Title of Invention	MASTER UNIT FOR A COMMUNICATION SYSTEM HAVING A MASTER-SLAVE STRUCTURE, COMMUNICATION SYSTEM AND METHOD FOR OPERATION THEREOF
Abstract	In read mode, a transmit control unit 16 of a master unit 1 transmits to a first transmitting unit 11 and a second transmitting unit 12 an information signal with a data field which has an associated data area for each connected user 3. The first transmitting unit 11 and the second transmitting unit 12 separately transmits the information signal in opposite directions on a first communication path 21 and a second communication path 22, wherein a processing unit 35 of each user writes into the associated data area during the processing of the information signal passing through. A receive control unit 18 of the master unit 1 superimposes the two information signals received by a first receiving unit 13 on the first communication path 21 and a the second receiving unit 14 on the second communication path 22.

Title of Invention

MASTER UNIT FOR A COMMUNICATION SYSTEM HAVING A MASTER-SLAVE STRUCTURE, COMMUNICATION SYSTEM AND METHOD FOR OPERATION THEREOF

Abstract

In read mode, a transmit control unit 16 of a master unit 1 transmits to a first transmitting unit 11 and a second transmitting unit 12 an information signal with a data field which has an associated data area for each connected user 3. The first transmitting unit 11 and the second transmitting unit 12 separately transmits the information signal in opposite directions on a first communication path 21 and a second communication path 22, wherein a processing unit 35 of each user writes into the associated data area during the processing of the information signal passing through. A receive control unit 18 of the master unit 1 superimposes the two information signals received by a first receiving unit 13 on the first communication path 21 and a the second receiving unit 14 on the second communication path 22.

