Abstract:
An arrangement for a motor vehicle includes a master controller and a slave controller that use different reference-earth voltages and respectively comprise a transmission and a reception unit for shared data interchange. The transmission and reception units are connected to one another via a shared communication line. The communication line contains a diode that prevents a flow of current from the slave controller to the master controller. The master controller contains a pull-up resistor via which the shared communication line can be connected to a first reference-earth potential of the reference-earth voltage of the master controller.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2014/063661, filed on Jun. 27, 2014, which claims the benefit of priority to Serial No. DE 10 2013 214 953.0, filed on Jul. 31, 2013 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to an arrangement including a master controller and a slave controller which are interconnected for exchanging data with each other via a communication line. The present disclosure also relates to the master controller and the slave controller. 
     Components in the motor vehicle which are controllable via a central controller, hereinafter master controller, and which operate at a reference voltage differing from the reference voltage of the master controller, each require separate power supply lines and ground connections in order to ensure that fault currents into the master controller are prevented in the event of a loss of ground, in the event of short circuits, or due to reversed polarities. 
     For the communication with the master controller, such components each have a transceiver unit, wherein the transceiver unit communicates with the transceiver unit of the master controller. In addition, such components include a consumer, for example, a positioning drive for a seat, a windshield wiper, a tailgate, or a window. Furthermore, such components include a device controller which is provided for the control of the consumer as a function of the data transmitted by the master controller. 
     In most cases, multiple such components are connectable to the master controller. In this case, the communication between the components and the master controller, i.e., which device accesses the communication line at which time, is controlled via a protocol in which the master controller in most cases acts as the master and the components in most cases act as slaves. Therefore, an electrical portion of each of the components, which includes its transceiver unit and its device controller, is hereinafter referred to as a slave controller. 
     A master controller conventionally functions at a 12V on-board electrical system voltage as the reference voltage. It provides the slave controllers, which often operate at a reference voltage of 12V, 24V, or 48V, with a pulse width modulated (PWM) communication signal for the communication via the communication line, which includes control commands for controlling the consumers. 
     In order to prevent fault currents which may damage the master controller, the connection of slave controllers which use a reference voltage other than that of the master controller has previously been allowable only via galvanic isolation. 
       FIG. 1  shows an arrangement having a first specific embodiment of a slave controller and a master controller, in which both the master controller and the slave controller use a 12V on-board electrical system voltage as the reference voltage. Therefore, in this specific embodiment according to the related art, no galvanic isolation is required. 
       FIG. 2  shows an arrangement according to the related art having a second specific embodiment of a slave controller which uses a reference voltage different from that of the master controller, for example, a 24V or 48V reference voltage instead of the 12V reference voltage. In order for no fault currents to be able to flow from the slave controller to the master controller in the event of a ground loss, due to short circuits, or due to reversed polarity, the transceiver unit of the slave controller is galvanically isolated from its device controller operating at the other reference voltage, for example, via optical couplers (not shown). 
     However, the use of galvanic isolation is expensive and requires extensive installation space. 
     SUMMARY 
     The object of the present disclosure is therefore to provide an arrangement for connecting a master controller using a first reference voltage to a slave controller of a motor vehicle using a second reference voltage, which requires no galvanic isolation and may therefore be manufactured in a more economical and compact manner. 
     The object is achieved via an arrangement made up of a master controller and a slave controller for a motor vehicle, wherein the master controller includes a first transceiver unit and the slave controller includes a second transceiver unit, and a communication line connecting the transceiver units, wherein the master controller operates at a first reference voltage and the slave controller operates at a second reference voltage, the voltages being different, and wherein a diode is provided in the communication line which prevents a current flow from the second transceiver unit in the direction of the first transceiver unit. In the event of a ground loss, in the event of a short circuit, or due to reversed polarity, a fault current flowing from the slave controller via the communication line into the master controller is therefore not able to flow. 
     It is preferred that the reference voltages are DC voltages. 
     Particularly preferably, the first reference voltage is lower than the second reference voltage, in order to ensure the current flow during the communication from the slave controller to the master controller. In this case, the first reference voltage and the second reference voltage each drop between a first reference potential and a second reference potential. 
