Abstract:
The invention relates to a network, in particular a PA PROFIBUS network having redundant properties and a linear topology. Two line ends are connected to a redundancy manager which disconnects said line ends in error-free conditions and connects them in error conditions. Said redundancy manager supplies voltage to the first line end for operating customers&#39; mobile telephones each of which is connected to the network by a branching unit. After receiving a supply voltage on one connection thereof, said branching units check the state of a cable which is connected to the other network connection and transmit the supply voltage in the faultless condition thereof only. When the redundancy manager does not detect the supply voltage on the other end of the line end at least during a predetermined space of time after voltage supply, said redundancy manager supplies the voltage thereto. A cable having a disturbance is then disconnected from the line by the adjacent branching units.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the German application No. 10356118.8, filed Nov. 27, 2003, to the German application No. 10 2004 055 53.0, filed Nov. 15, 2004 and to the International Application No. PCT/EP2004/013455, filed Nov. 26, 2004 which are incorporated by reference herein in their entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to a network, In particular a PA PROFIBUS network, with redundant properties, a branching element for a user device in said network, a redundancy manager for said network and a method for operating said network.  
       BACKGROUND OF INVENTION  
       [0003]     In automation systems for manufacturing or process technology with “classic” wiring of field devices, for example measurement converters and/or control elements, in which the field devices are connected in each case via a 4-wire master cable and subdistributors by a separate pair of wires to a programmable logic controller, the failure of one of the field devices or of a transmission link to this field device has no effect on the function of a field device since the individual field devices are operated physically separated from one another.  
       SUMMARY OF INVENTION  
       [0004]     With field devices which communicate via a field bus with the programmable logic controller, the bus cable forms a common component for all field devices and if it fails it affects all field devices. In addition errors in the field devices, for example a short circuit of the transmission line or sending out of noise signals on the transmission line, can adversely affect communication of the other field devices connected the bus cable. Because of the possibly reduced system availability field buses are not used in particularly critical applications or must be configured to provide redundancy.  
         [0005]     In principal a distinction must be made between two different redundancy concepts in automation technology systems. On the one hand system redundancy improves the availability through a redundant, that is largely duplicated structure of the complete system, consisting of field devices, a bus system and programmable logic controllers. Coordination, i.e. which of the components must actively be operated at any given time and which are in standby mode is undertaken at programmable logic controller level and the controllers must be configured for this purpose. All other components are standard components. The other concept is media redundancy in which with the transmission media only the part of the communication system is arranged redundantly of which the failure would have particularly serious effects on the system availability. For example a high-availability field bus system is known from EP 0 287 992 B2 which features two bus lines over which identical messages are transmitted serially in each case. With a detector logic which is located in the connected users, test characters are evaluated for testing the function of the busses. If there are errors in the test character there is a switchover to receive by the other error-free bus. The redundant configuration of the transmission medium thus increases the availability of the bus system.  
         [0006]     An Ethernet network with redundancy properties is known from EP 1 062 787 B1. The Ethernet network has a linear topology. The line ends are connected to a redundancy manager. The redundancy manager uses test telegrams to check the state of the network. If there is an interruption of the network the redundancy manager connects the line ends and thereby re-establishes a line structure and the operability of the network. The disadvantage here is that the test telegrams which are injected by the redundancy manager into the two ends of the line represent an additional network load for the network and thus reduce the transmission capacity of the network. This monitoring and switchover principal is also not simply transferable to bus systems in which, in addition to transmission of the data, the energy required for operating the user devices connected to the bus is transmitted over the bus.  
         [0007]     An object of the invention is to create a network, in particular a PROFIBUS PA network with redundant properties and the option of remote power feeding of user devices, a branching unit for a user device in said network, a redundancy manager for said network and a method for operating said net work, through which an increase in the availability of the network can be achieved with simple means.  
         [0008]     This object is achieved by the claims. Advantageous developments of the network, the branching unit and the redundancy manager are to be found in the dependent claims.  
