JP2015053633A - Communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a communication system hardly affected by ringing.SOLUTION: A communication system of the invention comprises: a trunk line having a pair of communication lines; two first communication devices, each of which has a termination resistor interposed between the pair of communication lines and is connected to each end part of the trunk line; a branch line 4 including another pair of communication lines connected to the pair of communication lines, respectively; and a second communication device 1 having a communication unit 16 connected to the branch line 4, where the two first communication devices and second communication device 1 communicate mutually. The second communication device 1 comprises a resistor 17a which is interposed between the other pair of communication lines included in the branch line 4 and has a resistance value corresponding to an impedance equal to or higher than a characteristic impedance of the branch line 4 and lower than an input impedance of the communication unit 16.

Description

  The present invention relates to a communication system in which a plurality of communication devices communicate via a communication line.

  Conventionally, a lot of electronic devices (communication devices) are mounted on a vehicle, and each electronic device is connected via a communication line and cooperates while exchanging information with each other. In addition, control and the like related to comfort in the passenger compartment are realized. When an electronic device mounted on a vehicle performs communication, CAN (Controller Area Network) is widely adopted as a communication protocol (see Non-Patent Document 1).

  Generally, in a communication system that employs a CAN communication protocol, a twisted pair cable formed by combining a pair of communication lines is used, and each electronic device performs communication using a differential signal, and a potential difference between the pair of communication lines. Dominant is detected when the threshold exceeds the threshold, and recessive is detected when the potential difference is below the threshold. In general, a communication system adopting a CAN communication protocol adopts a bus type network topology. In a bus-type network topology, each electronic device is connected to a branch line that branches from a main line used for communication in common with other electronic devices. For example, each electronic device associates dominant with data 0 and associates recessive with data 1 to transmit and receive data.

ISO 11898-1: 2003 Road vehicles--Controller area network (CAN)-Part1: Data link layer and physical signaling

  In a communication system employing the CAN communication protocol as described above, ringing occurs when an electronic device transmits a signal to another electronic device for communication. Ringing occurs due to factors such as impedance mismatching at the branching portion branching from the main line to the branch line and impedance mismatching of the communication device connected to the branch line.

  If the communication system has a large failure due to ringing, the communication device that receives the dominant signal transmitted from one communication device determines that it is a recessive signal, etc. There is a risk of communication failure.

  In recent years, a communication protocol in an in-vehicle network such as CAN-FD (CAN with Flexible Data Rate) capable of performing communication between electronic devices at a higher communication speed than a CAN communication protocol has been studied. In communication according to such a communication protocol, since it takes a short time to transmit a dominant or recessive signal, signal transmission / reception is sequentially performed before the influence of ringing is reduced, which affects communication. It is necessary to keep ringing lower.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a communication system that is less affected by ringing.

  The communication system according to the present invention has a trunk line having a pair of communication lines, a termination resistor, the termination resistor interposed between the pair of communication lines, and connected to each end of the trunk line. Two first communication devices, a branch line including a pair of communication lines respectively connected to the pair of communication lines, and a second communication device having a communication unit connected to the branch lines, the two communication devices In the communication system in which each of the first communication device and the second communication device performs communication, the second communication device includes a resistor interposed between a pair of communication lines included in the branch line, It has a resistance value equal to or higher than the characteristic impedance of the branch line and corresponding to an impedance lower than the input impedance of the communication unit.

  In the present invention, the second communication device includes a resistor interposed between a pair of communication lines included in the branch line. The resistor has a resistance value that is equal to or greater than the value of the characteristic impedance of the branch line and that is less than the relative impedance of the input impedance of the communication unit. Therefore, the second communication device can reduce the difference between the characteristic impedance of the branch line and the input impedance of the communication unit by the resistance, and thus can suppress the reflection of the signal and reduce the influence of the ringing.

  In the communication system according to the present invention, the two first communication devices and the second communication device communicate with each other at a communication speed of 1 Mbps or more and 8 Mbps or less, and the trunk line and the branch line are CAN (Controller Area Network). It is a twisted pair cable having a characteristic impedance larger than the lower limit value of the characteristic impedance of the bus.

