NETWORK COMMUNICATION SYSTEM WITH BIDIRECTIONAL POWER MODULATION FOR DATA TRANSMISSION

Description

[0001] The invention relates to a network communication system for a vehicle, comprising a communication bus, a first communication device, and a plurality of second communication devices that are connected to one another via the communication bus according to the preamble features of patent claim 1, as well as a method for bidirectional, current modulation-based communication in such a network communication system according to the preamble features of patent claim 12.

[0002] I amVarious types of network communication systems are known in the art. One type of network communication system is a so-called 1:N network communication system, which comprises a master network node, also referred to simply as the master, and a plurality of satellite network nodes, referred to below as slaves. The master can transmit information to and receive information from the slaves. However, the slaves cannot communicate with each other. In this case, the master corresponds to the first communication device, while the slave corresponds to the second communication device.

[0003] Information is transmitted from the master to the slaves and from each individual slave to the master via a transmission medium, which is an electrical connection line or a communication bus. The information to be transmitted in one direction or the other is modulated, or encoded, into electrical signals on the connection line. The master modulates data received from one or more slaves, and the slaves decode the data. The slaves also modulate their transmitted data, which is then received and decoded by the master.

[0004] Due to the physical properties of the connecting cables used to transmit information, electromagnetic radiation is generated whenever an electrical state of the connecting cable begins to change. This occurs when the electrical signals on the connecting cable are no longer constant but are modulated with data.

[0005] In Figure 1 Such a network communication system 10 is shown as an example. The network communication system 10 comprises, as described above, a master 12 and a plurality of slaves 14, of which Figure 1For the sake of simplicity, only one slave 14 is shown. The master 12 and the slave 14 are electrically connected to each other via a communication bus 16. Stray capacitances 18 arise along the communication bus 16, to which signal currents (alternating current) can flow, thereby generating electromagnetic radiation. Accordingly, a current from the master 12 to the slave 14 is reduced at each of the stray capacitances by the outgoing portion of the current, so that starting from an output current i 0 at the master 12, only an input current i 0 is received at the slave 14. Between different segments of the communication bus 16, Figure 1 also reduced currents i 1 and i 2 compared to the output current i 0 at the master 12.

[0006] A stray current is given by I = C * dV / dt, where "C" is the stray capacitance and "V" indicates a voltage across a corresponding section of the connecting line. The radiation of the connecting line R is proportional to dl / dt = C * d 2< V / dt 2< , where "dl / dt" is the first derivative of the stray current of the connecting line, and "d 2< V / dt 2< " is the second derivative of the voltage of the connecting line. The radiation R is therefore proportional to the rate of change of the bus current or proportional to the second derivative of the bus voltage.

[0007] To meet the stringent requirements of automotive EMC (electromagnetic compatibility) standards, as well as the stringent requirements of individual OEM manufacturers, radiation must be kept below certain limits defined in various frequency ranges. To comply with these standards, the data modulation is usually adjusted via the connecting line between the master and the slaves.

[0008] Implementing precise first-derivative current matching to limit radiation is relatively easy. This matching can therefore be easily implemented in network communication systems where data is modulated with current ("current shaping"). However, due to the peculiarities of common electronic circuits, in 1:N network communication systems with a master and N slaves, current modulation is only used in one direction, namely from the respective slave to the master.

[0009] Data transmission from the master to the slaves is usually carried out using voltage modulation encoding, whereby optimal voltage shaping based on second-derivative voltage matching is much more complex than first-derivative current matching. Furthermore, the equations cited above only apply to linear electronic circuits. If the communication bus also has non-linear impedances - e.g., the bus is not used solely for communication - precise second-derivative bus voltage matching becomes very difficult, if not impossible. In practice, therefore, the bus voltage modulation is not matched so precisely; instead, data transmission is usually slowed down. Therefore, the achievable data transmission rate from the master to the slaves is often several times lower than the data transmission rate from the respective slave to the master.

[0010] Such a network communication system 10 is described in detail in Figure 2 As previously described with reference to Figure 1 was executed, here a master 12 is connected to a plurality of slaves 14 via a communication bus 16.

