fig1 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 . 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 . 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 . 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 fig1 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 well 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 . 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 . the redundancy manager rm shown in the example in fig2 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 . 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 . fig1 ) 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 . 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 . 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 fig1 ; 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 . the termination of the cable with a terminating resistor at the two ends of the trunk cable is required for a number of reasons : 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 . 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 . fig3 shows an embodiment of the automation system as per fig1 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 . 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 fig4 . 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 . fig1 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 . the functioning of the branching units t 1 . . . t 4 is explained in greater detail below with reference to the state table shown in fig5 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 i 1 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 ( fig1 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 . 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 fig6 . 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 . 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 . 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 . the capacitor c shown in fig4 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 . 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 . 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 . in fig6 the states and state transitions of the table from fig5 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 . 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 . fig7 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 fig4 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 fig7 . 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 . alternatively to the branching unit shown in fig4 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 . a further alternative is to equip the branching units with a number of drop cable connections for field devices . 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 .