  In the present invention, the two first communication devices and the second communication device communicate at a communication speed of 1 Mbps to 8 Mbps. The trunk line and the branch line are twisted pair cables having a characteristic impedance having a value larger than the lower limit value of the characteristic impedance of the CAN bus.

  In the communication system according to the present invention, the second communication device includes a low-pass filter between the branch line and the communication unit.

  In the present invention, the second communication device includes a low-pass filter between the branch line and the communication unit. Therefore, since the second communication device can remove high-frequency ringing components by the low-pass filter, it is possible to further reduce the influence of ringing and improve the stability of communication.

  The communication system according to the present invention is characterized in that the low-pass filter includes a coil.

  In the present invention, the low pass filter includes a coil. Therefore, a low pass filter can be configured without grounding by interposing the coil in series between the branch line and the communication unit.

  The communication system according to the present invention includes a plurality of branch lines, and a plurality of second communication devices connected to the branch lines.

  In the present invention, a plurality of branch lines are provided. A plurality of second communication devices connected to the branch lines are provided. Therefore, communication in which ringing is suppressed can be performed between a plurality of second communication devices.

  According to the present invention, it is possible to provide a communication system that is less affected by ringing.

It is a schematic diagram which shows the structure of the whole communication system in this Embodiment. It is a block diagram which shows the structure of ECU. It is a block diagram which shows the structure of terminal ECU. It is a graph which shows the simulation result of the signal level which ECU received.

  Hereinafter, the present invention will be described in detail with reference to the drawings illustrating embodiments thereof.

  FIG. 1 is a schematic diagram showing an overall configuration of a communication system according to the present embodiment. The communication system includes ECUs (Electronic Control Units) 1a, 1b, and 1c that are mounted on the vehicle, and terminal ECUs 2a and 2b that are also mounted on the vehicle. The terminal ECU 2a is connected to one end of the trunk line 3 including the communication line. The terminal ECU 2b is connected to the other end of the trunk line 3. The ECU 1a is connected to a branch line 4a branched from a branch point A in the middle of the main line 3. Similarly, the ECU 1b and the ECU 1c are connected to the branch line 4b branched from the branch point B, and the ECU 1c is connected to the branch line 4c branched from the branch point C. The branch point B is in the middle of the trunk line 3 and closer to the terminal ECU 2b than the branch point A. The branch point C is in the middle of the trunk line 3 and closer to the terminal ECU 2b than the branch point B. The branch lines 4a, 4b, and 4c are configured to include communication lines.

  In addition, since ECU1a, 1b, and 1c are the same structures, they are also named ECU1 generically below. Since the end ECUs 2a and 2b have the same configuration, they are also collectively referred to as the end ECU 2 below. Since the branch lines 4a, 4b, and 4c have the same configuration, they are also collectively referred to as a branch line 4 below. The ECU 1 corresponds to the second communication device of the present invention, and the terminal ECU 2 corresponds to the first communication device of the present invention.

  In the communication system configured as described above, communication is performed between the ECU 1 and the terminal ECU 2 using the trunk line 3 as a common communication line. For example, when a signal is output to the branch line 4b by the ECU 1b, this signal reaches the ECU 1a from the branch line 4b through the trunk line 3 and the branch line 4a, and a reflected wave reflected by the connection portion etc. at which the ECU 1a connects to the branch line 4a. Arise. The reflected wave reaches the ECU 1b through the branch line 4a, the trunk line 3, and the branch line 4b (see the broken arrow in FIG. 1). The reflected wave that has reached the ECU 1b is reflected again at the connecting portion where the ECU 1b connects to the branch line 4a again. In this way, ringing occurs in the communication system due to repeated signal reflection. Moreover, although illustration is abbreviate | omitted in FIG. 1, the same reflected wave arises also in ECU1c. Further, the terminal ECU 2 may generate a similar reflected wave depending on the characteristic impedance values of the main line and the branch line. The ECU 1 corresponds to the communication device of the present invention.