[0011] The first communication device 12 comprises a modulatable voltage source 20 with which the data to be transmitted by the first communication device 12 is modulated into voltage and transmitted to the communication bus 16. The modulated voltage can be further measured in all slaves 14 connected to the communication bus 16 using a voltage measuring device 28. Thus, the data transmitted by the master 12 can be decoded in each slave 14.

[0012] Each of the respective slaves 14 also includes a modulatable current sink 30, which modulates the data sent by the slave 14 into current, which is drawn via the communication bus 16 from the voltage source 20 present in the master 12 via a current measuring device 26. The modulated current is measured by the current measuring device 26. Thus, the data sent by the respective slave 14 can be decoded in the master 12. Communication networks 10 constructed in this way can be implemented very cost-effectively.

[0013] In this context, WO 03 / 053018 A1 discloses a method for the bidirectional transmission of data in the form of signals on a communication path between at least two communication participants, each having at least one switchable consumer unit, each having at least one switchable power source unit which is assigned to the consumer unit, and each having at least one comparator unit which is assigned to the consumer unit, to which communication participants, at least temporarily, the function of a transmitter or receiver is assigned during the transmission of the data.at least temporarily assigned the function of a receiver by switching off the consumer unit of the transmitter before the signals are transmitted, clocking the power source unit of the transmitter accordingly to generate the signals to be transmitted via the communication path, transmitting the signals generated in this way via the communication path so that corresponding output signals are generated at the comparator unit assigned to the receiver, and switching the consumer unit of the transmitter on again after the transmission.

[0014] EP 1 371 045 B1 discloses a device for data transmission between vehicle sensors and a control unit, wherein the data transmission takes place asynchronously with a first data telegram from a respective vehicle sensor to the control unit. The control unit has an interface module that decodes the first data telegram containing sensor data from the respective vehicle sensor and reformats it into a second data telegram. The interface module transmits the second data telegram synchronously to the processor of the control unit, wherein the interface module has a memory for temporarily storing the sensor data. The second data telegram has an age bit for selecting the sensor data of the respective vehicle sensor, wherein the memory has a first data field for old sensor data and a second data field for new sensor data for each vehicle sensor, and the processor sets the age bit.

[0015] US 9,325,245 B2 relates to a bidirectionally isolated DC / DC converter comprising two ports, two voltage- and current-isolated sensing units, a processing module, two filter circuit units, and a bidirectional current conversion module. One of the two ports is selectively used as an input port of the bidirectionally isolated DC / DC converter, and another of the two ports is used as an output port. The two voltage- and current-isolated sensing units are respectively connected to the two ports to sample voltages and currents at the two ports and generate corresponding feedback signals. The processing module receives the feedback signals and outputs corresponding control signals according to the feedback signals.The bidirectional power conversion module is connected to the two terminals via the two voltage and current isolated detection units to perform the conversion of different voltages between the two terminals according to the control signals output from the processing module.

[0016] A pulse wave shaper for reducing the radiation emission level is known from US 9,496,969 B1. The pulse wave shaper comprises a first integrator, the first integrator receiving a first pulse wave and generating a second pulse wave, and a second integrator coupled to the first integrator, the second integrator receiving the second pulse wave and generating a third pulse wave with a pulse wave amplitude. The first pulse wave comprises a first pulse waveform, the second pulse wave comprises a second pulse waveform, and the third pulse wave comprises a third pulse waveform. The third pulse waveform, when transmitted over a bus, produces a reduced radiation emission level.

[0017] From DE 103 06 444 A1 a 2-line bus system with voltage modulation for transmitting data from a main device and current modulation for transmitting data from a secondary device as well as a method for operating such a bus system is known. Starting from the above-mentioned prior art, the invention is therefore based on the object of specifying a network communication system and a method of the above-mentioned type which enable fast and efficient communication in the transmission direction both from the master to the slaves and from each of the slaves to the master.

[0018] The object is achieved according to the invention by the features of the independent claims. Advantageous embodiments of the invention are specified in the subclaims.