         [0009]     The advantage of the invention is that the redundancy manager enables an error in the network to be detected and rectified comparatively rapidly. With an interruption or a short circuit in a cable segment no feed voltage is directed to this segment by the connected branching unit of a user device which in the positive case should be feeding over the segment the energy required for operation of the devices located beyond the segment, no feed voltage is routed on this segment or the forwarding of a feed voltage on this segment is interrupted. This means that the feed voltage no longer reaches the other end of the line which is connected to the redundancy manager. This is detected by the redundancy manager, which a short time after establishing the error state also feeds the required operating energy into the other end of the line. The defective cable segment is isolated by the two delimiting branching units and the network continues to be operable despite the error without any long interruption to operation. The communication in the network is also maintained in the event of an error without a higher-ranking network, especially a control system to which the network is connected, being disturbed or called on in any other way.  
         [0010]     By contrast with the method known from EP 1 062 787 B1 mentioned a above, in which the status of the network is checked with a test telegrams, the invention has the advantage that errors are able to be detected as soon as they have occurred and not just at the point at which test telegrams have been sent through the network. The reaction time of the known method of can be improved by increasing the frequency of the test telegrams, i.e. reducing the cycle time of the test telegram injection. However this would bring with it the disadvantage that the test telegrams would represent a significant additional network load. By contrast the invention advantageously completely avoids any additional load on the network with test telegrams.  
         [0011]     A further advantage can be seen in the fact that the redundancy manager and the branching units of the present invention do not have to participate in data traffic in the sense of data processing. Therefore the implementation effort is lower, the power requirement is reduced and the availability is increased because of the lower device complexity. The expansion of an existing network by redundancy properties is more simple to implement.  
         [0012]     Because the redundancy manager and the branching units are each provided with a termination element (terminating resistor) which can be connected in the case in which they are located at the end of the line in the relevant network topology, the signal transmission properties of the transmission link can be flexibly adapted to the prevailing topology after topology changes and thus the network is also suitable for higher baud rates.  
         [0013]     Advantageously and especially simple implementation of a branching unit is achieved if this is provided with at least two switches and with a control unit, with the control unit being able to set the two switches so that the user device connected by the relevant branching unit to the network can be connected through to the one, to the other or to both network connections of the branching unit to obtain operating energy and for data transmission.  
         [0014]     Advantageously it is made especially simple to test a cable connected to a branching unit for a short circuit or interruption if the branching unit features a resistor network in which the switches are arranged and when the switches can be controlled by the control unit such that current and/or voltage of the cable connected to the one or to the other network connection of the branching unit can be checked.  
         [0015]     Since the switching times of the switches are not infinitely short the operating energy injected at the line end or line ends can be made available to the user devices in the event of an error in an uninterruptible manner. To resolve this problem at least one of the branching units, but especially each of them, features an energy store which at least in the fault-free state can be charged by the feed voltage; The branching unit is embodied to record the voltage present at the connected user device and in the event of a voltage deficit to connect the energy store to the user device.  
         [0016]     As already mentioned, one advantage of the invention is that the redundancy manager and the branching unit do not have to take part in data traffic. An error, such as an interruption or a short circuit in a cable segment, can thus be displayed easily by the two branching units on both sides of the error location locally, for example using a light-emitting diode, however the redundancy manager can only detect the error status but not the error location. To enable the error location to be determined as well the redundancy manager advantageously features means which record the timing of the voltage and/or the current at the one end of the line during the forwarding of the feed voltage by the individual branching unit and from this determines the number of branching units up to the error location. The redundancy manager can display this information about the error location and/or notify a higher-ranking control system about it so that it can be established centrally where the error has occurred and is to be repaired. The redundancy manager thus preferably also has a communication interface for connection and exchange of data with a higher-ranking network in which the control system can be located.  
         [0017]     By recording changes in the voltage and/or the current at least one of the two line ends the redundancy manager can determine state transitions of the network and thus establish when an error has been repaired.  