Full Text	Description The invention relates to a user and to a master unit in a communication system having a number of users which are connected to one another via a dual-ring structure operating in opposite directions, to such a communication system having a master-slave structure and to a method for operating the user and the communication system, respectively. In production and automation technology, serial bus systems are increasingly used in which the remotely arranged devices of machine peripherals such as I/O modules, transducers, drives, valves and operator terminals communicate with automation, engineering or visual display systems via an efficient real-time communication system. In this arrangement, all users are networked together via a serial bus, preferably via a field bus, the data exchange via the bus being carried out, as a rule, on the basis of the master- slave principle. The active bus users on the bus system, the control devices, as a rule, possess a bus access authorization and determine the data transfer on the bus. The active bus users are called the master units in the serial bus system. In contrast, passive bus users are, as a rule, machine peripheral devices. They do not receive a bus access authorization, i.e. they are only allowed to acknowledge received information signals or transfer information signals to a master unit on request by the latter. Passive bus users are called slave units in the serial bus system. To avoid complex cabling, field bus systems having a master-slave structure are generally arranged in ring topology, all bus users being connected to a ring- shaped transmission path. An information signal generated by the master unit is fed into the ring- shaped transmission path by the master unit and successively passes through the slave units serially connected to the ring-shaped transmission path and is then received again by the master unit and evaluated. Master-slave systems can also be designed as multi- master systems. As a rule, the information signals are organized by the master unit into data packets which are composed of control data and useful data, preferably using the Ethernet standard which provides for data packets having a length of up to 1500 bytes with a transmission speed which, at the same time, is high at 100 Mbit/sec. Each of the slave units connected to the ring-shaped transmission path exchanges the useful data intended for it with the Ethernet message when the Ethernet message fed in by the master unit passes through on the ring-shaped transmission path. As a rule, the master-slave communication systems with ring structure are configured in such a manner that the master unit has a transmitting unit as data injection point and a receiving unit as data extraction point into a transmission medium. The individual slave units are then connected together on the transmission path to form a chain, wherein each user is connected to two neighbors and the first and last user in the chain is connected to the master unit. The data packets are transmitted in one direction starting from the master unit via its transmitting unit to the first slave unit connected and from there to the next one, until the last slave unit in the chain is reached, and then back to the receiving unit of the master unit. Each slave unit has, for receiving the circulating data packets from the previous user, an interface with a receiving unit and, for forwarding to the following user, an interface with a transmitting unit, a processing unit being arranged between receiving and transmitting unit in order to process the data packets passing through the slave unit, i.e. to exchange the useful data allocated to the slave unit with the data packets. The ring-shaped communication system with master-slave structure is often designed in such a manner that the master unit forms a physical line with the slave units arranged at it, the transmission medium having a dual- line structure and each slave unit having two ports with a combined transmitting/receiving unit, transmitting and receiving unit being short-circuited in the output port of the last slave unit in the transmission chain. The data packets injected into the first line by the master unit via its receiving unit are processed by the slave units on the forward path and are then simply forwarded only to the receiving unit of the master unit on the return path via the second line. A central requirement for master-slave communication systems, particularly when they are used in production and process automation, is a high fault tolerance, that is to say the capability of the communication system to maintain the required function, i.e., for example, the production of a workpiece, in spite of the occurrence of faults. In this context, faults in the communication system which must be overcome without impairment of the process are, in addition to faults in the data packets, also the failure of entire transmission links, in particular, for example due to physical separation of the transmission medium. To achieve a fault-tolerant master-slave communication system, particularly in the case of link faults, i.e. in the case of the failure of entire transmission sections, dual-ring structures operating in opposite directions are frequently used. Thus, a communication system having a master-slave structure in which the master unit is serially connected to a multiplicity of slave units via two communication paths operating in opposite directions, is described in US 4,663,748, wherein the master unit simultaneously sends out the data packets over two communication paths. The slave unit then has two processing units which are in each case connected between the two communication paths in order to process the data messages passing through. Furthermore, coupling units which can be activated are arranged in the users, so that when a link fault occurs, e.g. a break in a communication line, it reconfigures the communication system by monitoring the signals on both transmission rings and correspondingly switching over the communication system, in such a manner that a failure due to the link fault of a greater section of the communication system or even a total failure is prevented. In DE 103 12 907 A1, it is also proposed to arrange the slave unit in such a manner that on each communication path in the direction of data transmission, first a processing unit and then a multiplexer having two inputs and one output is arranged. The multiplexer is connected with its inputs in each case to the two processing units of the slave unit and connected with its output to the associated communication path. In fault-free normal operation, each of the two multiplexers switches through the processing unit arranged on the associated communication path. In fault mode, when a link fault occurs on the associated communication path, however, the processing unit on the other communication path is then switched through. This design of the slave unit enables the communication system to be reconfigured essentially in real time in the fault case. However, fault-tolerant master-slave communication systems having a dual-ring structure, in which the individual slave units in each case have two processing units for processing the data message passing through, provide for high hardware and switching complexity of the slave units and thus increase the cost. Furthermore, each slave unit must decide in normal operation which of the two data packets passing through the two processing units should be used for device control which greatly restricts the use of such communication systems at the required high data transmission rates. In addition, the known fault- tolerant communication systems with dual-ring topology require that the master unit responds separately to a link fault and switches from normal operation into fault operating mode. From US 2004/0008719, a master unit for a master-slave communication system having the features of claim 1 and a method for operating a master-slave communication system having the features of the preamble of claim 9 is known. A communication system having a fault- tolerant dual-ring structure is also represented in US 4,527,270, EP 0 605 795 A2 and GB 2 348 782 and EP 1 271 854. It is the object of the invention to provide a master unit for a master-slave communication system and a method for operating a master-slave communication system which provide for reconfiguration measures in real time on the occurrence of link faults in a fault- tolerant dual-ring topology, with minimum hardware and switching complexity in the master unit. This object is achieved by a master unit as claimed in claim 1 and a method 'as "claimed in claim 9. Preferred developments are specified in the dependent claims. According to the invention, a user in a communication system having a number of users which are connected to one another via a first communication path and a second communication path, the two communication paths operating as dual-ring structure in opposite directions to one another, is designed in such a manner that the user has in each case one receiving unit for receiving information signals on the associated communication path, and in each case one transmitting unit for transmitting information signals on the associated communication path, for each communication path. In addition, a single processing unit, having an input and an output, for processing information signals passing through the user, and a coupling unit which can be activated is provided. In normal operation, this coupling unit which can be activated connects the first receiving and transmitting unit belonging to the first communication path, the processing unit being interposed for processing the information signals passing through, and the second receiving and transmitting unit belonging to the second communication path, to one another. In the fault case of the first transmitting unit and/or the second receiving unit, i.e. when a link fault to the user occurs, the coupling unit which can be activated then connects the first receiving unit to the second transmitting unit via the processing unit and in fault mode of the first receiving unit and/or of the second transmitting unit, i.e. again when a link fault occurs to the adjoining user, the second receiving unit to the first transmitting unit via the processing unit. With this layout of the user in a fault-tolerant communication system with a dual-ring topology, a user reconfiguration can be carried out in real time when link faults occur to adjacent users on one or on both communication paths, in order to provide for faultless operation in spite of the link fault. Thus, the operation of the user, and thus the operation of the communication system to which the user is connected, can be maintained in spite of the link fault. The design according to the invention has the advantage that only a single processing unit is provided in each user which reduces the hardware complexity, and thus the costs. In addition, the behavior of the user in the case of redundancy, that is to say when a link fault occurs, does not differ from the behavior in normal operation since the information signals passing through are always interpreted and processed by the one processing unit present when passing through. This, at the same time, provides for a wide dynamic range during the switching process and thus for meeting the real- time requirements for the communication system. It is preferred in this context to design the coupling device which can be activated of the user in the form of a first multiplexer, the first input of which is connected to the first receiving unit, the second input of which is connected to the second receiving unit and the output of which is connected to the processing unit, and of a second multiplexer, the first input of which is connected to the second receiving unit, the second input of which is connected to the processing unit and the output of which is connected to the second transmitting unit, the first multiplexer, in normal operation, connecting its first input to its output and, in fault mode of the first receiving unit and/or the second transmitting unit, connecting its second input to its output, and the second multiplexer, in normal operation, connecting its first input to its output and in fault mode of the first transmitting unit and/or of the second receiving unit connecting its second input to its output. The design according to the invention of the coupling device which can be activated, with two 2-1 multiplexers which precede or follow the processing unit, ensures, in the fault case, that, depending on the position of the link fault occurring, the information signal is always conducted through the user in such a manner that the information signal, after passing through the processing unit, is fed back. Using 2-1 multiplexers only entails a slight hardware complexity and, in addition, provides for switching between normal operation and fault mode in a simple and highly dynamic manner. According to the invention, a master unit in a communication system having a master-slave structure, which has a multiplicity of users as slave units, is arranged in such a manner that the slave units are connected serially with the master unit via the dual- ring structure formed from the first and the second communication path and operating in opposite directions. In this arrangement, the master structure has for each communication path an associated transmitting and receiving unit, the transmitting units being connected to a transmit control unit and the receiving units being connected to a receive control unit. In this arrangement, the transmit control unit transfers to the two transmitting units of the master unit an information signal with a data field and a counter field, set to a predetermined value, for separately transmitting in opposite directions on the first and on the second communication path. When the information signal passes through, the processing units of each user alter the value of the counter field by a predetermined value. The control unit then evaluates the value of the counter fields of the two information signal received on the first and the second communication path by the receiving units. Given the design of the communication system according to the invention, the master unit has in a simple manner the capability of determining the freedom from faults in the communication system, especially in the case of a reconfiguration of the communication system after the occurrence of a link fault. After evaluation of the counter fields after reception of the two identical information signals circulating in opposite directions via the communication paths, the master unit can determine how many of the slave units connected are in operation. This is because the processing units in the active slave units in each case alter the value of the counter field of the information signal passing through with the aid of their processing unit, as a result of which conclusions regarding the operability of the slave units can then be drawn on evaluation of the counter fields of the two information signals received in the master unit. By correlating the values in the counter fields of the two information signals received, it can also be determined between which users the link fault has occurred in the communication system or, respectively, whether or where a user has completely failed. In this context, the counter fields of the two circulating information signals received on the first and second communication path are preferably evaluated by adding together the counter field values. The total value then immediately specifies whether all connected slave units are active, since their number is directly reflected in the aggregate value. According to the invention, the fault tolerance of the communication system is increased further in a simple manner due to the fact that in read mode, i.e. when the slave units are intended to transmit data to the master unit, an information signal with a data field which has an associated data area for each connected user at the communication system is transferred to the two transmitting units by the transmit control unit of the master unit. These two information signals are then sent separately and in opposite directions on the first and the second communication path, the users writing into the associated data area when the information signal passes through. The receive control unit of the master unit then superimposes the data fields of the two information signals received on the first communication path and the second communication path. Given this procedure, the operability of the communication system guarantees reliable read mode in a simple manner, particularly also in the case of redundancy, that is to say when the communication system is reconfigured due to a link fault and individual users have switched to fault mode. This is because, by superimposing the data fields of the two information signals received, i.e. particularly by oring them, a combined data field is generated in which regardless of how the information signals are fed back to the master unit via the communication paths, all data to be transmitted by the users are contained. Thus, the procedure according to the invention provides for high fault tolerance, particularly in the case of link faults in the dual-ring topology, in a simple manner. Furthermore, the evaluating method of the master unit in reconfiguration mode does not differ from that in normal mode. The invention will be explained in greater detail with reference to the attached drawings, in which: figure 1 shows a diagrammatic representation of a communication system according to the invention having a master-slave structure, wherein