  FIG. 2 is a block diagram showing the configuration of the ECU 1. The ECU 1 includes a control unit 11, a ROM (Read Only Memory) 12, a RAM (Random Access Memory) 13, an input unit 14, an output unit 15, a driver 16, a connection circuit 17, and the like. The control unit 11 is configured using an arithmetic processing device such as a CPU (Central Processing Unit) or an MPU (Micro Processing Unit), and performs various controls by reading and executing a control program stored in the ROM 12. Processing can be performed.

The ROM 12 is configured by a nonvolatile memory element such as an EEPROM (Electrically Erasable Programmable ROM) or flash memory, for example, and a control program executed by the control unit 11 and information necessary for processing performed by the control unit 11 Etc. are stored in advance. The RAM 13 is composed of a memory element such as SRAM (Static RAM) or DRAM (Dynamic RAM), for example, and information generated by the processing of the control unit 11 and information transmitted / received to / from another ECU 1. Etc. are stored.

  The input unit 14 receives a signal from an input device such as a sensor such as a vehicle speed sensor or a temperature sensor of the vehicle, or various switches for operation disposed inside and outside the vehicle, and performs sampling of the input signal or A / Information obtained by performing processing such as D conversion is given to the control unit 11. The output unit 15 is connected to a load such as a motor or a lamp, and outputs a drive signal for driving these loads in response to an instruction from the control unit 11. Note that the ECU 1 does not necessarily need to include both the input unit 14 and the output unit 15, and may be configured to include only one of them.

  The driver 16 transmits / receives information to / from other ECUs 1 according to the CAN protocol, and is connected to the branch line 4 via the connection circuit 17. The driver 16 converts the transmission information given from the control unit 11 into transmission data (frame) according to the CAN protocol and gives the data to the transmission unit 16a. The transmission unit 16a of the driver 16 outputs a signal to the branch line 4 according to the value of each bit of the given transmission data (0 (dominant) or 1 (recessive)). In the CAN protocol, the trunk line 3 and the branch line 4 are twisted pair cables formed by twisting a pair of communication lines. The transmitter 16 a outputs a differential signal to the branch line 4 connected via the connection circuit 17. When transmitting a dominant signal, the transmission unit 16a outputs a differential signal in which the potential difference between the pair of communication lines included in the branch line 4 is 2V, and when transmitting a recessive signal, the potential difference is 0V. A differential signal is output. The driver 16 corresponds to the communication unit of the present invention.

  Further, the driver 16 detects the signal level of the branch line 4 to determine whether the signal transmitted on the branch line 4 is a signal corresponding to dominant or recessive, and each bit is represented by dominant or recessive. A receiving unit 16b for receiving data. The signal level of the branch line 4 refers to a potential difference between a pair of communication lines included in the branch line 4. The driver 16 gives the data received by the receiving unit 16b to the control unit 11. Further, the driver 16 receives the data transmitted by the transmission unit 16a at the reception unit 16b, and when the transmission data and the reception data do not match (the recessiveness of the transmission data has changed to dominant in the reception data). ), It is detected that data is being transmitted by another ECU 1 or terminal ECU 2, and arbitration processing is performed. The arbitration process performed by the ECU 1 is the same as that performed by the conventional CAN protocol, and thus detailed description thereof is omitted.

  The connection circuit 17 is interposed in the branch line 4 and is configured to suppress a reflected wave due to a signal input from the branch line 4 to the driver 16. The connection circuit 17 includes a resistor 17 a interposed between a pair of communication lines included in the branch line 4. The resistor 17 a has a resistance value that is equal to or higher than the characteristic impedance of the branch line 4 and lower than the value of the input impedance to the driver 16. In general, the input impedance value of the driver 16 of the communication device corresponding to the ECU 1 is extremely higher than the characteristic impedance value of the communication line corresponding to the branch line 4. The signal level of the reflected wave is high when the degree of impedance mismatch is large, that is, when the difference in impedance between the two media transmitting signals is large. Therefore, since the resistance 17a has the resistance value as described above, the difference in impedance is reduced, so that signal reflection can be suppressed.

