished from a faultless communication session. First, the beginning of the execution of the communication sessions following in the bus cycle is not delayed by any evaluation of a special acknowledgement code. 
     The special acknowledgement code is advantageously converted into a normal acknowledgement code. This reduces the number of possible acknowledgement codes to be checked so that, in spite of the existence of an acknowledgement code which is additional in fact, the complexity of the analysis of the possible acknowledgement codes is not increased. The normal acknowledgement code is the acknowledgement code which identifies a communication session which has been faultlessly executed. The special acknowledgement code can be converted into a normal acknowledgement code, e.g. by so-called “masking”. If, e.g., the special acknowledgement code only differs from the normal acknowledgement code due to the first bit being set, the masking can begin by a logical AND operation on the special acknowledgement code, e.g. with the hexadecimal value “7FFF”. 
     If a retransmission counter is provided by which the number of retransmissions of the disturbed communication session is counted, it can be determined at any time how long the disturbance of the communication session already exists. 
     If a limit value is provided for the retransmission counter and any retransmission of the disturbed communication session is acknowledged with a fault acknowledgement code after the limit value has been reached, a disturbance of a communication session which lasts “too long” can be detected. 
     The limit value from which each retransmission of the disturbed communication session is acknowledged with a fault acknowledgement code can be suitably predetermined. Thus, the time interval after the passage of which a disturbance of a communication session exists for “too long” can be predetermined. The limit value can be preferably predetermined individually for each communication session. Thus, the limit value for a communication session which, e.g., supplies data for a critical first sampled-data control system can be set to a different limit value than that of a communication session by which only record data are transmitted. 
     If the retransmission counter can be read out by the communication station involved in the disturbed communication session, it can determine at any time whether and possibly for how long a communication session is disturbed. 
     If at least one threshold value is provided for the retransmission counter and planned measures are initiated by the communication station reading out the retransmission counter when the threshold value is reached, it is also possible to respond to the disturbance of the communication session in a suitable manner even before the limit value is reached. For example, when a first threshold value is reached, a corresponding note can be output, for instance on a screen or printer, when a second threshold value is reached a visual or oral warning can be initiated and when a third threshold value is reached, fault-limiting measures can be initiated. The fault-limiting measures can relate to, e.g., attempting to establish the communication session on a different path—for example with a redundant bus—, or placing the technical process—possibly the part process controlled by the communication station—into a safe state. 
     Advantageously, a structure is provided in a memory of at least one communication station which has for each communication session one field in which the value of the retransmission counter is stored in a first position, the limit value is stored in a second position and the at least one threshold value, together with a reference to the measure to be initiated when the threshold value is reached, is stored in a third position. The structure is used for the compact storage of the essential data which are provided for executing the communication method. Storing a reference to the measure to be initiated when the threshold value is reached makes it possible to directly call up a program routine in which the measure is programmed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  shows communication stations communicatively connected via a bus for controlling a technical process, 
         FIGS. 2   a  and  2   b  show communication sessions between individual communication stations, 
         FIG. 3  shows faultlessly executed communication sessions during two bus cycles, 
         FIG. 4  shows a message retransmission as a consequence of a disturbed communication session, 
         FIG. 5  shows a message retransmission in a later bus cycle, 
         FIG. 6  shows a layout of a data structure, and 
         FIG. 7  shows a flowchart. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  shows communication stations  1 ,  2 ,  3 , the communication station designated by the reference symbol  1  being a so-called master  1 —e.g. a stored-program control—and the communication stations designated by the reference symbols  2 ,  3  being so-called slaves  2 ,  3 —e.g. decentralized peripheral devices. The communication stations  1 ,  2 ,  3  are communicatively connected to one another via a bus  4 . 
     The master  1  is a communication station which has an active transmit authorization on the bus  4 . A slave  2 ,  3 , in contrast, only transmits after having first been addressed by the master  1 . Slaves  2 ,  3  are, therefore, lacking the active transmit authorization because they only respond to a request (being addressed) by the master  1 . 