[0019] According to the invention, a network communication system is thus specified, in particular for a vehicle, comprising a communication bus, a first communication device and a plurality of second communication devices which are connected to one another via the communication bus, wherein the first communication device comprises a bias voltage source for generating a constant bias voltage on the communication bus, the first communication device comprises a current measuring device for detecting the current on the communication bus, the network communication system has a terminating resistor which is arranged in parallel with the plurality of second communication devices, the first communication device comprises a first modulation device for modulating a current on the communication bus which is set by the bias voltage and the terminating resistor,each second communication device comprises a second modulation device for modulating the current set by the bias voltage and the terminating resistor, and each second communication device has a second voltage measuring device for detecting a voltage across the terminating resistor.

[0020] According to the invention, a method is also provided for bidirectional, current modulation-based communication in a network communication system, in particular for a vehicle, having a communication bus, a first communication device, a plurality of second communication devices that are connected to one another via the communication bus, and a terminating resistor that is arranged in parallel with the plurality of second communication devices, comprising the steps of generating a constant bias voltage on the communication bus with the first communication device, generating a modulated current on the communication bus with the first communication device, detecting the voltage at the terminating resistor caused by the current modulated by the first communication device with each of the second communication devices, generating a modulated current on the communication bus with one of the second communication devices,and detecting the current modulated with one of the second communication devices on the communication bus with the first communication device.,

[0021] The basic idea of ​​the present invention is therefore to use bidirectional current modulation to modulate the data to be transmitted during communication between a first communication device, also referred to as the master, and a plurality of second communication devices, also referred to as slaves. This has the advantage that the data transmission rate, which is usually slower with voltage modulation in the direction from the master to the slaves, can be increased with current modulation. Current modulation enables direct adjustment of the current, whereby requirements for the electromagnetic compatibility (EMC) of the network communication system can be met more easily. Since electromagnetic radiation is proportional to the first derivative of the current, i.e. to the change in current, the modulation can be carried out in such a way that the radiation can be reduced to a required or desired level.The simple control of radiation, in turn, allows communication between the master and the slaves to be improved and accelerated compared to communication based on voltage modulation. Limitations on the data transmission rate with voltage modulation, which are implemented in the prior art to limit radiation in voltage modulation, are not necessary.

[0022] The communication bus is typically implemented with two electrical lines, one of which can be connected to ground. The first and second communication devices are connected in parallel to the two lines.

[0023] The constant bias voltage generated by the bias voltage source enables the generation of a constant quiescent current on the communication bus, on which data is modulated by both the first communication device and each of the second communication devices. Modulation is performed by the first or the respective second modulation device based on the quiescent current. In particular, the second communication devices can perform their current modulation based on the quiescent current, for example, by diverting the current, so that they do not necessarily require their own power source.

[0024] In order to detect the modulation of the current on the communication bus performed by the first communication device, the current on the communication bus is determined by the second communication devices via a change in the voltage drop across the terminating resistor. Accordingly, the current modulation by the first communication device can be detected by the respective second communication device in order to decode the data modulated by the first communication device. Direct current measurement by the second communication devices would be difficult because the second communication devices are connected to the communication bus in parallel. However, each second communication device can measure the voltage at the terminating resistor in parallel, resulting in a current change. With knowledge of the terminating resistor, the current on the communication bus can be determined.In principle, it is not necessary to determine the exact values ​​for the current modulation. For example, increased or reduced levels can encode individual digital states without the need to precisely determine the modulation current.

[0025] Data transmission from the first communication device to one of the second communication devices, and from each of the second communication devices to the first communication device, occurs independently and, in principle, in any desired time sequence. It is simply necessary to prevent two of the communication devices from attempting to transmit data simultaneously.

[0026] According to the invention, the current measuring device comprises a measuring resistor through which the current modulated by the second communication device flows, and a first voltage measuring device for detecting a voltage drop across the measuring resistor. The measuring resistor is designed, for example, as a shunt with a small resistance value of one ohm or less. Such a shunt only slightly influences the current on the communication bus. The voltage measurement across the shunt can be performed very easily and reliably.

[0027] In an alternative embodiment of the invention, the current measuring device can comprise a current mirror, through which the current modulated by the second communication device flows, and a current meter. The current meter can thus measure, in particular, the current on the communication bus or bus current, specifically using the current mirror. By using the current mirror, the current measurement has no effect on the rest of the circuit.