  The connection circuit 17 includes a coil 17b interposed on one of the communication lines included in the branch line 4 on the side of the trunk line 3 from the resistor 17a, and a resistor 17c connected in parallel to the coil 17b. In the communication system according to the present embodiment, since the coil 17b and the resistor 17c function as a low-pass filter, high-frequency ringing components can be removed, so that ringing can be suppressed and communication stability can be improved. .

  FIG. 3 is a block diagram showing a configuration of the terminal ECU 2. The terminal ECU 2 includes a control unit 21, a ROM 22, a RAM 23, an input unit 24, an output unit 25, a driver 26, a connection circuit 27, and the like. Here, since the control part 21, ROM22, and RAM23 operate | move similarly to the control part 11, ROM12, and RAM13 in FIG. 2, description is abbreviate | omitted. The input unit 24, the output unit 25, and the driver 26 operate in the same manner on the input unit 14, the output unit 15, and the driver 16 in FIG. Note that the driver 26 of the terminal ECU 2 is connected to the main line 3 via the connection circuit 27.

  In the connection circuit 27, a series circuit including two resistors 27 a is interposed between a pair of communication lines in the branch line 4. The connection circuit 27 includes a capacitor 27b. One end of the capacitor 27b is connected between the two resistors 27a, and the other end is grounded. The connection circuit 27 configured in this way is a so-called split termination in the CAN protocol, and matches the input impedance to the driver 26 and the characteristic impedance of the main line 3. Note that the connection circuit 27 may be a termination resistor and may be configured other than split termination.

  In the communication system configured as described above, the connection circuit 17 of the ECU 1 matches the characteristic impedance of the branch line 4 and the input impedance to the driver 16, so that ringing can be suppressed.

  Next, a simulation performed to verify the effect of the communication system in the present embodiment will be described. As conditions for this simulation, the trunk line 3 had a total length of 14 m, and the branch line 4 had a total length of 2 m. Here, as a breakdown of the length of the trunk line 3, the length from the terminal ECU 2a to the branch point A and the length from the terminal ECU 2b to the branch point C are 2.5 m, and the length from the branch point A to the branch point B is as follows. The length from the branch point B to the branch point C was 4.5 m. The input impedance to the driver 16 of the ECU 1 (internal resistance of the ECU 1) and the input impedance to the driver 26 of the terminal ECU 2 (internal resistance of the terminal ECU 2) were set to 10 kΩ. The internal load capacities of the ECU 1 and the terminal ECU 2 were 50 pF. Further, in the ECU 1, the resistance value of the resistor 17a in the connection circuit 17 is 500Ω, the inductance of the coil 17b as a low-pass filter is 1 μH, and the resistance value of the resistor 17c is 30Ω. Furthermore, in the termination ECU 2, the resistance value of the resistor 27a in the connection circuit 27 is set to 60Ω, and the capacitance of the capacitor is set to 4.7 μF. Note that the values of the internal resistances of the ECU 1 and the terminal ECU 2 are values conforming to the ISO 11898-2 standard, and the connection circuit 27 that functions as split termination has a configuration conforming to the ISO 11898-5 standard.

  In this simulation, based on the above-described conditions, the ECU 1b transmits a dominant signal, that is, a differential signal having a potential difference of 2V from the transmission unit 16a for a certain period of time, and then a recessive signal, that is, a difference in which the potential difference becomes 0V. Continue to output motion signals. Moreover, ECU1b receives a signal in the receiving part 16b. ECU1a, ECU1c, and termination | terminus ECU2 do not output a signal. Therefore, in this simulation, the signal that the ECU 1b receives at the receiving unit 16b is only the reflected wave of the signal transmitted from the transmitting unit 16a. Therefore, in this simulation, the influence of ringing in the communication system of the present embodiment can be evaluated by evaluating the signal level of the signal received by the receiving unit 16b by the ECU 1b. In this simulation, according to the CAN protocol, the driver 16 determines that the received signal is a dominant signal when the signal level of the received signal is 0.9 V or higher, and determines that it is a recessive signal when the signal level is lower than that. To do.