     In addition, it is preferred that the first and second reference voltages are each connected to a central ground point, in particular of the motor vehicle. They thus have a shared second reference potential which is interconnected at the central ground point, i.e., ground. In this specific embodiment, the first reference potential of the first reference voltage is therefore lower than the first reference potential of the second reference voltage. 
     In a preferred specific embodiment which also achieves the object, the communication line is connectable to a first power supply line of the master controller via a pull-up resistor. It is preferred that the first reference potential of the first reference voltage is applied to the first power supply line. By connecting the communication line to the power supply line, approximately the first reference potential, reduced by the voltage drop at the pull-up resistor, is therefore applied to the communication line as the first voltage potential. 
     The arrangement of this specific embodiment is characterized by the pull-up resistor being situated in the master controller. As a result, the slave controller does not need the first reference voltage at all, and, in addition, no connection means are required for connecting a first power supply line and/or a first ground line for the first reference voltage at the slave controller. Since the first reference voltage is not provided at all at the slave controller, in addition, no galvanic isolation is required. 
     To connect the communication line to the first power supply line, it is preferred that the transceiver unit of the master controller includes a first electric switch. If the first electric switch is closed, it is preferred that the communication line is connected to the first power supply line. Preferably, a pulsing transmission signal may be generated on the communication line by alternately opening and closing the first electric switch. If the first electric switch is closed, the first voltage potential applied to the communication line is changed into a second voltage potential applied to the communication line. The second voltage potential is preferably approximately ground. By changing the time during which the first electric switch is closed or open, a pulse-width-modulated transmission signal may preferably be generated on the communication line. The master controller preferably includes a device controller with which the opening and closing of the first electric switch, in particular the period in which it is open or closed, is controllable. In this specific embodiment, a transmission from the master controller to the slave controller is therefore possible by opening and closing the first electric switch. 
     In addition, it is preferred that the transceiver unit of the slave controller includes a second electric switch. The second electric switch is provided for transmitting a communication signal from the slave controller to the master controller. Preferably, a pulsing transmission signal may also be generated on the communication line by alternately opening and closing the second electric switch. Also when transmitting from the slave controller to the master controller, it is preferred that the voltage applied to the communication line is provided via the power supply line of the master controller. For this purpose, it is preferred that the first electric switch of the first transceiver unit of the master controller is closed in a reception mode, in particular when transmitting the transmission signal from the slave controller to the master controller. When transmitting from the slave controller to the master controller, if the second switch is open, approximately the first reference potential of the first reference voltage, reduced by the voltage drop at the pull-up resistor, is then applied to the communication line. In this case as well, if the second electric switch is open, it is preferred that the first voltage potential applied to the communication line is changeable to a second voltage potential applied to the communication line, namely, also approximately to ground, by closing the second electric switch. Furthermore, it is preferred that a pulse-width-modulated transmission signal is able to be generated on the communication line via a change in the time in which the second electric switch is closed or open. Preferably, for this purpose, the slave controller also has a device controller via which the opening and closing of the second electric switch, in particular the period in which it is open or closed, is controllable. In this specific embodiment, a transmission from the slave controller to the master controller is therefore possible by opening and closing the second electric switch. 
     Since the first voltage potential required on the communication line both when transmitting from the slave controller to the master controller and when transmitting from the master controller to the slave controller is provided with the aid of the first reference potential, which is applied to the first power supply line, of the first reference voltage at which the master controller operates, only the power supply lines are provided at the slave controller which are required for the second reference voltage with which it operates. The master controller is therefore connected directly to the slave controller only via the communication line. 
     The first electric switch and/or the second electric switch are preferably formed as transistors, in particular as bipolar transistors. However, other electric switches, such as field-effect transistors, IGBTs (insulated-gate bipolar transistors), or relays are also usable as a first or second electric switch. 
     The diode is preferably situated in the transceiver unit of the slave controller. In principle, it may also be situated in the communication line or in the master controller. However, the placement in the slave controller has the advantage that the connection of multiple slave controllers to the communication line is possible via a simple parallel arrangement of the slave controllers. The cathode of the diode preferably points in the direction of the slave controller, so that a current flow is possible only from the master controller to the slave controller, or so that the current flow from the slave controller to the master controller is prevented. 