figure 1A represents normal mode, figure 1B represents a first communication system reconfiguration on the occurrence of a dual link fault, and figure 1C represents a second communication system reconfiguration in the case of the failure of a slave unit; and figure 2 shows a diagrammatic representation of a user according to the invention. In automation technology, field bus systems are increasingly used in which devices of the machine peripherals, arranged in distributed manner, communicate with automation, engineering and visual display systems via a field bus. As a rule, the field bus system has a serial bus which can be, for example, an electrical line, an optical waveguide or a radio cable. All bus users are then connected to this field bus, a distinction being made between active bus users and passive bus users. The active bus users on the field bus system are the master units which determine the data traffic on the bus. Such a master unit is, for example, an industrial PC which is used as process control computer in a production process. This master unit has a bus access authorization and can output data to the field bus without external request. The passive bus users on the bus system are peripheral machine devices, for example I/O devices, valves, drives and transducers. They are used as slave units and do not obtain a bus access authorization, i.e. they are only allowed to acknowledge received information signals or to transmit information signals to a master unit on request by the latter. The communication standard used for data transmission in the master-slave system is preferably the Ethernet concept. In Ethernet communication systems, the data to be transmitted are encapsulated in data packets, also called messages in the further text, having a predetermined format. The Ethernet messages can have a data length of up to 1500 bytes containing, additionally to the useful data, control data which have a start identifier, a destination and source address, the data packet type and a fault mechanism. Ethernet communication systems having a master-slave structure are preferably designed in such a manner that the individual slave units are connected together via the transmission medium to form a chain, each slave unit being connected to two neighbors and the first and the last user in the chain being connected to the master unit so that a ring structure is obtained. In this arrangement, the data are transmitted in one direction starting from the master unit to the first adjacent slave unit and from there to the next one until the last slave unit and then back to the master unit. To ensure high fault tolerance, particularly in the case of a link fault in the communication system, i.e. the failure of entire transmission sections with slave units, e.g. due to a cable break, the communication systems having a master-slave structure often have two communication paths which operate in opposite direction to one another. Due to the dual-ring structure operating in opposite directions, the possibility exists in the case of link faults to carry out reconfiguration measures in the communication system in order to maintain the operability of the communication system in spite of link faults. Figure 1 shows in a basic circuit diagram such a fault- tolerant communication system in an embodiment according to the invention. The communication system has a master unit 1 which is connected serially to N slave units 3 via a dual-ring structure 2 . The dual- ring structure comprises two unidirectional communication paths 21, 22 which pass through the connected slave units 3 in opposite directions. The master unit 1 is connected to the first communication path 21 as a data extraction point via a first transmitting unit TX11 and to the second communication path 22 as data extraction point via a second transmitting unit TX12. Furthermore, the master unit 1 has a first receiving unit RX13 as data injection point for the first communication path 21 and a second receiving unit RX14 as data injection point for the second communication path 22. The first transmitting unit TXll and the second transmitting unit TX12 are connected to a transmit control unit 16 via a first control line 15. The first receiving unit RX13 and the second receiving unit RX14 are connected via a second control line 17 and to a receive control unit 18. Each slave unit 3, in turn, has an interface, for receiving messages from a preceding user via the first communication path 21, with a first receiving unit RX31 and an interface with a first transmitting unit TX32 for forwarding to the next user via the first communication path 21. Furthermore, each slave unit 3 has for receiving a circulating Ethernet message via the second communication path 22 from a preceding user an interface with a second receiving unit RX33 and, for forwarding to the following user, an interface with a second transmitting unit TX34. Between the first receiving unit RX31, the second receiving unit RX32, the first transmitting unit TX33 and the second transmitting unit TX34, a processing unit 3 5 and a coupling device 3 7 which can be activated, is also connected in each slave unit 3. The basic circuit diagram of a slave unit 3 is shown in greater detail in figure 2. In the slave unit 3, the first receiving unit RX31 which is connected to the first communication path 21, and the second transmitting unit TX34 which is connected to the second communication path 22, are grouped as port 0. The second receiving unit RX33 which is connected to a second communication path 22, and the first transmitting unit TX32 which is connected to the first communication path 21, are organized as port 1. The coupling device 37 which can be activated has a first change-over switch 38 and a second change-over switch 39 which are in each case designed as 2-1 multiplexers. The receiving and transmitting unit 31, 32, 33, 34, the multiplexers 38, 39 of the coupling device 37 which can be activated, and the processing unit 3 5 are interconnected in this arrangement in the manner shown by arrows in figure 2 by a line network 40. The output of the first receiving unit RX31 is connected to the first input of the first multiplexer 38. The second input of the first multiplexer 38 is connected to the second receiving unit RX33. The output of the first multiplexer 3 8 is also connected to the processing unit 35. The second multiplexer 39, in turn, is connected with its first input to the second receiving unit RX33 and with its second input to the output of the processing unit 35. The output of the second multiplexer 39 is connected to the second transmitting unit TX34. In addition, the output of the processing unit 35 is also connected to the first transmitting unit TX32 via the line network 40. In the case of fault-free normal operation of the communication system as shown in figure 1A, the transmit control unit 16 of the master unit transfers to the first transmitting unit TXll and the second transmitting unit TX12 a message which is then sent by the two transmitting units simultaneously in opposite directions via the first communication path 21 and the second communication path 22. In this process, the messages pass in opposite directions through the connected slave units 3, all coupling devices 37 which can be activated in the slave units 3 being connected in such a manner that the input of the processing unit 35 is connected to the first receiving unit RX31, the output of the processing unit 3 5 is connected to the first transmitting unit TX32 and the second receiving unit RX33 is connected to the second transmitting unit TX34. In this operating mode of the slave units 3, the coupling device 37 which can be activated ensures that the two identical messages circulating in opposite directions on the first communication path 21 and on the second communication path 22 always pass through the slave unit in such a manner that only the messages transmitted via the first communication path 21 are processed by the processing unit 35. In contrast, the message circulating on the second communication path 22 is only passed through by the slave units 3 and arrives again unprocessed at the master unit 1. The two messages circulating in opposite directions via the first and second communication path 21, 22 are in this case recognized by the first receiving unit RX13 and the second receiving unit RX14 of the master unit 1 and forwarded via the second control line 17 to the receive control unit 18 for evaluation. In the design according to the invention, therefore, the coupling device 37 which can be activated and consists of the two 2-1 multiplexers 38, is controlled in fault-free normal operation in such a manner that of the two identical messages which circulate on the two communication paths 21, 22 simultaneously but in the opposite direction, only the message on the first communication path 21 is conducted through the processing unit 35 of the slave units 3 for processing. The message circulating on the second communication path 22 serves as redundancy and is fed back unchanged to the master unit 1. The communication system according to the invention, having a master-slave structure in which the slave units are serially connected to the master unit via two dual-ring structures which operate in opposite directions, only a single processing unit 35 being provided in each slave unit 3, also has the capability in the fault case, i.e. on the occurrence of a link fault, for reconfiguring the communication paths in the individual slave units in order to thus maintain the operability of the overall communication system. Figure 1B shows a dual link fault between the slave unit M and the slave unit M+1. Figure 1C represents a complete failure of the slave unit M which is equivalent to the occurrence of two dual link faults, one between slave unit M-1 and the slave unit M and between the slave unit M+1 and the slave unit M. When such a dual link fault occurs, the coupling device 37, which can be activated, of the slave units 3 is driven in such a manner that the message arriving either on the first communication path 21 or the second communication path 22 is fed back to the master unit 1 on the other communication path in each case, the message first always passing through the processing unit 35 of the slave unit 3. In the case of the dual link fault between the slave unit M and the slave unit M+1, shown in figure 1B, this occurs in such a manner that the slave units 1 to M-1 and M+2 to M are in normal operation whereas the slave units M and M+1 are reconfigured. In the fault case shown in figure 1C, in which the slave unit M completely fails, the slave units 1 to M-2 and the slave units M+2 to M are in normal operation. Slave units M-1 and M+1, in contrast, are reconfigured. The reconfiguration is preferably triggered by the two ports 0 and 1 in the slave units 3. These two ports 0 and 1 detect by means of a known detection process whether the slave unit can communicate with an adjoining slave unit. If a link fault is detected by port 0 or port 1, a corresponding fault mode is then carried out and the coupling device 34 which can be activated, of the slave unit is driven in the desired manner. In the case of a fault mode of port 1 as occurs in the dual link fault, shown in figure 1B, in the slave unit M and in the device failure, shown in figure 1C, in the slave unit M-1, the coupling device 37 which can be activated is driven in such a manner that the input of the processing unit 3 5 is connected to the first receiving unit RX31 and the output of the processing unit 35 is connected to the second transmitting unit TX34. The message circulating on the first communication path 21 is thus fed back to the second communication path 21 via the processing unit 35. In the case of the design of the coupling device 37 which can be activated, shown in figure 2, in the slave unit 3 with the first multiplexer 3 8 and the second multiplexer 39, this occurs in such a manner that the second input of the second multiplexer 39 is connected to its output. The first multiplexer 3 8 remains in normal mode, in contrast. In the case of a fault mode of ports 0 in the slave unit 3, i.e. when the first receiving unit RX31 and/or the second transmitting unit TX34 detect an interruption of the communication path to the adjacent slave unit which occurs in the case of the dual link fault, shown in figure 1B, in slave M+1 and in the device failure, shown in figure 1C, in slave M+1, the coupling device 34 which can be activated, in the slave unit 3 is driven in such a manner that the input of the processing unit 35 is connected to the second receiving unit RX33 and the output of the processing unit 35 is connected to the first transmitting unit TX32 so that the message passing through on the second communication path 22, after processing in the processing unit 35, is fed back to the master unit 1 on the first communication path 21. In the embodiment, shown in figure 2, of the coupling device 34 which can be activated, this occurs in such a manner that the first multiplexer 3 8 switches its second input to its output whereas the second multiplexer 39 remains in normal mode. The procedure according to the invention thus makes it possible to carry out reconfiguration measures in the communication system in a slave unit having only one processing unit, with the aid of a dual-ring structure and a coupling device which can be activated, in a simple manner in order to ensure the operability of the communication system in the case of a link fault, the behavior of the slave units with respect to message processing in the redundancy case not differing from that in normal mode. Apart from the dual link fault shown in figures 1B and 1C, in which the two communication paths to the adjacent user are interrupted, the procedure according to the invention also enables single link faults, in which only one communication path is interrupted, to be detected and to maintain the operability of the communication system by correspondingly reconfiguring the users adjoining the fault location. In the slave units 3, only one processing unit 3 5 is also always provided so that, in comparison with slave units having two processing units, no decision needs to be made about which processing unit is responsible for message processing. To achieve a high fault tolerance of the communication system with low hardware expenditure, particularly also in the master unit 1, the processing of the messages circulating in opposite directions on the first communication path 21 and the second communication path 22 is carried out in such a manner that the processing during fault-free normal operation does not significantly differ from that in the case of a fault mode in which the operability of the communication system is maintained by reconfiguring the individual slave units in the case of link faults. The slave unit can be designed in the manner according to the invention. However, the possibility also exists to use slave units having a different switching configuration which can be used as part of a master-slave system having a dual-ring structure. According to the invention, the two messages received by the first receiving unit RX13 on the first communication path 21 and by the second receiving unit RX14 on the second communication path 22 in the master unit 1 are superimposed in the receive control unit 18 in order to thus produce a single message. This is done preferably by oring the useful data of the two messages bit by bit. Furthermore, the messages in each case have in the control data area a counter field, the value of which is evaluated, preferably added together, in order to determine the operating state in the communication system, particularly the occurrence of a link fault. According to the invention, this is done in such a manner that the transmit control unit 16 of the master unit 1 transfers to the first transmitting unit TX11 and to the second transmitting unit TX12 in each case an identical message with a data field and a counter field, set to a predetermined value, for separately sending in opposite directions on the first communication path 21 and the second communication path 22. The processing unit 35 of each connected slave unit 3 is also designed in such a manner that when the message passes through, the value of the counter field is altered by a predetermined value. In the receive control unit 18, the value of the counter field of the two messages received by the first receiving unit RX13 on the first communication path 21 and by the second receiving unit RX14 on the second communication path 22 is then in each case evaluated. From the values of the two counter fields, it is then possible to determine by simple addition whether all connected slave units are active. This is preferably done in such a manner that the counter field of the message is set to the value 0 in the case of sending in opposite directions and each processing unit 35, when the message passes through the slave unit 3, increments the value of the counter field by 1. Since due to the design of the users according to the invention, only one message is ever processed by the processing unit both in normal operation and in fault mode with reconfiguration of the user circuit, the added value of the counter fields of the two messages fed back to the master unit 1 specifies the number of active users. It is thus possible to determine whether all connected users are active or whether a total failure of one user has occurred, e.g. due to a double dual link fault as shown in figure 1C. In addition, it is possible to determine the precise position of the link fault, for example its occurrence between the slave unit M and slave unit M+1 in figure 1B, by comparing the