[0028] According to the invention, the bias voltage source and the first modulation device are arranged in parallel in the first communication device, and a diode is connected upstream of the bias voltage source, blocking a current from the first modulation device into the bias voltage source. Thus, the diode prevents the modulated current from being smoothed or even completely absorbed by the bias voltage source. The quiescent current and the modulated current add up to a total current in which the transmitted data is modulated.

[0029] According to the invention, the bias voltage source and the current measuring device are connected in series. Thus, the first communication device can have a simple design. The bias voltage source, in combination with a resistor, for example, the terminating resistor, can provide the quiescent current on the communication bus. The current can flow through the voltage source and thus also through the current measuring device.

[0030] In an advantageous embodiment of the invention, the bias voltage source is designed as a constant voltage source. The constant voltage source is therefore not used to modulate the data on the communication bus. The quiescent current can be adjusted by selecting the constant voltage source in combination with the terminating resistor.

[0031] According to the invention, the first modulation device is connected in series with a first voltage source. The first voltage source presets the first modulation device. A circuit adaptation is performed to ensure the proper functioning of the first modulation device.

[0032] According to the invention, the first modulation device and / or the second modulation device are designed for analog modulation of the quiescent current. The current modulation thus occurs in accordance with an analog signal to be transmitted, so that the quiescent current is modulated based thereon.

[0033] According to the invention, the first modulation device and / or the second modulation device is / are designed for digital modulation of the current, preferably with a plurality of different modulation current levels. For example, in one implementation of the first communication device, the first modulation device can be designed to drive a current during its data transmission at a dominant data level, i.e., to feed it into the communication bus, and to block a current at a passive data level, i.e., the quiescent current is not changed. As a result, the modulated data appear on the communication bus above the quiescent current and thus with a voltage at the terminating resistor that is greater than the voltage of the constant voltage source. When multiple modulation current levels are used, correspondingly more data can be transmitted compared to binary data transmission.

[0034] According to the invention, the terminating resistor is arranged in the first communication device. A separate arrangement of the terminating resistor on the communication bus is not required, thus simplifying the network communication system overall. The arrangement of the bus resistor in the first communication device allows all second communication devices to have the same structure. This simplifies the construction and maintenance of the network communication system.

[0035] According to the invention, the terminating resistor is arranged in one of the second communication devices. A separate arrangement of the terminating resistor on the communication bus is not required, thereby simplifying the overall network communication system.

[0036] According to the invention, the terminating resistor is arranged directly on the communication bus. This allows the first and second communication devices to be provided without the need for a terminating resistor. Thus, all first and second communication devices can be manufactured identically.

[0037] The second modulation device is preferably designed as a current sink.

[0038] Based on the preset quiescent current on the communication bus by the first communication device, the modulation is carried out accordingly by reducing the quiescent current through the current sink. Accordingly, a smaller portion of the quiescent current set by the first communication device flows through the terminating resistor, so that the voltage applied to it drops accordingly. Designing the second communication devices with one current sink each is easy because no active power supply is required. When using current sinks, all current sinks must be blocked when not in use, i.e. while they themselves are not modulating data onto the quiescent current on the communication bus, in order to prevent unwanted fluctuations in the current through the terminating resistor.

[0039] The invention will be explained in more detail below with reference to preferred embodiments and the accompanying drawings. The features presented may represent an aspect of the invention both individually and in combination. Features of various embodiments are transferable from one embodiment to another.

[0040] It shows Fig. 1 a schematic view of a communication network from the prior art with a master and a slave, which are connected to each other via a connecting line, wherein the connecting line has stray capacitances, Fig. 2 a concrete embodiment of the communication network from Fig. 1, wherein the master has a modulatable voltage source and each slave has a modulatable current sink, Fig. 3A is a schematic representation of a network communication system according to a first, preferred embodiment with a first communication device, a plurality of second communication devices and a terminating resistor, which are connected in parallel to a communication bus, Fig. 3B is a schematic representation of a network communication system according to a second, alternative embodiment with a current mirror-based current measuring device, Fig. 4 is an exemplary timing diagram with a data transmission between the first and a second communication device with a voltage curve at the terminating resistor when using binary, digital modulation, and Fig.5 shows an exemplary timing diagram with a data transmission between the first and a second communication device with a voltage curve at the terminating resistor when using digital modulation with three different signal levels.