  FIG. 4 is a graph showing a simulation result of the signal level received by the ECU 1b. In the graph shown in FIG. 4, the horizontal axis represents time (unit: nsec), and the vertical axis represents signal level (unit: V). The thick line in FIG. 4 shows the transition of the signal level when the characteristic impedance of the main line 3 and the branch line 4 is 120Ω, and the thin line shows the transition of the signal level when the characteristic impedance is 140Ω. 4 indicates a signal level waveform when the characteristic impedance of the main line 3 and the branch line 4 is 280Ω.

  In FIG. 4, the transmission unit 16a of the ECU 1b continues to output a dominant signal until t1 seconds, and continues to output a recessive signal after t1 seconds. The reception unit 16b of the ECU 1b shows the waveform of the signal level of the dominant signal up to t1 seconds. After t1 seconds, the signal level by the reflected wave of the dominant signal transmitted by the transmitter 16a is indicated by the receiver 16b. In any of the waveforms when the characteristic impedances of the main line 3 and the branch line 4 are 120Ω, 140Ω, and 280Ω, the signal level does not exceed 0.9V after t1 seconds. That is, there is no mismatch between the transmission data of the transmission unit 16a and the reception data of the reception unit 16b. Therefore, since the ECU 1b does not mistakenly recognize recessive as a dominant, the communication system in the present embodiment can perform communication without being affected by ringing. Although not shown in FIG. 4, the simulation was also performed when the characteristic impedances of the main line 3 and the branch line 4 were 200Ω, 220Ω, 240Ω, and 260Ω under the above-described conditions. As a result, the mismatch between the transmission data of the transmission unit 16a and the reception data of the reception unit 16b did not occur in any characteristic impedance. Therefore, when the characteristic impedance of the main line 3 and the branch line 4 is 120Ω to 280Ω under the above-described conditions, the communication of the communication system is less affected by ringing.

  In the communication system configured as described above, the difference between the characteristic impedance of the branch line 4 and the input impedance of the driver 16 can be reduced by the resistor 17a of the connection circuit 17 of the ECU 1, and the coil 17b and the resistor 17c function as a low-pass filter. Therefore, communication with a low influence of ringing can be performed. Therefore, the communication system can perform communication with less influence of ringing even when performing communication at a higher speed than communication based on the CAN protocol, for example, communication of 1 Mbps to 8 Mbps.

  In the embodiment, it has been described that the connection circuit 17 is integrated with the ECU 1. However, the connection circuit 17 may be provided separately from the ECU 1.

  Furthermore, it should be thought that embodiment disclosed this time is an illustration and restrictive at no points. The scope of the present invention is defined not by the above-mentioned meaning but by the scope of claims for patent, and is intended to include all modifications within the scope and meaning equivalent to the scope of claims for patent.

1a, 1b, 1c ECU (second communication device)
2 End ECU (first communication device)
3 trunk line 4a, 4b, 4c branch line 16 driver (communication part)
17 Connection circuit 17a, 17c Resistance 17b Coil

Claims (5)

A trunk line having a pair of communication lines; and two first communication devices having a termination resistor, the termination resistor interposed between the pair of communication lines, and connected to each end of the trunk line; A branch line including a pair of communication lines each connected to the pair of communication lines; and a second communication apparatus having a communication unit connected to the branch lines, the two first communication apparatuses and the second communication. In a communication system in which each device communicates,
The second communication device is
Comprising a resistor interposed between a pair of communication lines included in the branch line;
The resistance is equal to or higher than a characteristic impedance of the branch line and has a resistance value corresponding to an impedance lower than an input impedance of the communication unit. The two first communication devices and the second communication device communicate at a communication speed of 1 Mbps or more and 8 Mbps or less,
The communication system according to claim 1, wherein the trunk line and the branch line are twisted pair cables having a characteristic impedance larger than a lower limit value of a characteristic impedance of a CAN (Controller Area Network) bus. The communication system according to claim 1, wherein the second communication device includes a low-pass filter between the branch line and the communication unit. The communication system according to claim 3, wherein the low-pass filter includes a coil. A plurality of branch lines;
The communication system according to any one of claims 1 to 4, further comprising a plurality of second communication devices each connected to the branch line.

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