values in the two counter fields, using the known number of connected slave units as a basis. A fault-tolerant operation of the communication system, particularly also in the case of a reconfiguration of the communication system by altering the course of the signal in the users on occurrence of a link fault, is also achieved due to the fact that the two identical messages circulating in opposite directions on the first communication path 21 and the second communication path 22 are designed in such a manner that in the useful data field, a data area is allocated to each connected slave unit. The processing unit 35 of each slave unit 3 carries out a data exchange in the associated data area with the message passing through. In the receive control unit 18 of the master unit 1, the useful data fields of the two messages received by the first receiving unit RX13 on the first communication path 21 and by the second receiving unit RX14 on the second communication path 22 are then superimposed so that a common message is obtained. This superimposed message is always identical, regardless of whether the communication system is in normal operation or in fault mode on occurrence of a link fault, as long as all slave units 3 are still active. In read mode, when the slave units 3 are intended to transmit data to the master unit 1, a message which is set to the value 0 in the entire useful data field is transferred by the transmit control unit 16 of the master unit 1 via the first control line 15 to the two transmit units TX11, TX12. The processing units 35 of the slave units 3 then write the desired data into the associated useful data areas. The receive control unit 18 of the master unit 1 subsequently ors the useful data fields of the two messages received by the first receiving unit RX13 on the first communication path 21 and by the second receiving unit RX14 on the second communication path 22 in order to form a common message. Regardless of whether the communication system is in normal mode or in reconfiguration mode, the ored message always contains all data of the connected slave units 3 requested by the master unit 1. In write mode, in contrast, when the master unit 1 wishes to transmit e.g. control commands to the slave units 3 via the first control line 15, the transmit control unit 16 of the master unit 1 transfers to the transmitting units TX11, TX12 a message having a useful data field which contains the data to be transmitted to the slave units 3 for simultaneous sending in opposite directions on the two communication paths 21, 22. The processing units 35 of the slave units 3 then extract the associated data from the message regardless of whether they are in normal mode or in reconfiguration mode. In principle, oring of the useful data fields of the two messages fed back to the master unit 1 and received by the first receiving unit RX13 and second receiving unit RX14 is no longer required. Such an oring process leads to a common message with a useful data field which corresponds to the useful data field of the message sent. Given the design of the master-slave communication system according to the invention, the possibility exists for the master unit, with an arbitrary arrangement of the individual slave unit, but particularly if the slave units are arranged and operated in the manner according to the invention, to determine, in a simple manner, the freedom of faults in the communication system particularly also in the case of a reconfiguration of the course of the signal in the dual-ring structure after occurrence of a link fault. Furthermore, a reliable read and write operation is guaranteed in the communication system even in the redundant case, that is to say when individual users in the communication system have switched to fault mode, by superimposing the useful data field of the two messages fed back. We claim: 1. A master unit for a communication system having a master-slave structure, comprising a first transmitting unit (11), connected to a first communication path (21), for transmitting information signals on the first communication path (21), a second transmitting unit (12), connected to a second communication path (22), for transmitting information signals on the second communication path (22), a first receiving unit (13), connected to the first communication path (21), for receiving information signals on the first communication path (21), and a second receiving unit (14), connected to the second communication path (22), for receiving information signals on the second communication path (22), a transmit control unit (16) connected to the first transmitting unit (11) and the second transmitting unit (12), and a receive control unit (18) connected to the first receiving unit (13) and the second receiving unit (14), wherein the transmit control unit (18), in read mode of the master unit, is designed for transferring to the first transmitting unit (11) and the second transmitting unit (12) an information signal with a data field which has for each connected user (3) an associated data area, for separately transmitting in opposite directions on the first communication path (21) and the second communication path (22), and wherein the receive control unit (18), in read mode, is designed for superimposing the data fields of the two information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22). 2. The master unit as claimed in claim 1, wherein the information signal transferred to the first transmitting unit (11) and the second transmitting unit (12) by the transmit control unit (18) in read mode has a data field set to the value 0, wherein the receive control unit (18), in read mode, is designed for oring the data fields of the two information signals received by the first receiving unit (13) and the first communication path (21) and by the second receiving unit (14) on the second communication path (22) . 3. A communication system comprising a master-slave structure which has a master unit (1) as claimed in claim 1 or 2 and a multiplicity of users as slave units (3) which are connected to one another via the first communication path (21) and the second communication path (22), the first communication path (21) and the second communication path (22) operating in opposite directions to one another. 4. The communication system as claimed in claim 3, wherein each user (3) has a first receiving unit (31), connected to the first communication path (21), for receiving information signals on the first communication path (21), a first transmitting unit (32), connected to the first communication path, for transmitting information signals on the first communication path (21), a second receiving unit (33) connected to the second communication path (22), for receiving inf orination signals on the second communication path (22) and a second transmitting unit (34), connected to the second communication path, for transmitting information signals on the second communication path (22), a processing unit (35), having an input and an output, for processing information signals, and a coupling device (37) which can be activated, the coupling device (37) which can be activated being designed for connecting the input of the processing unit (35) to the first receiving unit (31), the output of the processing unit to the first transmitting unit (32) and the second receiving unit (33) to the second transmitting unit (34) in normal mode, connecting the input of the processing unit (35) to the first receiving unit (31) and the output of the processing unit (35) to the second- transmitting unit (34) in fault mode of the first transmitting unit (32) and/or of the second receiving unit (33), and connecting the input of the processing unit (35) to the second receiving unit (33) and the output of the processing unit (35) to the first transmitting unit (32) in fault mode of the first receiving unit (31) and/or of the second transmitting unit (34). 5. The communication system as claimed in claim 4, wherein the coupling device (37), which can be activated, of each user (3) has a first multiplexer (38), the first input of which is connected to the first receiving unit (31), the second input of which is connected to the second receiving unit (33) and the output of which is connected to the input of the processing unit (35) , and a second multiplexer (39), the first input of which is connected to the second receiving unit (35), the second input of which is connected to the output of the processing unit (35) and the output of which is connected to the second transmitting unit (34), the first multiplexer (38) being designed for connecting its first input to its output in normal mode and its second input to its output in fault mode of the first receiving unit (31) and/or of the second transmitting unit (34), and the second multiplexer (39) being designed for connecting its first input to its output in normal mode and its second input to its output in fault mode of the first transmitting unit (32) and/or of the second receiving unit (33). 6. The communication system as claimed in claim 4 or 5, wherein in each user (3), the first transmitting unit (32) and the second transmitting unit (34) and/or the first receiving unit (31) and the second receiving unit (33) are designed for detecting a link fault on the connected communication path (21, 22) and initiating a corresponding fault mode. 7. The communication system as claimed in one of claims 4 to 6, wherein the transmit control unit (16) of the master unit (1) is designed for transferring to the first transmitting unit (11) and the second transmitting unit (12) the information signal having a counterfield set to a predetermined value for separately transmitting in opposite directions on the first communication path (21) and the second communication path (22), wherein the processing unit (35) of each user is designed for altering the value of the counterfield by a predetermined value when the information signal passes through, and the receive control unit (18) of the master unit (1) is designed for in each case evaluating the value of the counterfields of the two information signals received by the first receiving unit (13) on the first communication path (31) and by the second receiving unit (14) on the second communication path (22). 8. The communication system as claimed in claim 7, wherein the receive control unit (18) of the master unit (1) is designed for adding the value of the counterfields of information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22). 9. A method for operating a communication system having a master-slave structure which has a master unit (1) and a multiplicity of users (3) as slave units, wherein the slave units (3) are connected serially to the master unit (1) via a dual-ring structure (2) formed from the first communication path (21) and the second communication path (22) and operating in opposite directions, wherein the master unit (1) has a first transmitting unit (11), connected to the first communication path, for transmitting information signals on the first communication path (21), a second transmitting unit (12), connected to the second communication path, for transmitting information signals on the second communication path (22), a first receiving unit (13), connected to the first communication path, for receiving information signals on the first communication path (21), a second receiving unit (14) connected to the second communication path (22), for receiving information signals on the second communication path (22), a transmit control unit (16) connected to the first transmitting unit (11) and the second transmitting unit (12), and a receive control unit (18) connected to the first receiving unit (13) and the second receiving unit (14), the method comprising the steps: transferring, in read mode, from the transmit control unit (16) of the master unit (1) to the first transmitting unit (11) and the second transmitting unit (12) an information signal with a data field which has an associated data area for each connected user (3), separately transmitting the information signal in opposite directions on the first communication path (21) and the second communication path (22), wherein a processing unit (35) of each user writes into the associated data area during the processing of the information signal passing through, and superimposing by the receive control unit (18) of the master unit (1) the data fields of the two information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22) . 10. The method as claimed in claim 9, wherein the information signal transferred to the first transmitting unit (11) and the second transmitting unit (12) by the transmit control unit (18) of the master unit (1) in read mode has a data field set to the value 0, wherein the receive control unit (18) of the master unit (1), in read mode, ores the data fields of the two information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22). 11. The method as claimed in claim 9 or 19, wherein the transmit control unit (16) of the master unit (1) transfers to the first transmitting unit (11) and the second transmitting unit (12) the information signal having a counterfield set to a predetermined value, for separately transmitting in opposite directions on the first communication path (21) and the second communication path (22), wherein the processing unit (35) of each user changes the value of the counterfield by a predetermined value when the information signal passes through, and the receive control unit (18) in each case evaluates the value of the counterfields of the two information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22) . 12. The method as claimed in claim 11, wherein the receive control unit (18) of the master unit (1) adds together the value of the counterfields of the information signals received by the first receiving unit (13) on the first communication path (21) and by the second receiving unit (14) on the second communication path (22) . 13. The method as claimed in any one of claims 9 to 12, wherein each user (3) has a first receiving unit (31), connected to the first communication path, for receiving information signals on the first communication path (21), a first transmitting unit (32), connected to the first communication path, for transmitting information signals on the first communication path (21), a second receiving unit (33) connected to the second communication path for receiving information signals on the second communication path (22) and a second transmitting unit (34), connected to the second communication path for transmitting information signals on the second communication path (22), and a coupling device (37) which can be activated, wherein the coupling device (37) which can be activated, connects an input of the processing unit (35) to the first receiving unit (21), an output of the processing unit (35) to the first transmitting unit (38) and the second receiving unit (33) to the second transmitting unit (34) in normal mode, the input of the processing unit (35) to the first receiving unit (31) and the output of the processing unit (35) to the second transmitting unit (34) in fault mode of the first transmitting unit (32) and/or second receiving unit (33) , and the input of the processing unit (35) to the second receiving unit (33) and the output of the processing unit to the first transmitting unit (32) in fault mode of the first receiving unit (31) and/or second transmitting unit (34). 14. The method as claimed in claim 13, wherein the coupling device (37) which can be activated has a first multiplexer (38), the first input of which is connected to the first receiving unit (31), the second input of which is connected to the second receiving unit (33) and the output of which is connected to the input of the processing unit (35), and a second multiplexer (39), the first input of which is connected to the second receiving unit (33) , the second input of which is connected to the output of the processing unit (35) and the output of which is connected to the second transmitting unit (34), wherein the first multiplexer (38) connects its first input to its output in normal mode and its second input to its output in fault mode of the first receiving unit (31) and/or of the second transmitting unit (34), and wherein the second multiplexer (39) connects its first input to its output in normal mode and its second input to its output in fault mode of the first transmitting unit (31) and/or the second receiving unit (34). 15. The method as claimed in claim 13 or 14, wherein the first transmitting unit (32) and the second transmitting unit (34) and/or the first receiving unit (31) and the second receiving unit (33) are designed for detecting a link fault on the connected communication path (21, 22) and activating a corresponding fault mode. 16. The method as claimed in any one of claims 9 to 15, which is used for controlling and regulating a multiplicity of servomotors. In read mode, a transmit control unit 16 of a master unit 1 transmits to a first transmitting unit 11 and a second transmitting unit 12 an information signal with a data field which has an associated data area for each connected user 3. The first transmitting unit 11 and the second transmitting unit 12 separately transmits the information signal in opposite directions on a first communication path 21 and a second communication path 22, wherein a processing unit 35 of each user writes into the associated data area during the processing of the information signal passing through. A receive control unit 18 of the master unit 1 superimposes the two information signals received by a first receiving unit 13 on the first communication path 21 and a the second receiving unit 14 on the second communication path 22.