     The object is also achieved via a master controller for such an arrangement. It is preferred that the master controller includes the pull-up resistor. The first voltage potential on the communication line is therefore provided via the first reference voltage, which is applied to the first power supply line of the master controller. Therefore, the slave controller does not require the first reference voltage at all. 
     Therefore, in addition, no galvanic isolation is required. 
     It is preferred that the master controller includes a transceiver unit including a first electric switch which is provided for connecting the communication line to the first power supply line. In this case, the arrangement made up of the pull-up resistor connected to the first power supply line and the first electric switch acts like a current source which provides the signal provided on the power supply line. In principle, this arrangement is therefore replaceable by a current source. 
     The voltage potential available on the communication line is preferably determined via a voltage divider which includes the pull-up resistor and a second resistor. 
     Furthermore, the object is achieved via a slave controller for such an arrangement. It is preferred that the slave controller includes the diode in order to prevent the fault currents from the slave controller to the master controller. 
     The master controller and the slave controller are interconnected for transmission and reception in both directions only via a single communication line. In addition, no galvanic isolation is necessary between the device controller and the transceiver unit of the slave controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described below based on figures. The figures are merely exemplary and do not limit the general idea of the present disclosure. 
         FIG. 1  shows, in (a), an arrangement for connecting a slave controller to a master controller according to the related art, wherein both the first slave controller and the master controller use the same reference voltage, and in (b), by way of example, a pulse-width-modulated transmission signal applied to a communication line; 
         FIG. 2  shows an additional arrangement for connecting a slave controller to the master controller of  FIG. 1  according to the related art, wherein the slave controller uses a reference voltage which is different from that of the master controller; 
         FIG. 3  shows an arrangement according to the present disclosure for connecting a slave controller to a master controller, wherein the slave controller uses a reference voltage which is different from that of the master controller; and 
         FIG. 4  schematically depicts, in (a), a loss of ground on the side of the slave controller, and in (b), a reversed polarity of a second reference voltage of the slave controller. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an arrangement  3  for the connection of a slave controller  2  to a master controller  1 . Both the master controller  1  and the slave controller  2 , which are also depicted below as components of the arrangement  3 , use a 12V on-board electrical system voltage as a first reference voltage. Therefore, in this exemplary embodiment according to the related art, no galvanic isolation is required. Since the slave controller  2  operates at the same reference voltage as the master controller, a shared ground line  42  from the slave controller  2  to the master controller  1  is provided and is allowable. 
     The master controller  1  is situated in a controller housing  10 . It includes a device controller  11  and a transceiver unit  14 . The device controller includes a transmission line  132  for transmitting a transmission signal PWM from the master controller  1  to the slave controller  2 , and a reception line  131  for receiving a transmission signal PWM from the slave controller  2  to the master controller  1 . 
     The device controller  11  is connected via a first power supply line  44  and a ground line  42  to a direct-current source (not shown), which provides a first reference voltage U 1 . A first reference potential of the first reference voltage U 1 , here, a 12V on-board electrical system voltage of a motor vehicle, is applied to the first power supply line  44 . A second reference potential of the first reference voltage U 1 , i.e., ground, is applied to the ground line  42 . 
     In addition, the device controller  11  is connected to an ignition switch line  46  which is connected to an ignition key (not shown) of the motor vehicle, so that the master controller  1  is active only if the ignition is switched on. As a result, it is ensured that the master controller  1  and/or the slave controller  2  connected to the master controller  1  are not inadvertently active. The ignition switch line  46  is also at the first reference potential of the first reference voltage U 1 , i.e., at 12V. 
     For transmission, the transceiver unit  14  includes a first switch  12 , which is designed here as a bipolar transistor. The bipolar transistor is situated in a common-emitter circuit. The terms first switch  12  and first transistor are used synonymously below. For this reason, the emitter E of the first transistor  12  is connected to the ground line  42 , the base B of the first transistor  12  is connected to the transmission line  132  of the device controller  11 , and the collector C is connected to a communication line  43  between the master controller  1  and the slave controller  2 . The communication line  43  is also connected to the reception line  131  of the device controller  11  and is designed to have a high resistance. 