[0041] The Figure 3A shows a network communication system 50 according to a first preferred embodiment.

[0042] The network communication system 50 of the first embodiment comprises a first communication device 52, a plurality of second communication devices 54, and a communication bus 56 via which the first communication device 52 and the plurality of second communication devices 54 are connected to one another. The communication bus 56 is configured with two electrical lines 58.

[0043] The communication bus 56 also includes a terminating resistor 60 connected to the communication bus 56 in parallel with the communication devices 52, 54. In an alternative embodiment, the terminating resistor 60 can be arranged in series with a constant terminating voltage source 62, so that the value of the terminating resistor 60 can be kept small.

[0044] The first communication device 52 includes a bias voltage source 64 for generating a constant bias voltage on the communication bus 56. The constant bias voltage generates a quiescent current through the terminating resistor 60. The bias voltage source 64 is embodied here as a constant voltage source 64.

[0045] Each of the second communication devices 54 comprises a second modulation device 78 for modulating a sink current on the communication bus 56 and a second voltage measuring device 80 for measuring a voltage at the terminating resistor 60. By measuring the voltage at the terminating resistor 60 with the second voltage measuring device 80, each second communication device 54 can determine the total current on the communication bus 56 with knowledge of the terminating resistor 60.

[0046] The first communication device 52 includes a current measuring device 66 connected in series with the bias voltage source 64. In this embodiment of the Fig. 3AThe current measuring device 66 comprises a measuring resistor 68 through which the quiescent current flows together with the current modulated by the current sink 78 of the respective second communication device 54. This current corresponds to a current on the communication bus 56. The first communication device 52 or the current measuring device 66 further comprises a first voltage measuring device 70 for measuring a voltage drop across the measuring resistor 68. Knowing the measuring resistor value 68, the value of the modulated current on the communication bus 56 can be determined.

[0047] In an alternative embodiment, the current measuring device 66 can also be implemented by other means—without the measuring resistor 68 and voltage measuring device 70. For example, in a second, alternative embodiment, the current measuring device 66 can be implemented by means of a current mirror 69 and an ammeter 71, as symbolically shown in Figure 3BThe current measuring device 66 thus comprises a current mirror 69, through which the quiescent current flows together with the current modulated by the current sink 78 of the respective second communication device 54. This current also corresponds to a current on the communication bus 56 or bus current. The first communication device 52 or the current measuring device 66 further comprises a current meter 71 for measuring the current on the communication bus via the current mirror. By using the current mirror, the current measurement has no effect on the rest of the circuit.

[0048] The following description refers to both the design of the Figure 3A as well as the design of the Figure 3BA first modulation device 72 is arranged in series with a first supply voltage source 74 in parallel with the bias voltage source 64 and the current measuring device 66. The first modulation device 72 is embodied here as a modulatable current source. The first supply voltage source 74 enables the modulatable current source 72 to introduce a modulated current into the communication bus 56 as an addition to the constant quiescent current.

[0049] In this context, a diode 76 is connected upstream of the series circuit comprising the bias voltage source 64 and the current measuring device 66, blocking a current from the first modulation device 72 into the bias voltage source 64. The diode 76 thus ensures that the current modulated by the first modulation device 72 flows only into the communication bus 56 and is not smoothed or even completely absorbed by the bias voltage source 64. The quiescent current and the modulated current add up to a total current (bus current) that contains the transmitted data.

[0050] During operation, in the network communication system 50, a constant quiescent current is initially generated on the communication bus 56 by the first communication device 52 from the bias voltage source 64. Both the first modulation device 72 of the first communication device 52 and all second communication devices 54 are inactive, i.e., no currents are generated or drawn on the communication bus 56 in addition to the quiescent current. Based on this, data can be modulated by current modulation at a time offset by both the first communication device 52 and each of the second communication devices 54, as explained in detail below.