Full Text

Description
The invention relates to a user and to a master unit in
a communication system having a number of users which
are connected to one another via a dual-ring structure
operating in opposite directions, to such a
communication system having a master-slave structure
and to a method for operating the user and the
communication system, respectively.
In production and automation technology, serial bus
systems are increasingly used in which the remotely
arranged devices of machine peripherals such as I/O
modules, transducers, drives, valves and operator
terminals communicate with automation, engineering or
visual display systems via an efficient real-time
communication system. In this arrangement, all users
are networked together via a serial bus, preferably via
a field bus, the data exchange via the bus being
carried out, as a rule, on the basis of the master-
slave principle.
The active bus users on the bus system, the control
devices, as a rule, possess a bus access authorization
and determine the data transfer on the bus. The active
bus users are called the master units in the serial bus
system. In contrast, passive bus users are, as a rule,
machine peripheral devices. They do not receive a bus
access authorization, i.e. they are only allowed to
acknowledge received information signals or transfer
information signals to a master unit on request by the
latter. Passive bus users are called slave units in the
serial bus system.

To avoid complex cabling, field bus systems having a
master-slave structure are generally arranged in ring
topology, all bus users being connected to a ring-
shaped transmission path. An information signal
generated by the master unit is fed into the ring-
shaped transmission path by the master unit and
successively passes through the slave units serially
connected to the ring-shaped transmission path and is
then received again by the master unit and evaluated.
Master-slave systems can also be designed as multi-
master systems.
As a rule, the information signals are organized by the
master unit into data packets which are composed of
control data and useful data, preferably using the
Ethernet standard which provides for data packets
having a length of up to 1500 bytes with a transmission
speed which, at the same time, is high at 100 Mbit/sec.
Each of the slave units connected to the ring-shaped
transmission path exchanges the useful data intended
for it with the Ethernet message when the Ethernet
message fed in by the master unit passes through on the
ring-shaped transmission path.
As a rule, the master-slave communication systems with
ring structure are configured in such a manner that the
master unit has a transmitting unit as data injection
point and a receiving unit as data extraction point
into a transmission medium. The individual slave units
are then connected together on the transmission path to
form a chain, wherein each user is connected to two
neighbors and the first and last user in the chain is
connected to the master unit. The data packets are
transmitted in one direction starting from the master
unit via its transmitting unit to the first slave unit
connected and from there to the next one, until the
last slave unit in the chain is reached, and then back
to the receiving unit of the master unit. Each slave

unit has, for receiving the circulating data packets
from the previous user, an interface with a receiving
unit and, for forwarding to the following user, an
interface with a transmitting unit, a processing unit
being arranged between receiving and transmitting unit
in order to process the data packets passing through
the slave unit, i.e. to exchange the useful data
allocated to the slave unit with the data packets.
The ring-shaped communication system with master-slave
structure is often designed in such a manner that the
master unit forms a physical line with the slave units
arranged at it, the transmission medium having a dual-
line structure and each slave unit having two ports
with a combined transmitting/receiving unit,
transmitting and receiving unit being short-circuited
in the output port of the last slave unit in the
transmission chain. The data packets injected into the
first line by the master unit via its receiving unit
are processed by the slave units on the forward path
and are then simply forwarded only to the receiving
unit of the master unit on the return path via the
second line.
A central requirement for master-slave communication
systems, particularly when they are used in production
and process automation, is a high fault tolerance, that
is to say the capability of the communication system to
maintain the required function, i.e., for example, the
production of a workpiece, in spite of the occurrence
of faults. In this context, faults in the communication
system which must be overcome without impairment of the
process are, in addition to faults in the data packets,
also the failure of entire transmission links, in
particular, for example due to physical separation of
the transmission medium.

To achieve a fault-tolerant master-slave communication
system, particularly in the case of link faults,
i.e. in the case of the failure of entire transmission
sections, dual-ring structures operating in opposite
directions are frequently used. Thus, a communication
system having a master-slave structure in which the
master unit is serially connected to a multiplicity of
slave units via two communication paths operating in
opposite directions, is described in US 4,663,748,
wherein the master unit simultaneously sends out the
data packets over two communication paths. The slave
unit then has two processing units which are in each
case connected between the two communication paths in
order to process the data messages passing through.
Furthermore, coupling units which can be activated are
arranged in the users, so that when a link fault
occurs, e.g. a break in a communication line, it
reconfigures the communication system by monitoring the
signals on both transmission rings and correspondingly
switching over the communication system, in such a
manner that a failure due to the link fault of a
greater section of the communication system or even a
total failure is prevented.
In DE 103 12 907 A1, it is also proposed to arrange the
slave unit in such a manner that on each communication
path in the direction of data transmission, first a
processing unit and then a multiplexer having two
inputs and one output is arranged. The multiplexer is
connected with its inputs in each case to the two
processing units of the slave unit and connected with
its output to the associated communication path. In
fault-free normal operation, each of the two
multiplexers switches through the processing unit
arranged on the associated communication path. In fault
mode, when a link fault occurs on the associated
communication path, however, the processing unit on the
other communication path is then switched through. This

design of the slave unit enables the communication
system to be reconfigured essentially in real time in
the fault case.
However, fault-tolerant master-slave communication
systems having a dual-ring structure, in which the
individual slave units in each case have two processing
units for processing the data message passing through,
provide for high hardware and switching complexity of
the slave units and thus increase the cost.
Furthermore, each slave unit must decide in normal
operation which of the two data packets passing through
the two processing units should be used for device
control which greatly restricts the use of such
communication systems at the required high data
transmission rates. In addition, the known fault-
tolerant communication systems with dual-ring topology
require that the master unit responds separately to a
link fault and switches from normal operation into
fault operating mode.
From US 2004/0008719, a master unit for a master-slave
communication system having the features of claim 1 and
a method for operating a master-slave communication
system having the features of the preamble of claim 9
is known. A communication system having a fault-
tolerant dual-ring structure is also represented in
US 4,527,270, EP 0 605 795 A2 and GB 2 348 782 and
EP 1 271 854.
It is the object of the invention to provide a master
unit for a master-slave communication system and a
method for operating a master-slave communication
system which provide for reconfiguration measures in
real time on the occurrence of link faults in a fault-

tolerant dual-ring topology, with minimum hardware and
switching complexity in the master unit.
This object is achieved by a master unit as claimed in
claim 1 and a method 'as "claimed in claim 9. Preferred
developments are specified in the dependent claims.
According to the invention, a user in a communication
system having a number of users which are connected to
one another via a first communication path and a second
communication path, the two communication paths
operating as dual-ring structure in opposite directions

to one another, is designed in such a manner that the
user has in each case one receiving unit for receiving
information signals on the associated communication
path, and in each case one transmitting unit for
transmitting information signals on the associated
communication path, for each communication path. In
addition, a single processing unit, having an input and
an output, for processing information signals passing
through the user, and a coupling unit which can be
activated is provided. In normal operation, this
coupling unit which can be activated connects the first
receiving and transmitting unit belonging to the first
communication path, the processing unit being
interposed for processing the information signals
passing through, and the second receiving and
transmitting unit belonging to the second communication
path, to one another. In the fault case of the first
transmitting unit and/or the second receiving unit,
i.e. when a link fault to the user occurs, the coupling
unit which can be activated then connects the first
receiving unit to the second transmitting unit via the
processing unit and in fault mode of the first
receiving unit and/or of the second transmitting unit,
i.e. again when a link fault occurs to the adjoining
user, the second receiving unit to the first
transmitting unit via the processing unit.
With this layout of the user in a fault-tolerant
communication system with a dual-ring topology, a user
reconfiguration can be carried out in real time when
link faults occur to adjacent users on one or on both
communication paths, in order to provide for faultless

operation in spite of the link fault. Thus, the
operation of the user, and thus the operation of the
communication system to which the user is connected,
can be maintained in spite of the link fault. The
design according to the invention has the advantage
that only a single processing unit is provided in each
user which reduces the hardware complexity, and thus
the costs. In addition, the behavior of the user in the
case of redundancy, that is to say when a link fault
occurs, does not differ from the behavior in normal
operation since the information signals passing through
are always interpreted and processed by the one
processing unit present when passing through. This, at
the same time, provides for a wide dynamic range during
the switching process and thus for meeting the real-
time requirements for the communication system.
It is preferred in this context to design the coupling
device which can be activated of the user in the form
of a first multiplexer, the first input of which is
connected to the first receiving unit, the second input
of which is connected to the second receiving unit and
the output of which is connected to the processing
unit, and of a second multiplexer, the first input of
which is connected to the second receiving unit, the
second input of which is connected to the processing
unit and the output of which is connected to the second
transmitting unit, the first multiplexer, in normal
operation, connecting its first input to its output
and, in fault mode of the first receiving unit and/or
the second transmitting unit, connecting its second
input to its output, and the second multiplexer, in
normal operation, connecting its first input to its
output and in fault mode of the first transmitting unit
and/or of the second receiving unit connecting its
second input to its output.

The design according to the invention of the coupling
device which can be activated, with two 2-1
multiplexers which precede or follow the processing
unit, ensures, in the fault case, that, depending on
the position of the link fault occurring, the
information signal is always conducted through the user
in such a manner that the information signal, after
passing through the processing unit, is fed back. Using
2-1 multiplexers only entails a slight hardware
complexity and, in addition, provides for switching
between normal operation and fault mode in a simple and
highly dynamic manner.
According to the invention, a master unit in a
communication system having a master-slave structure,
which has a multiplicity of users as slave units, is
arranged in such a manner that the slave units are
connected serially with the master unit via the dual-
ring structure formed from the first and the second
communication path and operating in opposite
directions. In this arrangement, the master structure
has for each communication path an associated
transmitting and receiving unit, the transmitting units
being connected to a transmit control unit and the
receiving units being connected to a receive control
unit. In this arrangement, the transmit control unit
transfers to the two transmitting units of the master
unit an information signal with a data field and a
counter field, set to a predetermined value, for
separately transmitting in opposite directions on the
first and on the second communication path. When the
information signal passes through, the processing units
of each user alter the value of the counter field by a
predetermined value. The control unit then evaluates
the value of the counter fields of the two information
signal received on the first and the second
communication path by the receiving units.