     The communication stations  1 ,  2 ,  3  are provided for controlling or monitoring a technical process  50  shown diagrammatically. The technical process  50  comprises a reactor  51  with an inlet  52  and an outlet  53 . The reactor  51  is fed by the inlet  52 . A reagent  54  leaves the reactor  51  via the outlet  53 . The inlet  52  is controlled by a valve  55 . A filling level meter  56  is used for determining a filling level  54 ′ of the reactor  51 . 
     A simple control and/or monitoring (automation) of the technical process  50 , which will be used by way of an example in the text which follows, relate to an assumption that the valve  55  is controlled with a view to a constant filling level  54 ′ of the reactor  51 . 
       FIGS. 2   a  and  2   b  show the data exchange or, respectively, communication sessions  12 ,  21 ,  13 ,  31  between communication stations  1 ,  2 ,  3  for this automation. The data exchange takes place under control of a program  6 , stored in a memory  5 , which is executed by the master  1 . For this purpose, the program  6  comprises a task  7 ,  7 ′ which is executed in a fixed timing pattern and is called up, e.g., every 500 ms. With each execution of the task  7 ,  7 ′, a data exchange  12 ,  21  and  13 ,  31 , respectively, takes place between the master  1  and the slave  2  ( FIG. 2   a ) and between master  1  and slave  3  ( FIG. 2   b ). 
     In accordance with a planned communication session, a data exchange takes place between the relevant communication stations  1 ,  2 ,  3 . The data exchange takes place by a message  12 ,  21 ,  13 ,  31 . In the text which follows, therefore, the terms communication/communication session and message are used synonymously. If the master  1  addresses a slave  2 ,  3 , this takes place by a message  12 ,  13 . The slave  2 ,  3 , in turn, responds to this stimulus with a message  21 ,  31 . 
       FIG. 2   a  shows a message  12  sent by the master  1  to the slave  2 . The message  12  comprises e.g. output and control data in the form of digital and/or analog values, e.g. a maximum value for the filling level  54 ′. However, the message  12  also causes the slave  2  to send its input data to the master  1  in a message  21 . The message  21  thus contains, in particular, a value representing the filling level  54 ′ of the reactor  51  which is picked up by the filling level meter  56 . The message  12  implicitly issues to the slave  2 , which is not actively authorized to transmit, an authorization for transferring the data requested by the master  1 . This is done by the message  21 . 
       FIG. 2   b  shows a message  13  by which the master  1  sends to the slave  3  the respective output data. The message  13  comprises output and/or control data in the form of digital or analog values, among them also a value predetermining the position of the valve  55 . The message  13  causes the slave  3  to send its input data, e.g. also a value representing the actual volume flowing through the inlet  12 , to the master  1  in a message  31 . 
     A message  12 ,  13 ,  21 ,  31  is always acknowledged  12 ′,  13 ′,  21 ′,  31 ′. The faultless undisturbed transmission of a message  12 ,  13 ,  21 ,  31  is acknowledged by a normal acknowledgement which specifies that the communication  12 ,  13 ,  21 ,  31  has been executed faultlessly. The normal acknowledgement comprises, e.g., a normal acknowledgement code (not shown) with the value “0x00”. 
     Automation of the technical process  50  ( FIG. 1 ) requires a closed-loop control for keeping the filling level  54 ′ of the reactor  51  constant. Since the message  21  comprises the measurement value of the filling level  54 ′ and the latter is thus “sampled” only when the slave  2  transmits the corresponding message  21  to the master  1 , this is a sampled-data control system. The quality/stability of such a sampled-data control system is mainly determined by the time interval between two samples of a process parameter (in this case filling level  54 ′). To achieve stable closed-loop control, the basic mathematical control models require sampling at equidistant times. 
       FIG. 3  diagrammatically shows the times t 1   n , t 2   f , t 1   n+1 , t 2   n+1  cluttered along a time axis, at which the messages  21 ,  13  are transmitted to and from the corresponding slave  2 ,  3 . 