     The slave controller  2  is also situated in a slave controller housing  20 . In order to be able to connect the slave controller  2  to the master controller  1 , connecting plugs  19 ,  29  are provided in each of the housings  10 ,  20  of the components  1 ,  2 , which are connected to each other in the interconnected state via connecting lines  42 - 44 ,  46 . 
     The slave controller  2  also includes a device controller  21  which includes a transmission line  232  and a reception line  231 . Here as well, a transceiver unit  24  including a second switch  22  is provided, which is designed as a bipolar transistor. This second transistor  22  is also connected in a common-emitter circuit. The terms second switch  22  and second transistor are used synonymously below. 
     The transmission line  232  of the slave controller  2  is connected to the base B of the second transistor  22 . The emitter of the second transistor  22  is connected to the ground line  42 . The collector C of the second transistor  22  is connected to the communication line  43 . In addition, it is connected to a defined voltage via a pull-up resistor R 1 , namely, here, to the first reference voltage U 1  via the ignition switch line  46 . In principle, a connection of the pull-up resistor R 1  to the first reference voltage U 1  is also possible via the first power supply line  44 . However, the connection via the ignition switch line  46  has the aforementioned advantage with respect to the connection via the first power supply line  44  that a communication is then possible only if the ignition is switched on. 
     The reception line  231  of the slave controller  2  is connected to a voltage divider R 3 R 4  which includes a third resistor R 3  and a fourth resistor R 4 . The third resistor R 3  connects the communication line  43  to the reception line  231 . The fourth resistor R 4  connects the reception line  231  to ground M. As a result, when receiving the transmission signal PWM from the master controller  1  to the slave controller  2 , a voltage potential U 231  which is applied to the reception line  231  and to the device controller  21  is adjusted in such a way that it is processable by the device controller  21  of the slave controller  2 . 
     To transmit the transmission signal PWM from the master controller  1  to the slave controller  2 , the first electric switch  12  is alternately opened and closed. For that purpose, for closing, a voltage (not shown) is provided by the device controller  11  of the master controller  1  at the base B of the first transistor  12 , which makes a current flow I from the collector C to the emitter E possible. The first electric switch  12  is reopened in that this voltage is not provided, and therefore no current flow I is possible from the collector C to the emitter E. In this case, the second electric switch  22  of the transceiver unit  24  of the slave controller  2  is open. 
     If the first electric switch  12  is open, the first reference voltage U 1  across the pull up-resistor R 1  and the voltage divider R 3 R 4  in the slave controller  2  drops to ground. The communication line  43  is then at a first voltage potential P which corresponds to the first reference potential 12V of the first reference voltage U 1 , reduced by the voltage drop U R1  at the pull up resistor R 1 . 
     If the first electric switch  12  is closed, the communication line  43  is connected to ground M via the first electric switch  12 . A second voltage potential P′ is then applied to the communication line  43 , i.e., approximately ground M or a second voltage potential P′ which is slightly higher than ground M. The voltage U 43  applied to the reception line  43  therefore pulses between the first voltage potential P and the second voltage potential P′. 
     The voltage U 132  applied to the base B of the first transistor  12 , in particular the time t in which this voltage U 132  is high enough for the first transistor  12  to be enabled or for the first switch  12  to be closed, is controllable via the device controller  11  of the master controller  1 . This control therefore produces a pulse-width-modulated transmission signal PWM, U 43  on the communication line  43 .  FIG. 1( b )  shows such a signal PWM, U 43 . 
     The transmission signal PWM is also applied to the reception line  231  of the slave controller  2 , wherein the first and second voltage potentials P, P′ are reduced by the voltage U R3  dropping across the third resistor R 3  of the voltage divider R 3 R 4 . As a result, the device controller  21  of the slave controller  2  sees the transmission signal PWM transmitted by the master controller  1  and is able to process it. 