Given the design of the communication system according
to the invention, the master unit has in a simple
manner the capability of determining the freedom from
faults in the communication system, especially in the
case of a reconfiguration of the communication system
after the occurrence of a link fault. After evaluation
of the counter fields after reception of the two
identical information signals circulating in opposite
directions via the communication paths, the master unit
can determine how many of the slave units connected are
in operation. This is because the processing units in
the active slave units in each case alter the value of
the counter field of the information signal passing
through with the aid of their processing unit, as a
result of which conclusions regarding the operability
of the slave units can then be drawn on evaluation of
the counter fields of the two information signals
received in the master unit. By correlating the values
in the counter fields of the two information signals
received, it can also be determined between which users
the link fault has occurred in the communication system
or, respectively, whether or where a user has
completely failed.
In this context, the counter fields of the two
circulating information signals received on the first
and second communication path are preferably evaluated
by adding together the counter field values. The total
value then immediately specifies whether all connected
slave units are active, since their number is directly
reflected in the aggregate value.
According to the invention, the fault tolerance of the
communication system is increased further in a simple
manner due to the fact that in read mode, i.e. when the
slave units are intended to transmit data to the master
unit, an information signal with a data field which has
an associated data area for each connected user at the

communication system is transferred to the two
transmitting units by the transmit control unit of the
master unit. These two information signals are then
sent separately and in opposite directions on the first
and the second communication path, the users writing
into the associated data area when the information
signal passes through. The receive control unit of the
master unit then superimposes the data fields of the
two information signals received on the first
communication path and the second communication path.
Given this procedure, the operability of the
communication system guarantees reliable read mode in a
simple manner, particularly also in the case of
redundancy, that is to say when the communication
system is reconfigured due to a link fault and
individual users have switched to fault mode. This is
because, by superimposing the data fields of the two
information signals received, i.e. particularly by
oring them, a combined data field is generated in which
regardless of how the information signals are fed back
to the master unit via the communication paths, all
data to be transmitted by the users are contained.
Thus, the procedure according to the invention provides
for high fault tolerance, particularly in the case of
link faults in the dual-ring topology, in a simple
manner. Furthermore, the evaluating method of the
master unit in reconfiguration mode does not differ
from that in normal mode.
The invention will be explained in greater detail with
reference to the attached drawings, in which:
figure 1 shows a diagrammatic representation of a
communication system according to the invention having
a master-slave structure, wherein
figure 1A represents normal mode,

figure 1B represents a first communication system
reconfiguration on the occurrence of a dual link fault,
and
figure 1C represents a second communication system
reconfiguration in the case of the failure of a slave
unit; and
figure 2 shows a diagrammatic representation of a user
according to the invention.
In automation technology, field bus systems are
increasingly used in which devices of the machine
peripherals, arranged in distributed manner,
communicate with automation, engineering and visual
display systems via a field bus. As a rule, the field
bus system has a serial bus which can be, for example,
an electrical line, an optical waveguide or a radio
cable. All bus users are then connected to this field
bus, a distinction being made between active bus users
and passive bus users. The active bus users on the
field bus system are the master units which determine
the data traffic on the bus. Such a master unit is, for
example, an industrial PC which is used as process
control computer in a production process. This master
unit has a bus access authorization and can output data
to the field bus without external request. The passive
bus users on the bus system are peripheral machine
devices, for example I/O devices, valves, drives and
transducers. They are used as slave units and do not
obtain a bus access authorization, i.e. they are only
allowed to acknowledge received information signals or
to transmit information signals to a master unit on
request by the latter.
The communication standard used for data transmission
in the master-slave system is preferably the Ethernet

concept. In Ethernet communication systems, the data to
be transmitted are encapsulated in data packets, also
called messages in the further text, having a
predetermined format. The Ethernet messages can have a
data length of up to 1500 bytes containing,
additionally to the useful data, control data which
have a start identifier, a destination and source
address, the data packet type and a fault mechanism.
Ethernet communication systems having a master-slave
structure are preferably designed in such a manner that
the individual slave units are connected together via
the transmission medium to form a chain, each slave
unit being connected to two neighbors and the first and
the last user in the chain being connected to the
master unit so that a ring structure is obtained. In
this arrangement, the data are transmitted in one
direction starting from the master unit to the first
adjacent slave unit and from there to the next one
until the last slave unit and then back to the master
unit.
To ensure high fault tolerance, particularly in the
case of a link fault in the communication system,
i.e. the failure of entire transmission sections with
slave units, e.g. due to a cable break, the
communication systems having a master-slave structure
often have two communication paths which operate in
opposite direction to one another. Due to the dual-ring
structure operating in opposite directions, the
possibility exists in the case of link faults to carry
out reconfiguration measures in the communication
system in order to maintain the operability of the
communication system in spite of link faults.
Figure 1 shows in a basic circuit diagram such a fault-
tolerant communication system in an embodiment
according to the invention. The communication system

has a master unit 1 which is connected serially to N
slave units 3 via a dual-ring structure 2 . The dual-
ring structure comprises two unidirectional
communication paths 21, 22 which pass through the
connected slave units 3 in opposite directions. The
master unit 1 is connected to the first communication
path 21 as a data extraction point via a first
transmitting unit TX11 and to the second communication
path 22 as data extraction point via a second
transmitting unit TX12. Furthermore, the master unit 1
has a first receiving unit RX13 as data injection point
for the first communication path 21 and a second
receiving unit RX14 as data injection point for the
second communication path 22. The first transmitting
unit TXll and the second transmitting unit TX12 are
connected to a transmit control unit 16 via a first
control line 15. The first receiving unit RX13 and the
second receiving unit RX14 are connected via a second
control line 17 and to a receive control unit 18.
Each slave unit 3, in turn, has an interface, for
receiving messages from a preceding user via the first
communication path 21, with a first receiving unit RX31
and an interface with a first transmitting unit TX32
for forwarding to the next user via the first
communication path 21. Furthermore, each slave unit 3
has for receiving a circulating Ethernet message via
the second communication path 22 from a preceding user
an interface with a second receiving unit RX33 and, for
forwarding to the following user, an interface with a
second transmitting unit TX34. Between the first
receiving unit RX31, the second receiving unit RX32,
the first transmitting unit TX33 and the second
transmitting unit TX34, a processing unit 3 5 and a
coupling device 3 7 which can be activated, is also
connected in each slave unit 3.

The basic circuit diagram of a slave unit 3 is shown in
greater detail in figure 2. In the slave unit 3, the
first receiving unit RX31 which is connected to the
first communication path 21, and the second
transmitting unit TX34 which is connected to the second
communication path 22, are grouped as port 0. The
second receiving unit RX33 which is connected to a
second communication path 22, and the first
transmitting unit TX32 which is connected to the first
communication path 21, are organized as port 1.
The coupling device 37 which can be activated has a
first change-over switch 38 and a second change-over
switch 39 which are in each case designed as 2-1
multiplexers. The receiving and transmitting unit 31,
32, 33, 34, the multiplexers 38, 39 of the coupling
device 37 which can be activated, and the processing
unit 3 5 are interconnected in this arrangement in the
manner shown by arrows in figure 2 by a line network
40.
The output of the first receiving unit RX31 is
connected to the first input of the first multiplexer
38. The second input of the first multiplexer 38 is
connected to the second receiving unit RX33. The output
of the first multiplexer 3 8 is also connected to the
processing unit 35. The second multiplexer 39, in turn,
is connected with its first input to the second
receiving unit RX33 and with its second input to the
output of the processing unit 35. The output of the
second multiplexer 39 is connected to the second
transmitting unit TX34. In addition, the output of the
processing unit 35 is also connected to the first
transmitting unit TX32 via the line network 40.
In the case of fault-free normal operation of the
communication system as shown in figure 1A, the
transmit control unit 16 of the master unit transfers

to the first transmitting unit TXll and the second
transmitting unit TX12 a message which is then sent by
the two transmitting units simultaneously in opposite
directions via the first communication path 21 and the
second communication path 22. In this process, the
messages pass in opposite directions through the
connected slave units 3, all coupling devices 37 which
can be activated in the slave units 3 being connected
in such a manner that the input of the processing unit
35 is connected to the first receiving unit RX31, the
output of the processing unit 3 5 is connected to the
first transmitting unit TX32 and the second receiving
unit RX33 is connected to the second transmitting unit
TX34.
In this operating mode of the slave units 3, the
coupling device 37 which can be activated ensures that
the two identical messages circulating in opposite
directions on the first communication path 21 and on
the second communication path 22 always pass through
the slave unit in such a manner that only the messages
transmitted via the first communication path 21 are
processed by the processing unit 35. In contrast, the
message circulating on the second communication path 22
is only passed through by the slave units 3 and arrives
again unprocessed at the master unit 1. The two
messages circulating in opposite directions via the
first and second communication path 21, 22 are in this
case recognized by the first receiving unit RX13 and
the second receiving unit RX14 of the master unit 1 and
forwarded via the second control line 17 to the receive
control unit 18 for evaluation.
In the design according to the invention, therefore,
the coupling device 37 which can be activated and
consists of the two 2-1 multiplexers 38, is controlled
in fault-free normal operation in such a manner that of
the two identical messages which circulate on the two

communication paths 21, 22 simultaneously but in the
opposite direction, only the message on the first
communication path 21 is conducted through the
processing unit 35 of the slave units 3 for processing.
The message circulating on the second communication
path 22 serves as redundancy and is fed back unchanged
to the master unit 1.
The communication system according to the invention,
having a master-slave structure in which the slave
units are serially connected to the master unit via two
dual-ring structures which operate in opposite
directions, only a single processing unit 35 being
provided in each slave unit 3, also has the capability
in the fault case, i.e. on the occurrence of a link
fault, for reconfiguring the communication paths in the
individual slave units in order to thus maintain the
operability of the overall communication system.
Figure 1B shows a dual link fault between the slave
unit M and the slave unit M+1. Figure 1C represents a
complete failure of the slave unit M which is
equivalent to the occurrence of two dual link faults,
one between slave unit M-1 and the slave unit M and
between the slave unit M+1 and the slave unit M. When
such a dual link fault occurs, the coupling device 37,
which can be activated, of the slave units 3 is driven
in such a manner that the message arriving either on
the first communication path 21 or the second
communication path 22 is fed back to the master unit 1
on the other communication path in each case, the
message first always passing through the processing
unit 35 of the slave unit 3.
In the case of the dual link fault between the slave
unit M and the slave unit M+1, shown in figure 1B, this
occurs in such a manner that the slave units 1 to M-1
and M+2 to M are in normal operation whereas the slave