         [0018]     To extend the redundancy of the inventive network beyond the redundancy manager through to a higher-ranking network, for example one containing a control system, the redundancy manager is preferably able to be connected via at least two segment couplers to at least two communication channels of the higher-ranking redundant network and is further embodied to monitor the functionality of the segment couplers and depending on the lists, to select one of the segment couplers for connection with the network. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention, along with its embodiments and advantages, is explained in greater detail below with reference to the drawings in which an exemplary embodiment of the invention is shown. The figures show:  
         [0020]      FIG. 1 a  block diagram of a part of a automation technology system,  
         [0021]      FIG. 2 a  block diagram of a redundancy manager,  
         [0022]      FIG. 3 a  block diagram of a part of a automation technology system in an alternate embodiment to  FIG. 1 ,  
         [0023]      FIG. 4 a  block diagram of a branching unit,  
         [0024]      FIG. 5 a  state table of a control unit of a branching unit,  
         [0025]      FIG. 6 a  state diagram corresponding to the state table shown in  FIG. 5  and  
         [0026]      FIG. 7 a  basic circuit diagram of an expanded branching unit. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0027]      FIG. 1  shows a part of an automation technology system. A control system  1  is connected to a bus system  2  in accordance with the PROFIBUS DP Specification. The bus system  2  can be configured as a simple system, or to provide redundancy as shown here. At each of the two communication channels  3 ,  4  of the bus system  2 , as well as other devices not shown here, for example automation units, a link  5  or  6  is connected in each case. The link  5  is linked to a segment coupler  7  which is connected via a drop cable  8  to a port A 1  of a redundancy manager RM. The other link  6  is linked to a further segment coupler  9  which is connected via a further drop cable  10  to a port A 2  of the redundancy manager RM. A network  11  which complies with the PROFIBUS PA specification and has a linear topology is connected to two further ports B 1  and B 2  of the redundancy manager RM. The one line end E 1  of the network  11  is formed by the end of the trunk cable H 3  connected to the port B 1 , which at its other end is connected to a network connection of a branching unit T 2 . The branching unit T 2  and further branching units T 1 , T 3 , T 4  serve to connect field devices F 1 , F 2 , F 3 , F 4  as user devices to the network  11 . In this case the field devices F 1  . . . F 4  are each connected via drop cables SK 1 , SK 2 , SK 3  or SK 4  to the relevant branching units T 1  . . . T 4 .  
         [0028]     So that a continuous line as a structure of the network  11  is achieved the other network connection of the branching unit T 2  is linked by a trunk cable H 1  to a network connection of the branching unit T 1 , the other network connection of the branching unit T 1  by a trunk cable H 2  to a network connection of the branching unit and T 3  and the other network connection of the branching unit T 3  by a trunk cable H 4  to the network connection of the branching unit T 4 . The other network connection of the branching unit T 4  is connected by a trunk cable H 5  to the port B 2  of the redundancy manager RM. The end of the trunk cable H 5  located at the port B 2  represents in the error-free case a second line end E 2  of the linear network  11 .  
         [0029]     Via the trunk cables H 3 , H 1 , H 2  and H 4 , as well as the data, energy to operate the field devices F 1  . . . F 4  is also transmitted. To this end each of the two segment couplers  7 ,  9  contains a direct current source and feeds direct current into both wires of the associated stub line  8  or  10 . The redundancy manager RM selects one of the two redundant segment couplers  7 ,  9 , in this case for example the segment coupler  7 , and when the system starts up switches the relevant port A 1  directly to port B 1  so that the feed voltage made available by this segment coupler  7  is also present at port B 1 . In the error-free case the branching units T 1  . . . T 4  forward the feed voltage arriving in each case on the one network connection to the other network connection. This means that the feed voltage is successively switch through to the line end E 2 , which is located at port B 2  of the redundancy manager RM. The redundancy manager RM monitors the incoming voltage at its port B 2 . If, after a delay depending on the network configuration, this does not comply with a predetermined required value, it is clear that there is an error present in network  11 . This can for example be a short circuit or an interruption in one of the trunk cables H 1  . . . H 5 .  