     To transmit the transmission signal PWM from the slave controller  2  to the master controller  1 , the second electric switch  22  situated in the slave controller  2  is alternately opened and closed. For this purpose, a sufficiently high voltage U 232  is provided at the base B of the second electric switch  21  via the transmission line  232  of the device controller  21  of the slave controller  2 , so that the second transistor  22  is enabled and a current is able to flow from the collector C to the emitter E. In this case, the first electric switch  12  in the master controller  1  is open. 
     If the second switch  22  is open, the first reference voltage U 1  drops across the pull-up resistor R 1  and the voltage divider R 3 R 4  in the slave controller  2 . The communication line  43  is then at the first voltage potential P, which corresponds to the first reference potential 12V of the first reference voltage U 1 , reduced by the voltage drop U R1  at the pull-up resistor R 1 . 
     If the second electric switch  22  is closed, the communication line  43  is connected to ground M via the second electric switch  22 . The communication line  43  is then at the second voltage potential P′, i.e., also approximately at ground M. The voltage U 43  applied to the reception line  43  during the transmission from the slave controller  2  to the master controller  1  therefore pulses between the first voltage potential P and the second voltage potential P′, in a manner similar to the transmission from the master controller  1  to the slave controller  2 . 
     Since the communication line  43  is connected to the reception line  131  device controller  11  of the master controller  1 , the device controller  11  of the master controller  1  sees the transmission signal PWM from the slave controller  2  and is able to process it. 
     In this arrangement  3  according to the related art, both controllers  1 ,  2  operate at the same reference voltage U 1 . The interconnection of the power supply lines  42 ,  44 ,  46  of the controllers  1 ,  2  is therefore non-critical. 
     In order to prevent fault currents which could damage the master controller  1 , the connection of slave controllers  2  which use a reference voltage U 2  which is different from that of the master controller  1  has previously been allowed only via a galvanic isolation  25 , for example, via optical couplers (not shown).  FIG. 2  shows such an arrangement  3  according to the related art. 
     In this arrangement  3 , the device controller  21  of the slave controller  2  operates at a second reference voltage U 2  which is different from the first reference voltage U 1 , i.e., for example, at 24V or at 48V. The slave controller  2  is therefore connected via a second power supply line  40  and a second ground line  41  to a second DC voltage source (not shown) which provides the second reference voltage U 2 . 
     The slave controller  2  is also different from the slave controller  2  of  FIG. 1  in that a galvanic isolation  25  is provided between the transceiver unit  24  of the slave controller  2  of  FIG. 2  and its device controller  21 . As a result, the reception line  231 , which is directly connected to the communication line  43  in the arrangement  3  of  FIG. 1 , is galvanically isolated from it here. In addition, as a result, the transmission line  232 , which is directly connected to the base B of the second transistor  22  in the arrangement  3  of  FIG. 1 , is also galvanically isolated from it here. As a result, all electrical connections  231 ,  232  between the device controller  21  operating at the second reference voltage U 2  and the transceiver unit  24  operating at the first reference voltage U 1  are galvanically isolated from each other here. Fault currents due to a short circuit, loss of ground, or reversed polarity on the side of the device controller  21  of the slave controller  2  are therefore not able to flow at all via the transceiver unit  24  of the slave controller  2  to the master controller  1 . 
     Moreover, the design of the transceiver unit  24  of the slave controller  2  of this specific embodiment is identical to that of the slave controller  2  of  FIG. 1 . This is schematically depicted by a line  5 . In addition, in this arrangement  3 , the same master controller  1  is used as in  FIG. 1 . The transmission from the master controller  1  to the slave controller  2  and from the slave controller  2  to the master controller  1  is therefore carried out in a manner similar to that already described in  FIG. 1 . 
       FIG. 3  shows an arrangement  3  of a master controller  1  and a slave controller  2 , in which a galvanic isolation  25  (see  FIG. 2 ) of the connecting lines  231 ,  232  from the device controller  21  to the transceiver unit  24  of the slave controller  2  may be omitted. 
     In this specific embodiment, on the one hand, a fault current from the slave controller  2  via the communication line  43  to the master controller  1  is prevented with the aid of a diode D 1  which is situated in the communication line  43 . Here, the diode D 1  is situated in the slave controller  2 , but in principle may also be situated anywhere else in the communication line  43 , also in the master controller  1 . 