units M and M+1 are reconfigured. In the fault case
shown in figure 1C, in which the slave unit M
completely fails, the slave units 1 to M-2 and the
slave units M+2 to M are in normal operation. Slave
units M-1 and M+1, in contrast, are reconfigured.
The reconfiguration is preferably triggered by the two
ports 0 and 1 in the slave units 3. These two ports 0
and 1 detect by means of a known detection process
whether the slave unit can communicate with an
adjoining slave unit. If a link fault is detected by
port 0 or port 1, a corresponding fault mode is then
carried out and the coupling device 34 which can be
activated, of the slave unit is driven in the desired
manner.
In the case of a fault mode of port 1 as occurs in the
dual link fault, shown in figure 1B, in the slave unit
M and in the device failure, shown in figure 1C, in the
slave unit M-1, the coupling device 37 which can be
activated is driven in such a manner that the input of
the processing unit 3 5 is connected to the first
receiving unit RX31 and the output of the processing
unit 35 is connected to the second transmitting unit
TX34. The message circulating on the first
communication path 21 is thus fed back to the second
communication path 21 via the processing unit 35. In
the case of the design of the coupling device 37 which
can be activated, shown in figure 2, in the slave unit
3 with the first multiplexer 3 8 and the second
multiplexer 39, this occurs in such a manner that the
second input of the second multiplexer 39 is connected
to its output. The first multiplexer 3 8 remains in
normal mode, in contrast.
In the case of a fault mode of ports 0 in the slave
unit 3, i.e. when the first receiving unit RX31 and/or
the second transmitting unit TX34 detect an

interruption of the communication path to the adjacent
slave unit which occurs in the case of the dual link
fault, shown in figure 1B, in slave M+1 and in the
device failure, shown in figure 1C, in slave M+1, the
coupling device 34 which can be activated, in the slave
unit 3 is driven in such a manner that the input of the
processing unit 35 is connected to the second receiving
unit RX33 and the output of the processing unit 35 is
connected to the first transmitting unit TX32 so that
the message passing through on the second communication
path 22, after processing in the processing unit 35, is
fed back to the master unit 1 on the first
communication path 21. In the embodiment, shown in
figure 2, of the coupling device 34 which can be
activated, this occurs in such a manner that the first
multiplexer 3 8 switches its second input to its output
whereas the second multiplexer 39 remains in normal
mode.
The procedure according to the invention thus makes it
possible to carry out reconfiguration measures in the
communication system in a slave unit having only one
processing unit, with the aid of a dual-ring structure
and a coupling device which can be activated, in a
simple manner in order to ensure the operability of the
communication system in the case of a link fault, the
behavior of the slave units with respect to message
processing in the redundancy case not differing from
that in normal mode.
Apart from the dual link fault shown in figures 1B and
1C, in which the two communication paths to the
adjacent user are interrupted, the procedure according
to the invention also enables single link faults, in
which only one communication path is interrupted, to be
detected and to maintain the operability of the
communication system by correspondingly reconfiguring
the users adjoining the fault location.

In the slave units 3, only one processing unit 3 5 is
also always provided so that, in comparison with slave
units having two processing units, no decision needs to
be made about which processing unit is responsible for
message processing.
To achieve a high fault tolerance of the communication
system with low hardware expenditure, particularly also
in the master unit 1, the processing of the messages
circulating in opposite directions on the first
communication path 21 and the second communication path
22 is carried out in such a manner that the processing
during fault-free normal operation does not
significantly differ from that in the case of a fault
mode in which the operability of the communication
system is maintained by reconfiguring the individual
slave units in the case of link faults. The slave unit
can be designed in the manner according to the
invention. However, the possibility also exists to use
slave units having a different switching configuration
which can be used as part of a master-slave system
having a dual-ring structure.
According to the invention, the two messages received
by the first receiving unit RX13 on the first
communication path 21 and by the second receiving unit
RX14 on the second communication path 22 in the master
unit 1 are superimposed in the receive control unit 18
in order to thus produce a single message. This is done
preferably by oring the useful data of the two messages
bit by bit.
Furthermore, the messages in each case have in the
control data area a counter field, the value of which
is evaluated, preferably added together, in order to
determine the operating state in the communication
system, particularly the occurrence of a link fault.

According to the invention, this is done in such a
manner that the transmit control unit 16 of the master
unit 1 transfers to the first transmitting unit TX11
and to the second transmitting unit TX12 in each case
an identical message with a data field and a counter
field, set to a predetermined value, for separately
sending in opposite directions on the first
communication path 21 and the second communication path
22. The processing unit 35 of each connected slave unit
3 is also designed in such a manner that when the
message passes through, the value of the counter field
is altered by a predetermined value. In the receive
control unit 18, the value of the counter field of the
two messages received by the first receiving unit RX13
on the first communication path 21 and by the second
receiving unit RX14 on the second communication path 22
is then in each case evaluated. From the values of the
two counter fields, it is then possible to determine by
simple addition whether all connected slave units are
active.
This is preferably done in such a manner that the
counter field of the message is set to the value 0 in
the case of sending in opposite directions and each
processing unit 35, when the message passes through the
slave unit 3, increments the value of the counter field
by 1. Since due to the design of the users according to
the invention, only one message is ever processed by
the processing unit both in normal operation and in
fault mode with reconfiguration of the user circuit,
the added value of the counter fields of the two
messages fed back to the master unit 1 specifies the
number of active users. It is thus possible to
determine whether all connected users are active or
whether a total failure of one user has occurred,
e.g. due to a double dual link fault as shown in
figure 1C. In addition, it is possible to determine the
precise position of the link fault, for example its

occurrence between the slave unit M and slave unit M+1
in figure 1B, by comparing the values in the two
counter fields, using the known number of connected
slave units as a basis.
A fault-tolerant operation of the communication system,
particularly also in the case of a reconfiguration of
the communication system by altering the course of the
signal in the users on occurrence of a link fault, is
also achieved due to the fact that the two identical
messages circulating in opposite directions on the
first communication path 21 and the second
communication path 22 are designed in such a manner
that in the useful data field, a data area is allocated
to each connected slave unit. The processing unit 35 of
each slave unit 3 carries out a data exchange in the
associated data area with the message passing through.
In the receive control unit 18 of the master unit 1,
the useful data fields of the two messages received by
the first receiving unit RX13 on the first
communication path 21 and by the second receiving unit
RX14 on the second communication path 22 are then
superimposed so that a common message is obtained. This
superimposed message is always identical, regardless of
whether the communication system is in normal operation
or in fault mode on occurrence of a link fault, as long
as all slave units 3 are still active.
In read mode, when the slave units 3 are intended to
transmit data to the master unit 1, a message which is
set to the value 0 in the entire useful data field is
transferred by the transmit control unit 16 of the
master unit 1 via the first control line 15 to the two
transmit units TX11, TX12. The processing units 35 of
the slave units 3 then write the desired data into the
associated useful data areas. The receive control unit
18 of the master unit 1 subsequently ors the useful
data fields of the two messages received by the first

receiving unit RX13 on the first communication path 21
and by the second receiving unit RX14 on the second
communication path 22 in order to form a common
message. Regardless of whether the communication system
is in normal mode or in reconfiguration mode, the ored
message always contains all data of the connected slave
units 3 requested by the master unit 1.
In write mode, in contrast, when the master unit 1
wishes to transmit e.g. control commands to the slave
units 3 via the first control line 15, the transmit
control unit 16 of the master unit 1 transfers to the
transmitting units TX11, TX12 a message having a useful
data field which contains the data to be transmitted to
the slave units 3 for simultaneous sending in opposite
directions on the two communication paths 21, 22. The
processing units 35 of the slave units 3 then extract
the associated data from the message regardless of
whether they are in normal mode or in reconfiguration
mode. In principle, oring of the useful data fields of
the two messages fed back to the master unit 1 and
received by the first receiving unit RX13 and second
receiving unit RX14 is no longer required. Such an
oring process leads to a common message with a useful
data field which corresponds to the useful data field
of the message sent.
Given the design of the master-slave communication
system according to the invention, the possibility
exists for the master unit, with an arbitrary
arrangement of the individual slave unit, but
particularly if the slave units are arranged and
operated in the manner according to the invention, to
determine, in a simple manner, the freedom of faults in
the communication system particularly also in the case
of a reconfiguration of the course of the signal in the
dual-ring structure after occurrence of a link fault.
Furthermore, a reliable read and write operation is

guaranteed in the communication system even in the
redundant case, that is to say when individual users in
the communication system have switched to fault mode,
by superimposing the useful data field of the two
messages fed back.