         [0030]     An interruption of the trunk cable H 2  between the branching units T 1  and T 3  will now be considered by way of an example, as is indicated in  FIG. 1  by a dashed interruption line  12 . This type of interruption is detected by the branching unit T 1  which subsequently does not forward the feed voltage so that the branching units T 3  and T 4  as w ell as the port B 2  of the redundancy manager RM are no longer reached. The redundancy manager RM detects the absence of the feed voltage at port B 2  and subsequently applies a voltage to supply the field devices F 3  and F 4  which lie beyond the error location, i.e. in the example described, beyond the interruption  12 , to its port B 2 . It does this by connecting the ports B 1  and B 2  and thereby the line ends E 1  and E 2  to each other. The supply voltage is switched through from port B 2  via the branching unit T 4  to the branching unit T 3  which lies immediately beyond the error location, detects the error  12  and thus does not forward the supply voltage. With the establishment of the supply voltage for all field devices F 1  . . . F 4  the data transmission in the network  11  and thereby the further operation of the network  11  is safeguarded despite the error  12 .  
         [0031]     The method of operation described on start-up of the network  11  can also contain further steps in which data will be exchanged between the branching units and the redundancy manager and/or in the reverse direction with a method not described in any greater detail here. Such an exchange of data enables the reliability of the network  11  to be increased and its start up and error detection also simplified.  
         [0032]     The redundancy manager RM shown in the example in  FIG. 2  contains a first control unit RCMA assigned to the ports A 1  and A 2 , a second control unit RCMB assigned to the ports B 1  and B 2  and higher-ranking controller RMC. The ports A 1 , A 2 , B 1  and B 2  are interconnected via a switching network with switched RMS 1 , RMS 2  and RMS 3 , with switch RMS 1  which can be controlled by the first control unit RMCA being used for selection of the two ports A 1 , A 2  and the switches RMS 2  and RMS 3  which can be controlled by the second control unit RMCB connecting the relevant selected port A 1  or A 2  either with one of the two ports B 1  and B 2 , with both ports B 1 , B 2  or with neither of the ports B 1  and B 2 . The ports B 1  and B 2  have termination elements BT 1 , BT 2  in the form of terminating resistors, which can be activated or deactivated.  
         [0033]     The first control unit RMCA monitors the currents and/or voltages at the ports A 1  and A 2  and in this way can monitor the segment couplers  7  and  9  (cf.  FIG. 1 ) and in the event of an error initiate the switchover from the faulty segment couplers to the others. The second control unit RMCB monitors the currents and/or voltages at the ports B 1  and B 2  and thus, as already explained above, can detect whether an error is present in the network  11  and whether accordingly one of the two ports B 1  and B 2  is to be connected to the relevant selected port A 1  or A 2 . In addition, as will be explained later, the second control unit RMCB can detect whether the error has been rectified in the network  11  and accordingly actuate the switches RMS 2  and RMS 3 .  
         [0034]     The higher-ranking control RMC is connected to the two control units RMCA and RMCB and has a communication interface RMI, in this case a PROFIBUS slave interface, for connection to the relevant selected port A 1  or A 2 . This enables the redundancy manager RM to communicate with the higher-ranking control system  1  in order for example to transmit status information so that suitable measures can be taken to rectify the error, or to receive configuration commands.  
         [0035]     Like the redundancy manager RM The branching units T 1  . . . T 4  also have connectible termination elements which are connected in if they are located at the end of a line in a linear topology to avoid signal reflections at the line end. In the error-free case the termination element BT 2  is connected in the redundancy manager RM at the port B 2  in the example explained on the basis of  FIG. 1 ; The termination element BT 1  is separated from the corresponding port B 1 . If an error occurs as has been explained in the example by the interruption  12  the redundancy manager RM separates the termination element BT 2  from the port B 2  and the branching units T 1  and T 3  lying on either side of the error location activate their relevant termination element. This means that even if the line ends are shifted signal reflections are effectively suppressed.  