[0051] When the first communication device 52 wishes to transmit data to one or more of the second communication devices 54, the first communication device 52 modulates its data into modulated current using the first modulation device 72 and allows its modulated current to flow into the communication bus 56 as an addition to the constant quiescent current. The second modulation devices 78 of the second communication devices 54 remain inactive. As a result, the current flowing via the communication bus 56, i.e., the quiescent current together with the current modulated by the first modulation device 72, flows entirely through the terminating resistor 60. The current modulated by the first modulation device 72 causes an additional voltage drop across the terminating resistor 60 compared to the quiescent current.The second communication devices 54 measure the voltage at the terminating resistor 60 with their respective second voltage measuring device 80 in order to decode the modulated data. Accordingly, the data modulated by the first communication device 52 can be decoded in each of the second communication devices 54.

[0052] Accordingly, the second communication devices 54 can each draw a modulated current from the communication bus 56 with the corresponding second modulation device 78 to transmit data to the first communication device 52. This is enabled by the bias voltage and the resulting quiescent current, so that the second modulation device 78 of each second communication device 54 can draw a modulated sink current that correspondingly increases the quiescent current flowing from the bias voltage source 64.

[0053] The modulated sink current drawn by the respective second modulation device 78 causes a change in the voltage drop across the sense resistor 68, which reciprocates the current modulated by one of the second communication devices 54. The first communication device 52 measures the voltage across the sense resistor 68, thus enabling the data modulated by one of the second communication devices 54 to be decoded.

[0054] The data transmission from the first communication device 52 to one of the second communication devices 54 and from each of the second communication devices 54 to the first communication device 52 occurs independently and, in principle, in any desired temporal sequence. It is simply necessary to prevent two of the communication devices 52, 54 from attempting to transmit data simultaneously.

[0055] Figure 4shows a modulation of the current on the communication bus 56 by the first or one of the second communication devices 52, 54 in accordance with the first embodiment. During a first transmission phase 82, the first communication device 52 transmits data that is binary digital data with a dominant and a passive level. Accordingly, the current on the communication bus 56 is increased by the first modulation device 72 to modulate a dominant level. As a result, an instantaneous value of the voltage on the communication bus 56 is also increased, which can be detected and decoded by the second voltage measuring device 80. At a passive level, the current on the communication bus 56 remains unchanged; the first modulation device 72 digitally encodes the passive level with zero (0).At an active level, the current on the communication bus 56 is changed accordingly; the first modulation device 72 digitally encodes the active level with one (1).

[0056] The same applies to the modulation and transmission of data from one of the second communication devices 54 to the first communication device 52. While one of the second communication devices 54 modulates its data with its second modulation device 78, the total current drawn by the bias voltage source 64 is increased. Due to the series-connected measuring resistor 68, the instantaneous value of the voltage on the communication bus 56 is reduced.

[0057] Figure 5shows a modulation of the current on the communication bus 56 by the first communication device 52 according to a second embodiment. The data to be modulated here can have three different levels: two dominant levels and one passive level. Accordingly, the current on the communication bus 56 is increased by the first modulation device 72 in accordance with the first or second dominant level to modulate the data on the communication bus 56.

[0058] In a further embodiment, the data to be sent can be encoded with multiple modulation levels in both directions and decoded accordingly after reception on the other side.

[0059] In an alternative embodiment, the terminating resistor 60 is arranged in the first communication device 52 or in the second communication device 54.

[0060] Based on the exclusive use of current modulation for data transmission in the network communication system 50, the configuration of the modulated current can be adapted to meet electromagnetic compatibility (EMC) requirements. The actual adaptation of the modulation to reduce EMC emissions by the network communication system 50 is outside the scope of the invention and is therefore not explained here. The network communication system 50 is provided here to enable this modulation adaptation. List of reference symbols