We claim:
1. A master unit for a communication system having a
master-slave structure, comprising
a first transmitting unit (11), connected to a first
communication path (21), for transmitting information
signals on the first communication path (21), a second
transmitting unit (12), connected to a second
communication path (22), for transmitting information
signals on the second communication path (22),
a first receiving unit (13), connected to the first
communication path (21), for receiving information signals
on the first communication path (21), and
a second receiving unit (14), connected to the second
communication path (22), for receiving information signals
on the second communication path (22),
a transmit control unit (16) connected to the first
transmitting unit (11) and the second transmitting unit
(12), and
a receive control unit (18) connected to the first
receiving unit (13) and the second receiving unit (14),
wherein the transmit control unit (18), in read mode of
the master unit, is designed for transferring to the first
transmitting unit (11) and the second transmitting unit
(12) an information signal with a data field which has for
each connected user (3) an associated data area, for
separately transmitting in opposite directions on the
first communication path (21) and the second communication
path (22),
and wherein the receive control unit (18), in read mode,
is designed for superimposing the data fields of the two
information signals received by the first receiving unit

(13) on the first communication path (21) and by the
second receiving unit (14) on the second communication
path (22).
2. The master unit as claimed in claim 1, wherein the
information signal transferred to the first transmitting
unit (11) and the second transmitting unit (12) by the
transmit control unit (18) in read mode has a data field
set to the value 0,
wherein the receive control unit (18), in read mode, is
designed for oring the data fields of the two information
signals received by the first receiving unit (13) and the
first communication path (21) and by the second receiving
unit (14) on the second communication path (22) .
3. A communication system comprising a master-slave
structure which has a master unit (1) as claimed in claim
1 or 2 and a multiplicity of users as slave units (3)
which are connected to one another via the first
communication path (21) and the second communication path
(22), the first communication path (21) and the second
communication path (22) operating in opposite directions
to one another.
4. The communication system as claimed in claim 3,
wherein each user (3) has a first receiving unit (31),
connected to the first communication path (21), for
receiving information signals on the first communication
path (21), a first transmitting unit (32), connected to
the first communication path, for transmitting information
signals on the first communication path (21), a second
receiving unit (33) connected to the second communication

path (22), for receiving inf orination signals on the second
communication path (22) and a second transmitting unit
(34), connected to the second communication path, for
transmitting information signals on the second
communication path (22), a processing unit (35), having an
input and an output, for processing information signals,
and a coupling device (37) which can be activated, the
coupling device (37) which can be activated being designed
for connecting the input of the processing unit (35) to
the first receiving unit (31), the output of the
processing unit to the first transmitting unit (32) and
the second receiving unit (33) to the second transmitting
unit (34) in normal mode, connecting the input of the
processing unit (35) to the first receiving unit (31) and
the output of the processing unit (35) to the second-
transmitting unit (34) in fault mode of the first
transmitting unit (32) and/or of the second receiving unit
(33), and connecting the input of the processing unit (35)
to the second receiving unit (33) and the output of the
processing unit (35) to the first transmitting unit (32)
in fault mode of the first receiving unit (31) and/or of
the second transmitting unit (34).
5. The communication system as claimed in claim 4,
wherein the coupling device (37), which can be activated,
of each user (3) has a first multiplexer (38), the first
input of which is connected to the first receiving unit
(31), the second input of which is connected to the second
receiving unit (33) and the output of which is connected
to the input of the processing unit (35) , and a second
multiplexer (39), the first input of which is connected to
the second receiving unit (35), the second input of which

is connected to the output of the processing unit (35) and
the output of which is connected to the second
transmitting unit (34), the first multiplexer (38) being
designed for connecting its first input to its output in
normal mode and its second input to its output in fault
mode of the first receiving unit (31) and/or of the second
transmitting unit (34), and the second multiplexer (39)
being designed for connecting its first input to its
output in normal mode and its second input to its output
in fault mode of the first transmitting unit (32) and/or
of the second receiving unit (33).
6. The communication system as claimed in claim 4 or 5,
wherein in each user (3), the first transmitting unit (32)
and the second transmitting unit (34) and/or the first
receiving unit (31) and the second receiving unit (33) are
designed for detecting a link fault on the connected
communication path (21, 22) and initiating a corresponding
fault mode.
7. The communication system as claimed in one of claims
4 to 6, wherein the transmit control unit (16) of the
master unit (1) is designed for transferring to the first
transmitting unit (11) and the second transmitting unit
(12) the information signal having a counterfield set to a
predetermined value for separately transmitting in
opposite directions on the first communication path (21)
and the second communication path (22), wherein the
processing unit (35) of each user is designed for altering
the value of the counterfield by a predetermined value
when the information signal passes through, and the
receive control unit (18) of the master unit (1) is

designed for in each case evaluating the value of the
counterfields of the two information signals received by
the first receiving unit (13) on the first communication
path (31) and by the second receiving unit (14) on the
second communication path (22).
8. The communication system as claimed in claim 7,
wherein the receive control unit (18) of the master unit
(1) is designed for adding the value of the counterfields
of information signals received by the first receiving
unit (13) on the first communication path (21) and by the
second receiving unit (14) on the second communication
path (22).
9. A method for operating a communication system having
a master-slave structure which has a master unit (1) and a
multiplicity of users (3) as slave units, wherein the
slave units (3) are connected serially to the master unit
(1) via a dual-ring structure (2) formed from the first
communication path (21) and the second communication path
(22) and operating in opposite directions, wherein the
master unit (1) has a first transmitting unit (11),
connected to the first communication path, for
transmitting information signals on the first
communication path (21), a second transmitting unit (12),
connected to the second communication path, for
transmitting information signals on the second
communication path (22), a first receiving unit (13),
connected to the first communication path, for receiving
information signals on the first communication path (21),
a second receiving unit (14) connected to the second
communication path (22), for receiving information signals

on the second communication path (22), a transmit control
unit (16) connected to the first transmitting unit (11)
and the second transmitting unit (12), and a receive
control unit (18) connected to the first receiving unit
(13) and the second receiving unit (14),
the method comprising the steps:
transferring, in read mode, from the transmit control unit
(16) of the master unit (1) to the first transmitting unit
(11) and the second transmitting unit (12) an information
signal with a data field which has an associated data area
for each connected user (3),
separately transmitting the information signal in opposite
directions on the first communication path (21) and the
second communication path (22), wherein a processing unit
(35) of each user writes into the associated data area
during the processing of the information signal passing
through, and
superimposing by the receive control unit (18) of the
master unit (1) the data fields of the two information
signals received by the first receiving unit (13) on the
first communication path (21) and by the second receiving
unit (14) on the second communication path (22) .
10. The method as claimed in claim 9, wherein the
information signal transferred to the first transmitting
unit (11) and the second transmitting unit (12) by the
transmit control unit (18) of the master unit (1) in read
mode has a data field set to the value 0, wherein the
receive control unit (18) of the master unit (1), in read
mode, ores the data fields of the two information signals
received by the first receiving unit (13) on the first
communication path (21) and by the second receiving unit

(14) on the second communication path (22).
11. The method as claimed in claim 9 or 19, wherein the
transmit control unit (16) of the master unit (1)
transfers to the first transmitting unit (11) and the
second transmitting unit (12) the information signal
having a counterfield set to a predetermined value, for
separately transmitting in opposite directions on the
first communication path (21) and the second communication
path (22), wherein the processing unit (35) of each user
changes the value of the counterfield by a predetermined
value when the information signal passes through, and the
receive control unit (18) in each case evaluates the value
of the counterfields of the two information signals
received by the first receiving unit (13) on the first
communication path (21) and by the second receiving unit
(14) on the second communication path (22) .
12. The method as claimed in claim 11, wherein the
receive control unit (18) of the master unit (1) adds
together the value of the counterfields of the information
signals received by the first receiving unit (13) on the
first communication path (21) and by the second receiving
unit (14) on the second communication path (22) .
13. The method as claimed in any one of claims 9 to 12,
wherein each user (3) has a first receiving unit (31),
connected to the first communication path, for receiving
information signals on the first communication path (21),
a first transmitting unit (32), connected to the first
communication path, for transmitting information signals
on the first communication path (21), a second receiving

unit (33) connected to the second communication path for
receiving information signals on the second communication
path (22) and a second transmitting unit (34), connected
to the second communication path for transmitting
information signals on the second communication path (22),
and a coupling device (37) which can be activated, wherein
the coupling device (37) which can be activated, connects
an input of the processing unit (35) to the first
receiving unit (21), an output of the processing unit (35)
to the first transmitting unit (38) and the second
receiving unit (33) to the second transmitting unit (34)
in normal mode, the input of the processing unit (35) to
the first receiving unit (31) and the output of the
processing unit (35) to the second transmitting unit (34)
in fault mode of the first transmitting unit (32) and/or
second receiving unit (33) , and the input of the
processing unit (35) to the second receiving unit (33) and
the output of the processing unit to the first
transmitting unit (32) in fault mode of the first
receiving unit (31) and/or second transmitting unit (34).
14. The method as claimed in claim 13, wherein the
coupling device (37) which can be activated has a first
multiplexer (38), the first input of which is connected to
the first receiving unit (31), the second input of which
is connected to the second receiving unit (33) and the
output of which is connected to the input of the
processing unit (35), and a second multiplexer (39), the
first input of which is connected to the second receiving
unit (33) , the second input of which is connected to the
output of the processing unit (35) and the output of which
is connected to the second transmitting unit (34), wherein

the first multiplexer (38) connects its first input to its
output in normal mode and its second input to its output
in fault mode of the first receiving unit (31) and/or of
the second transmitting unit (34), and wherein the second
multiplexer (39) connects its first input to its output in
normal mode and its second input to its output in fault
mode of the first transmitting unit (31) and/or the second
receiving unit (34).
15. The method as claimed in claim 13 or 14, wherein the
first transmitting unit (32) and the second transmitting
unit (34) and/or the first receiving unit (31) and the
second receiving unit (33) are designed for detecting a
link fault on the connected communication path (21, 22)
and activating a corresponding fault mode.
16. The method as claimed in any one of claims 9 to 15,
which is used for controlling and regulating a
multiplicity of servomotors.

In read mode, a transmit control unit 16 of a master unit 1
transmits to a first transmitting unit 11 and a second
transmitting unit 12 an information signal with a data field
which has an associated data area for each connected user 3.
The first transmitting unit 11 and the second transmitting
unit 12 separately transmits the information signal in
opposite directions on a first communication path 21 and a
second communication path 22, wherein a processing unit 35 of
each user writes into the associated data area during the
processing of the information signal passing through. A
receive control unit 18 of the master unit 1 superimposes the
two information signals received by a first receiving unit 13
on the first communication path 21 and a the second receiving
unit 14 on the second communication path 22.

MASTER UNIT FOR A COMMUNICATION SYSTEM HAVING A MASTER-SLAVE STRUCTURE, COMMUNICATION SYSTEM AND METHOD FOR OPERATION THEREOF

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