     The slave  2  transmits the current filling level  54 ′ of the reactor  51  to the master  1  with the message  21 . At times t 1   n , t 1   n+1 , the process parameter “filling level”  54 ′ is thus sampled. The new position of valve  55  is predetermined by master  1  for slave  3  with message  13 . Messages  12  ( FIG. 2   a ) and  31  ( FIG. 2   b ) are not shown for reasons of clarity. 
     To obtain stable control of the filling level  54 ′, it is important that the time interval between two successive times t 1   n , t 1   n+1 , at which the filling level  54 ′ is sampled, remains constant. The time difference between time t 1   n  and t 2   n+1 , i.e. the picking up of the filling level  14 ′ and the outputting of the resultant control value to the process  50 , represents a dead time which can be easily taken into consideration and compensated for mathematically. 
     In the case of an undisturbed faultless communication, the equidistance between two successive times t 1   n , t 1   n+1  is guaranteed by the fixed timing pattern in which the task  7 ,  7 ′ ( FIG. 2   a ,  2   b ) is executed under the control of which the communication sessions are executed. 
     The time interval Δt designates the duration of a bus cycle, the terms bus cycle and duration of a bus cycle being used synonymously in the text which follows. The duration of a bus cycle (bus cycle time) Δt is constant. During a bus cycle Δt, all planned communication sessions are executed. If the starting time of task  7 ,  7 ′ ( FIG. 2   a ,  2   b ), under the control of which the data exchange  21 ,  13  between the master  1  and the slaves  2 ,  3  is executed, falls into a bus cycle Δt, the communication sessions  21 ,  13  belong to the communication sessions planned for this bus cycle Δt. 
       FIG. 3  shows two bus cycles Δt which in each case comprise the communication sessions  21 ,  13 . These two bus cycles Δt do not follow one another directly in time—indicated by the broken timeline. Between the two bus cycles shown, one or more other bus cycles are executed which do not include the communication sessions  21 ,  13 . 
       FIG. 4  shows the effect of a disturbed communication session  21   s  on the duration of a bus cycle Δt. In the second bus cycle Δt s  shown, a disturbance which impairs the execution of the communication session  21  occurs at time t 1   n+1 . The disturbed communication session  21   s  is acknowledged with a fault acknowledgement  21 ″. Each acknowledgement  21 ′,  21 ″ comprises an acknowledgement code  21 ′,  21 ″ so that this fault acknowledgement  21 ″, too, comprises a fault acknowledgement code  21 ″ which unambiguously species the type of the fault. The disturbed communication relation  21   s  is followed by a retransmission  21   w  of the disturbed communication session. The first retransmission  21   w  of the disturbed communication session cannot be executed faultlessly, either, and is, therefore, acknowledged with a fault acknowledgement  21 ″. It is only after the second retransmission  21   w  that it can be faultlessly executed. The faultlessly executed second retransmission  21   w  is correspondingly acknowledged with a normal acknowledgement  21 ′. The normal acknowledgement  21 ′ comprises a normal acknowledgement code  21 ′ which specifies the faultless execution. 
     Overall, the communication session  13  is thus executed correspondingly later in time, namely only at time t 2 ′ n+1 . In the case of a bus cycle Δt ( FIG. 3 ) which is not encumbered by a disturbed communication session, in contrast, the execution occurs at time t 2   n+1 . The time offset by which the communication session  13  is executed later corresponds to the duration of the two retransmissions  21   w  of the disturbed communication session  21   s . 
     The new value for the position of the valve  55  ( FIG. 1 ) is transmitted with the message  13 . The intervention in the closed-loop control is thus delayed, i.e. no longer at equidistant times so that it may no longer be possible to keep the filling level  54 ′ ( FIG. 1 ) constant. The more dynamic the controlled system the stronger this will affect the quality of the closed-loop control. In the extreme case, even the stability of the closed-loop control can be put in question. Furthermore, the duration Δts of the bus cycle with the disturbed communication  21   s  is extended in comparison with the duration Δt of the bus cycle with the faultlessly executed communication  21 . This leads to communication sessions executed in each bus cycle also no longer being equidistant. 