         [0036]     The termination of the cable with a terminating resistor at the two ends of the trunk cable is required for a number of reasons:  
         [0037]     The bus signal is defined as a current signal with +10 mA which creates via the terminating resistors with two parallel-switched 100 Ohm resistors which correspond to a 50 Ohm resistance a defined voltage drop of +0.5 V.  
         [0038]     The maximum echo delay time in the cable of appr. 20 μs lies at 2 km in the order of magnitude of a signal half wave with approximately 16 μs, so that strong reflections would lead to bit errors. The overlapping should have settled down after a maximum of 20% of the duration of a half wave, corresponding to a line length of less than 300 m. Accordingly the IEC standard only allows drop lines of up to max. 120 m in length, with inherently secure networks of up to max. 30 m in length.  
         [0039]      FIG. 3  shows an embodiment of the automation system as per  FIG. 1  in which the redundancy manager RM is embodied in two parts. The first redundancy manager part RM 1  features the ports A 1  and B 1  with which it is connected to segment coupler  7  or to the line end E 1  of the network  11 . The other redundancy manager part RM 2  features the ports A 2  and B 2 , with which it is connected to the segment coupler  9  or to the line end E 2  of the network  11 . As is indicated by the dashed line, the redundancy manager RM 1  and the associated segment coupler  7  including the direct current source contained in it and if nec. the link  5 , can be grouped together in a first component  13  and the other redundancy manager RM 2  with the segment coupler  9  and the associated direct current source and if ne c. the link  6 , can be grouped together in a second component  14 . The exemplary embodiment shown here has the advantage that the network  11  does not have to be in the form of a ring because the ring is not closed at the line ends E 1  and E 2  but via the redundancy manager parts  13 ,  14 , the segment couplers  7 ,  9 , the links  5 ,  6  and the field bus system  2 .  
         [0040]     The basic structure of a branching unit is described in greater detail below with reference to the example of the branching unit T 1  in  FIG. 4 . The branching unit T 1  establishes the connection between the pairs of wires of the trunk cables H 1  and H 2  and the drop cable SK 1 . The trunk cables H 1  and H 2  are connected to the network connections NW 1  or NW 2  of the branching unit T 1 . As well as the data, the energy to operate the field devices is also transmitted over two pairs of copper wires H 1   a  and H 1   b , H 2   a  and H 2   b , as well as Sa and Sb of the cables H 1 , H 2  or SK 1 . To this end, as already mentioned, the segment couplers  7  and  9  (cf.  FIGS. 1 and 3 ) each contain a direct current source, with the segment coupler  7  selected by the redundancy manager RM injecting a direct current into the two wires of the transmission cable. The field devices F 1  . . . F 4  each take a proportion of the direct current and overlay the direct current with an alternating current which contains the information to be transmitted. The branching unit T 1  features a control unit ST, which with the aid of currents I 1 , I 2  and/or voltages U 1 , U 2 , which are measured on the trunk cables H 1  and H  2 , monitors the state of the connected cables H 1  and H 2  and also monitors the voltage U 3  on the drop cable leading to the field device F 1 . Furthermore the branching unit T 1  contains a resistance element BT, four switches S 0 , S 1 , S 2  and S 3 , a resistance net work consisting of resistors R 0 , R 1  and R 2  for voltage measurement and an energy accumulator C in the form of a capacitor. The position of the switches S 0  . . . S 3  is predetermined by the control unit ST depending on the recorded currents I 1  and I 2  and/or of the measured voltages U 1 , U 2  and U 3 . The terminating element BT which can be connected with the aid of the switch S 0  for the case in which the branching unit T 1  is located at the end of a line, corresponds to a standard terminating resistor of the PROFIBUS PA bus system. The size of the resistors R 0 , R 1  and R 2  is selected so that the state of the connected cables H 1  and H 2  can be determined in the optimum way. They are in this case preferably arranged to be of such high resistance that the current flowing over them is very small by comparison with the current which flows in normal operation over the cables H 1  and H 2 . In this case the line resistances of the cables H 1  and H 2  are negligibly small.  