     In addition, when transmitting over the communication line  43 , the first voltage potential P applied to it is also provided via the first power supply line  44  or the ignition switch line  46 . However, here, the pull-up resistor R 1  is not situated in the slave controller  2 , but rather in the master controller  1 . As a result, the first reference voltage U 1  is not required at all in the slave controller  2 , so that the master controller  1  and the slave controller  2  are interconnected only via the communication line  43 . 
     In this arrangement as well, the master controller  1  is situated in a controller housing  10 . It also includes a device controller  11  and a transceiver unit  14 . The device controller includes a transmission line  132  for transmitting a transmission signal PWM from the master controller  1  to the slave controller  2  and a reception line  131  for receiving a transmission signal PWM from the slave controller  2  to the master controller  1 . 
     The device controller  11  is connected via a first power supply line  44  and a ground line  42  to the DC voltage source (not shown), which provides the first reference voltage U 1 . The first reference potential of the first reference voltage U 1 , i.e., the 12V on-board electrical system voltage, is applied to the first power supply line  44 . The second reference potential of the first reference voltage U 1 , i.e., ground, is applied to the ground line  42 . 
     The ignition switch line  46  is not shown here. 
     For transmission, the transceiver unit  14  includes the first switch  12 , which is also designed here as a bipolar transistor. It is also situated in a common-emitter circuit. For this purpose, the emitter E of the first transistor  12  is connected to the communication line  43 , the base B of the first transistor  12  is connected to the transmission line  132  of the device controller  11 , and the collector C of the first transistor  12  is connected via the pull-up resistor R 1  to the first power supply line  44 . In addition, the communication line  43  is connected to the ground line  42  via a second resistor R 2 . In addition, it is connected to the reception line  131  of the device controller  11 , which is also designed here to have a high resistance. 
     The slave controller  2  is also situated in a slave controller housing  20 . The slave controller  2  also includes the device controller  21 , which includes the transmission line  232  and the reception line  231 . Here as well, the transceiver unit  24  is provided including a second switch  22  which is designed as a bipolar transistor. This second transistor  22  is also connected in a common-emitter circuit. 
     The transmission line  232  of the slave controller  2  is connected to the base B of the second transistor  22 . The emitter of the second transistor  22  is connected to ground M. The collector C of the second transistor  22  is connected to the communication line  43 . 
     Here as well, the reception line  231  of the slave controller  2  is connected to a voltage divider R 3 R 4  which includes a third resistor R 3  and a fourth resistor R 4 . The third resistor R 3  connects the communication line  43  to the reception line  231 ; the fourth resistor R 4  connects the reception line  231  to ground M. As a result, when receiving the transmission signal PWM from the master controller  1  to the slave controller  2 , a voltage potential U 231  applied to the reception line  231  or to the device controller  21  is also adjusted here in such a way that it is processable by the device controller  21  of the slave controller  2 . 
     The device controller  21  of the slave controller  2  operates at the second reference voltage U 2 , and is therefore connected here via the second power supply line  40  and the second ground line  41  to the second DC voltage source, which provides this voltage. 
     To transmit the transmission signal PWM from the master controller  1  to the slave controller  2 , the first electric switch  12  is alternately opened and closed. For this purpose, for closing, the voltage (not shown) is also provided here by the device controller  11  of the master controller  1  at the base B of the first transistor  12 , which makes the current flow I from the collector C to the emitter E possible, and for opening, it is correspondingly reduced. In this case, the second electric switch  22  of the transceiver unit  24  of the slave controller  2  is open. 
     If the first electric switch  12  is closed, the first reference voltage U 1  across the pull-up resistor R 1  and the voltage divider R 3 R 4  in the slave controller  2  drops to ground. This first electrical connection V 1  is schematically depicted by dashed lines in  FIG. 3 . The communication line  43  is then at the first voltage potential P, which corresponds to the first reference potential 12V of the first reference voltage U 1 , reduced by the voltage drop U R1  at the pull-up resistor R 1 . In order to provide a defined voltage potential P on the communication line  43  in the case of a closed first electric switch  12 , a second resistor R 2  is provided, which, along with the pull-up resistor R 1 , forms a voltage divider R 1 R 2 . 