       FIG. 5  shows how equidistance is guaranteed even in the case of a disturbed communication session  21   s . It shows two bus cycles Δt s , Δt immediately following one another. In the first bus cycle Δt s , a disturbance  21   s  occurs. A third bus cycle Δt is executed later in time—indicated by the broken timeline. 
     Analogously, a disturbance  21   s  occurs during the execution of the communication session  21 —at time t 1   n+1 . The disturbed communication session  21   s  is acknowledged with a special acknowledgement  21 ′ which comprises a special acknowledgement code  21 ′ like a faultlessly executed communication session. i.e., the special acknowledgement code  21 ′ is converted into a normal acknowledgement code  21 ′ or the special acknowledgement code  21 ′ is evaluated like a normal acknowledgement code  21 ′. Thus, there is no immediate retransmission of the disturbed communication session  21   s  in the same bus cycle Δt s . On the contrary, the retransmission of the disturbed communication session  21   s  is planned for the next bus cycle Δt. 
     Thus, there is no time offset in the transmission of the new value for the position of the valve  55  by the message  13  even in the bus cycle Δt s  with the disturbed communication session  21   s . The message  13  is still executed at time t 2   n+1  as before. 
     The retransmission  21   w  of the disturbed communication session  21   s  is executed at time t 1   n+1   w  in the bus cycle Δt following immediately. For such a retransmission  21   w , a special communication section t 30  is provided at the end of each bus cycle Δt. Planned communication sessions  13 ,  21  are executed at the beginning of each bus cycle Δt in a normal communication section t 20 . 
     The number of retransmissions  21   w  of a disturbed communication session  21   s  is counted in a retransmission counter  111  according to  FIG. 6 . If the retransmission counter  111  reaches a predetermined limit value  112 , each retransmission  21   w  of the disturbed communication session  21   s  is acknowledged with a fault acknowledgement (not shown) which comprises a fault acknowledgement code, when the limit value  112  is reached. Thus, permanently disturbed communication sessions can be recognized as such and the communication station which can no longer be reached can be flagged as failed. 
     Dividing a bus cycle Δt into the normal communication section t 20  and the special communication section t 30  leads to a decoupling between messages  13 ,  21  of correspondingly planned communication sessions and message retransmissions  21   w  due to disturbed communication sessions  21   s . The message retransmission  21   w  in bus cycle Δt—i.e. in the second bus cycle in FIG.  5 —is decoupled from planned communication sessions (shown dashed) to be executed in the normal communication section t 20 . 
     Since the duration of the bus cycle Δt is predetermined and constant, either the duration of the normal communication section t 20  or the duration of the special communication section t 30  is also predetermined. If the duration of the normal communication section t 20  is predetermined, the fixed bus cycle time Δt will produce the duration of the special communication section t 30  and vice versa. The duration of the normal communication section t 20  and special communication section t 30  is dimensioned in such a manner that at least one message retransmission  21   w  can take place during the special communication section t 30 . 
       FIG. 6  shows a structure  100  which is provided in the memory  5  ( FIG. 2   a ,  2   b ) of a communication station  1 ,  2 ,  3 . The structure  100  has a separate field  110 ,  120 ,  130  for each communication session—at least for each communication session in which the relevant communication station  1 ,  2 ,  3  is involved. In each field  110 ,  120 ,  130 , the value of the retransmission counter  111 ,  121  is stored in a first position, the limit value  112 ,  122  is stored in a section position and the at least one threshold value  114 ,  124 , together with a reference  115 ,  125  to the measure to be initiated when the threshold value  114 ,  124  is reached, is stored in a third position  113 ,  123 . 
     The structure is used for compact storage of the essential data which are provided for executing the communication method. Storing a reference  115 ,  125  to the measure to be initiated when the threshold value  114 ,  124  is reached enables a program routine to be called up directly in which the measure is programmed. 