         [0041]     The functioning of the branching units T 1  . . . T 4  is explained in greater detail below with reference to the state table shown in  FIG. 5  for the control unit ST of the branching unit T 1 . For the sake of simplicity it is assumed that the three resistors R 0 , R 1  and R 2  have the same resistance value. Beginning from the left, the current state of the control unit ST, test criteria for a state transition relating to the voltage U 1  and a power ratio K=U 2 /U 1 , the settings of the switches S 0 , S 1  and S 2 , the next state and remarks about the case concerned are entered in the columns of the table. The IDLE state specified in the table corresponds to the basic state which is assumed if the two trunk cables H 1  and H 2  are not carrying any voltage; I.e. the checked voltages U 1  and U 2  are equal to zero or at least smaller than a comparison voltage U 0 , which is to be defined in a suitable manner depending on at the relevant feed voltage. The same applies to a comparison current I 0  with which the currents I 2  and  11  can be compared. In this state the switches S 1  and S 2  are in the “on” position, as is specified in the table in the columns in the relevant row of the IDLE state of belonging to the switches S 1  and S 2 . Since the IDLE state is retained the IDLE state is again specified in this case in the column “next state”. In the column “K=U 2 /U 1 ” examples of the voltage ratio between the voltages U 2  and U 1  are specified for which overshoots and undershoots are monitored by the control unit ST. Depending on the result of the comparison, a transition is made from a current state into a next state. For example the entry in the column “K=U 2 /U 1 ” of the first row of the state TEST′, means that there will be a transfer into the follow-up state TEST 2  if the voltage ratio K lies between 1/10 and ⅖. The determination of the comparison values, here for example 1/10 and ⅖, with which the current voltage ratio K determined by the control unit ST is compared, depends on various peripheral conditions, especially the size of the resistors R 0 , R 1  and R 2  and is only specified here as an example. When a feed voltage US is switched on in the segment coupler  7  ( FIGS. 1 and 3 ) this voltage is fed via the redundancy manager RM, the branching unit T 2  and the trunk cable H 1  to the network connection NW 1  of the branching unit T 1  and U 1 =US&gt;U 0  applies. The control unit ST thus switches from the IDLE state into the TEST 1  state and measures the two voltages U 1  and U 2 . If the trunk cable in the subsequent segment, here the trunk cable H 2  is short-circuited, the voltage drop over the resistor R 0  will be very much greater than the voltage U 2  measurable at the network connection NW 2  or at the cable H 2 ; i.e. K=U 2 /U 1 &lt; 1/10. This corresponds to the second row of the state TEST 1  in the table. Because of the result of this test the state SHORT is assumed as the next state. In this case the switch S 2  remains in the “on” position and the short-circuited trunk cable H 2  is disconnected from the cable segment lying in front of it, the trunk cable H 1 . The connection via the high-impedance resistor R 0  can in this case be ignored. Simultaneously the line end produced in this way is terminated by switching over the switch S 0  to the position “on” via the termination element BT with the correct surge resistance.  
         [0042]     In accordance with first row of the state TEST 2  there is a transition from this state into the state OK if the voltage ratio K lies between 1/10 and ⅖. In the state OK the switches S 0 , S 1 , S 2  are in the “off” position and both trunk cables H 1  and H 2  connected to the branching unit T 1  are in order. Further distinctions between cases and state transitions which are produced by the various measurements of the voltages U 1  and U 2  by the control unit ST can be seen from the state diagram in  FIG. 6 .  
         [0043]     In the example explained above the feed voltage was fed to the branching unit T 1  via the trunk cable H 1 . If this voltage is fed alternately via the trunk cable H 2  the associated state table can be simply obtained by swapping over the indices for the voltages U 1  and U 2 .  