     If the first electric switch  12  is open, the first connection V 1  is interrupted and the communication line  43  is connected to ground M via the voltage divider R 3 R 4 . A second voltage potential P′ is then applied to the communication line  43 , i.e., approximately ground M or a second voltage potential P′ which is slightly higher than ground M. The voltage U 43  applied to the reception line  43  therefore pulses between the first voltage potential P and the second voltage potential P′ in a manner similar to the transmission signal PWM shown in  FIG. 1( b ) . 
     Here as well, the voltage U 132  applied to the base B of the first transistor  12 , in particular the time t in which this voltage U 132  is high enough for the first transistor  12  to be enabled or for the first switch  12  to be closed, is controllable via the device controller  11  of the master controller  1 . This control therefore also produces a pulse-width-modulated transmission signal PWM, U 43  on the communication line  43 . 
     The transmission signal PWM is also applied to the reception line  231  of the slave controller  2 , wherein the first and second voltage potentials P, P′ are reduced by the voltage U R3  dropping across the third resistor R 3  of the voltage divider R 3 R 4 . As a result, the device controller  21  of the slave controller  2  sees the transmission signal PWM transmitted by the master controller  1  and is able to process it. 
     To transmit the transmission signal PWM from the slave controller  2  to the master controller  1 , the second electric switch  22  situated in the slave controller  2  is alternately opened and closed. For this purpose, a sufficiently high voltage U 232  is provided at the base B of the second electric switch  21  via the transmission line  232  of the device controller  21  of the slave controller  2 , so that the second transistor  22  is enabled and a current is able to flow from the collector C to the emitter E. In this case, the first electric switch  12  in the master controller  1  is closed. 
     If the second electric switch  22  is open, the first reference voltage U 1  across the pull-up resistor R 1  and the voltage divider R 3 R 4  in the slave controller  2  drops to ground. This second electrical connection in the case of closed first and second switches  12 ,  22  is schematically depicted in  FIG. 3  by a dashed line V 2 . The communication line  43  is then at the first voltage potential P, which corresponds to the first reference potential 12V of the first reference voltage U 1 , reduced by the voltage drop U R1  at the pull-up resistor R 1 . 
     If the second electric switch  22  is closed, the communication line  43  is connected to ground M via the second electric switch  22 . The communication line  43  is then at the second voltage potential P′, i.e., also approximately ground M. The voltage U 43  applied to the reception line  43  during the transmission from the slave controller  2  to the master controller  1  therefore pulses between the first voltage potential P and the second voltage potential P′, in a manner similar to the transmission from the master controller  1  to the slave controller  2 . 
     Since the communication line  43  is connected to the reception line  131  of the device controller  11  of the master controller  1 , the device controller  11  of the master controller  1  sees the transmission signal PWM from the slave controller  2  and is able to process it. 
     In all aforementioned specific embodiments, the ground potential or the ground of the DC voltage sources is provided via a central ground point M in the motor vehicle. The central ground point M and the ground or the ground potential are therefore depicted here having the same reference numeral M. However, in order to prevent a current flow via the ground lines  41 ,  42  which are provided at the controllers  1 ,  2 , they are not interconnected. 
     The arrangement of  FIG. 3  requires only a single connection between the two controllers  1 ,  2 , i.e., the communication line  43 . In addition, it requires no galvanic isolation  25 . As a result, it may be manufactured more economically and is easier to install. 
       FIGS. 4 ( a )  and  4  ( b ) schematically depict fault cases.  FIG. 4 ( a )  shows a loss of ground or a short circuit between the second power supply line  40  and the second ground line  41  of the slave controller  2 . As a result, the first reference potential of the second reference voltage, i.e., for example, 24V or 48V, is at ground. However, a fault current via the communication line  43  to the master controller  1  is prevented by the diode D 1 . 
       FIG. 4 ( b )  schematically depicts a reversed polarity of the second power supply line  40  and the second ground line  41 . As a result, the first reference potential of the second reference voltage, i.e., for example, 24V or 48V, is also at ground. However, in this case, a fault current via the communication line  43  to the master controller  1  is also prevented by the diode D 1 .