     The omission points “ . . . ” in the structure  100 , on the one hand, and, on the other hand, in the field  130  indicate that the structure can comprise other fields  110 ,  120 ,  130 , depending on the number of communication sessions, and that the field  130 , like any other fields, basically has the same layout as the field  110 ,  120 . 
       FIG. 7  shows in a flowchart an algorithm for essential aspects of the communication method, which begins in step  1001  if the retransmission  21   w  of the disturbed communication session  21   s  could not be executed faultlessly either. In step  1010 , the retransmission counter  11 ,  121  ( FIG. 6 ) is incremented with each retransmission  21   w  of a disturbed communication session  21   s  ( FIG. 5 ). 
     In step  1020 , a check is made whether the retransmission counter  11 ,  121  has reached the limit value  112 ,  122  ( FIG. 6 ). If this is so, the system branches to step  1040  and the retransmission  21   w  of the disturbed communication session is acknowledged with a fault acknowledgement code  21 ″ ( FIG. 4 ). After execution of step  1040 , the algorithm is ended in step  1002 . If it is found in step  1020  that the retransmission counter  11 ,  121  has not yet reached the limit value  112 ,  122 , the algorithm is continued in step  1030 . 
     In step  1030 , the retransmission  21   w  of the disturbed communication relation  21   s  is acknowledged with the special acknowledgement code  21 ′ ( FIG. 5 ). The acknowledgement of the retransmission  21   w  of the disturbed communication session  21   s  with the special acknowledgement code  21 ′ has the effect that a failed retransmission  21   w  of the disturbed communication session  21   s  is also treated like a faultlessly executed communication and a next retransmission  21   w  is planned for a following bus cycle. 
     In step  1050 , a check is made whether the retransmission counter  111 ,  121  has reached the threshold value  114 ,  124  (FIG.  6 )—possibly one of a number of threshold values. If this is so, the system branches to step  1060  and a measure is triggered, the reference (address) of which is stored at position  115 ,  125  ( FIG. 6 ). If, e.g., the limit value  112 ,  122  is set to the value “20”, the threshold value  114 ,  124  can be set e.g. to value “10”. I.e. after ten unsuccessful retransmissions  21   w  of a disturbed communication session  215 , the threshold value  114 ,  124  is reached and a corresponding measure  115 ,  125  can be initiated. This measure  115 ,  125  can include, e.g., outputting a warning message on a display device (not shown) in order to indicate the disturbed communication session  21   s . The actual measure is implemented as program routine (subroutine). 
     As a reference  115 ,  125 , its start address is stored at position  115 ,  125 . When the threshold value is reached, the measure  115 ,  125  can be triggered directly on the basis of the stored reference  115 ,  125 . After step  1060  has been executed, the algorithm is ended in step  1002 . If it is found in step  1050  that the retransmission counter  111 ,  121  has not yet reached the threshold value  114 ,  124 , the algorithm is ended immediately in step  1002 . 
     The algorithm is started every time in step  1001  even if the retransmission  21   w  of the disturbed communication session  21   s  could not be executed faultlessly. If, in contrast, the first retransmission  21   w  of the disturbed communication session  21   s  has already been faultlessly executed, this is acknowledged with the normal acknowledgement code  21 ′ analogously to step  1030 . In this case, evaluation of the limit or threshold value  111 ,  121  and  114 ,  124  is not required. 
     Thus, a method for cyclic communication between communication stations  1 ,  2 ,  3 , provided for controlling or monitoring a technical process  50 , via a bus  4 , is specified in which communication sessions  12 ,  13 ,  21 ,  31 , which have been planned for the communication stations  1 ,  2 ,  3 , are executed during in each case one bus cycle of predeterminable duration Δt. In the case of a disturbed communication session, its retransmission  21   w  is planned for a subsequent bus cycle and the disturbed communication session is acknowledged with a special acknowledgement code. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.