[0061] 10Network communication system (state of the art) 12Master (state of the art) 14Slave (state of the art) 16Communication bus (state of the art) 18Stray capacitance (state of the art) 20Modulatable voltage source (state of the art) 22Measuring resistor (state of the art) 24First voltmeter (state of the art) 26Current measuring device (state of the art) 28Second voltmeter (state of the art) 30Current modulation device, modulatable current sink (state of the art) I n Current modulated by the slave (state of the art) UM Voltage modulated by the master (state of the art) 50Network communication system 52First communication device, master 54Second communication device, slave 56Communication bus 58Line 60Termination resistor 62Termination voltage source, constant voltage source 64Bias source, constant voltage source 66Current measuring device 68Measuring resistor 69Current mirror 70First voltage measuring device 71Ammeter 72First modulation device, modulatable current source 74Supply voltage source 76Diode 78Second modulation device, modulatable current sink 80Second voltage measuring device 82First transmission phase 84Second transmission phase IM Current modulated by the master

Claims

1. Network communication system (50) for a vehicle, the network communication system comprising: a communication bus (56), a first communication device (52) and a plurality of second communication devices (54) which are connected to one another via the communication bus (56), wherein the first communication device (52) comprises a bias voltage source (64) for generating a constant bias voltage on the communication bus (56), the first communication device (52) comprises a current measurement apparatus (66) for capturing the current on the communication bus (56), the network communication system (50) has a terminating resistor (60) which is arranged in parallel with the plurality of second communication devices (54), the first communication device (52) comprises a first modulation apparatus (72) for modulating a current set by the bias voltage and the terminating resistor (60) on the communication bus (56), each second communication device (54) comprises a second modulation apparatus (78) for modulating the current set by the bias voltage and the terminating resistor (60), each second communication device (54) has a second voltage measurement apparatus (80) for capturing a voltage at the terminating resistor (60), characterized in that the bias voltage source (64) and the current measurement apparatus (66) are connected in series, the bias voltage source (64) and the first modulation apparatus (72) are arranged in parallel in the first communication device (52), and a diode (76) is connected upstream of the bias voltage source (64) and blocks a current from the first modulation apparatus (72) into the bias voltage source (64) .

2. Network communication system (50) according to Claim 1, characterized in that the current measurement apparatus (66) has a measurement resistor (68), through which the current modulated by the second communication device (54) flows, and a first voltage measurement apparatus (70) for capturing a voltage drop across the measurement resistor (68).

3. Network communication system (50) according to Claim 1, characterized in that the current measurement apparatus (66) has a current mirror (69), through which the current modulated by the second communication device (54) flows, and an ammeter (71) .

4. Network communication system (50) according to one of the preceding claims, characterized in that the bias voltage source (64) is in the form of a constant voltage source.

5. Network communication system (50) according to one of the preceding claims, characterized in that the first modulation apparatus (72) is connected in series with a first voltage source (74).

6. Network communication system (50) according to one of the preceding claims, characterized in that the first modulation apparatus (72) and / or the second modulation apparatus (78) is / are designed for analogue modulation of the current.

7. Network communication system (50) according to one of the preceding claims, characterized in that the first modulation apparatus (72) and / or the second modulation apparatus (78) is / are designed for digital modulation of the current, preferably with a plurality of different modulation current levels.

8. Network communication system (50) according to one of the preceding claims, characterized in that the terminating resistor (60) is arranged in the first communication device (52).

9. Network communication system (50) according to one of the preceding claims, characterized in that the terminating resistor (60) is arranged in one of the second communication devices (54).

10. Network communication system (50) according to one of the preceding claims, characterized in that the terminating resistor (60) is directly arranged on the communication bus (56).

11. Network communication system (50) according to one of the preceding claims, characterized in that the second modulation apparatus (78) is in the form of a current sink which can be modulated.

12. Method for bidirectional communication based on current modulation in a network communication system (50) according to one of Claims 1 to 11, comprising the steps of: generating a constant bias voltage on the communication bus (56) using the first communication device (52), generating a modulated current on the communication bus (56) using the first communication device (52), capturing the voltage caused by the current modulated using the first communication device (52) at the terminating resistor (60) using each of the second communication devices (54), generating a modulated current on the communication bus (56) using one of the second communication devices (54), and capturing the current modulated using one of the second communication devices (54) on the communication bus (56) using the first communication device (52).