         [0044]     The states SHORT (trunk cable H 2  short circuited) and OPEN (trunk cable H 2  open circuited) are error states which lead to the injection of the feed voltage by the redundancy manager RM via both ports B 1  and B 2 . If the error concerned is rectified the branching unit T 1  involved is initially switched to the state IDLE. Since the switch S 0  is in the “off” position and in this case, the linear network  11  fed from both sides has no line termination. This leads to an increase in the signal amplitude which is detected by the second control unit RMCB of the redundancy manager RM and which causes the latter to open the switch RMS 3  and thus cancel the power feed at the port B 2  again. As a result of the now error-free Network  11  the actual operating state OK is reached via the state TEST 1 .  
         [0045]     In the exemplary embodiment described the determination of the state of the cable segment to be monitored uses the ratio between output and input voltage of the branching unit. Instead this state can also for example be determined from the absolute values of the voltages and the currents.  
         [0046]     The capacitor C shown in  FIG. 4  is used to ensure an interruptible power supply for the user device F 1  connected to the branching unit T 1  even in the period caused by switching delays between the occurrence of an error and the establishment of the alternate energy supply. To this end the capacitor C is charged with the feed voltage in the operating phase if at least one of the switches S 1 , S 2  is in the “off” position via a decoupling diode D 1  and a charge resistor R 3 . The control unit ST monitors the voltage U 3  present on the drop cable SK 1  to the user device F 1  and switches the capacitor C via the switch S 3  to the drop cable SK 1  if it detects a voltage U 3  which is too low. The decoupling diode D 1  and a further decoupling diode D 2  prevent a flow back of energy into the network  11 .  
         [0047]     A RESET of the system can be initiated manually automatically by the redundancy manager RM and briefly switching off the feed voltage to the ports B 1  and B 2 , after which via the IDLE state a new test cycle is initiated, running through the TEST′ and TEST 2  states.  
         [0048]     The embodiment of a network described has the advantages that the branching units can draw their comparatively low operating energy from the trunk cable, the control units ST of the branching units operate independently and the signal path can be embodied as a passive path since there are only resistors and switches between the network connections of the branching units. An active solution, for example with a signal refresh as with repeaters is however also possible.  
         [0049]     In  FIG. 6  the states and state transitions of the table from  FIG. 5  are shown once again for a better overview in the form of a state diagram which describes the same behavior of the control unit ST as the table. The following applies to all states: For U 1 &lt;U 0  a RESET is performed by the redundancy manager RM and the sequence begins with the IDLE state.  
         [0050]     For the exemplary embodiment described above a short circuit in the drop cable SK 1  is not dealt with. It are can however be expanded in a simple manner so that all cables connected to a branching unit can be monitored.  FIG. 7  shows a basic circuit diagram of a branching unit T 5  expanded in this way which by comparison with the branching unit T 1  explained with reference to  FIG. 4  has been expanded by a switch S 4  and resistors R 4 , R 5  and R 6 . The principle of monitoring three cables is similar to the monitoring of two cables described above and can thus be understood per se by person skilled and the art with reference to  FIG. 7 .  
         [0051]     In the exemplary embodiment described the branching units are set up separately from the field devices and merely connected to the latter via a drop cable in each case. As an alternative to this a branching unit can be integrated into the housing of the field device concerned.  
         [0052]     Alternatively to the branching unit shown in  FIG. 4  with a drop cable connection it is possible to embody the unit without a drop cable connection or not connect a drop cable. This makes it possible to divide the network line up into predetermined line segments which can be monitored individually. Errors can be localized and rectified more simply in this way.  
         [0053]     A further alternative is to equip the branching units with a number of drop cable connections for field devices.  
         [0054]     If the error is to be repaired after the error location has been determined, unstable states for example in the form of intermittent contacts can occur by which operation of the system will be adversely affected. To avoid this provision can be made for fixing the states at the network connections of the branching unit which can for example be done by a short circuit connectors which are then removed again after repair. This state fixing can also be used with the corresponding layout for explicitly deactivating individual cable segments and thus makes maintenance work easier in an explosion-hazard area. In this case there is the option of mechanically covering the terminals of the network connections which may not be worked on, with the coverings being designed so that when they are removed both the short circuit mentioned above the occurence of sparks which are capable of ignition arising is prevented.