JP2013098871A - Communication system, communication device, and communication method - Google Patents

Abstract

A communication system, a communication apparatus, and a communication method capable of reducing the influence of ringing and realizing high-speed communication.
When transmission data is dominant, ECU1 outputs a signal of a first signal level to a communication line 5 over a first time Tp, and then outputs a second signal level (0 V over a second time Th after that). ) Is output, the transmission data is recessive and the signal level of the communication line 5 is recessive, the signal is not output to the communication line 5 over the first time Tp and the second time Th, and the signal is transmitted in a high impedance state. When the data is recessive and the signal level of the communication line 5 is dominant, the signal is not output for the first time Tp and is set to the high impedance state, and then the signal of the second signal level is output for the second time Th. . The ECU 1 is configured to be able to connect / cut off the resistor with respect to the communication line 5 by switching the switch, and the resistor is connected after the first time Tp has elapsed in the 1-bit transmission time Tb.
[Selection] Figure 2

Description

  The present invention relates to a communication system, a communication apparatus, and a communication method that allow a plurality of communication apparatuses connected to a common communication line to transmit and receive information to each other and to detect a collision of information transmission with respect to the communication line.

Conventionally, communication between a plurality of electronic devices (communication devices) mounted on a vehicle is performed using a CAN (Controller
(Area Network) communication protocol is widely adopted (see Non-Patent Documents 1 and 2). In the CAN communication protocol, since a plurality of communication devices are connected to a common CAN bus, when a plurality of communication devices transmit information simultaneously and a collision occurs, an arbitration process ( Arbitration) is performed, and information transmission with a high priority is executed. In order to perform arbitration, each communication device outputs a transmission signal to the CAN bus and simultaneously detects the signal level of the CAN bus, and the signal level of the detected signal relative to the transmission signal output by itself is detected. When it changes from recessive (inferior value) to dominant (dominant value), it judges that communication collision has occurred and stops the transmission process. The dominant signal is dominant over the recessive signal on the CAN bus. Therefore, even if a communication collision occurs, the electronic device that outputs the dominant signal can continue the transmission process.

  Generally, in a communication system adopting a CAN communication protocol, a twisted line is used as a communication line, and each communication device performs communication using a differential signal, and a case where a potential difference between twisted lines exceeds a threshold is dominant. And a recessive case where the potential difference does not exceed the threshold value. In the digital processing in the communication apparatus, the dominant is associated with data 0 and the recessive is associated with data 1. FIG. 11 is a schematic diagram illustrating an example of a signal waveform in a conventional communication system, in which a vertical axis represents a potential difference between twist lines and a horizontal axis represents time. The signal waveform shown in FIG. 11 shows the waveform when the signal is changed from dominant to recessive on the CAN bus (dominant transmission from 0 ns to 1000 ns, recessive transmission from 1000 ns to 2000 ns). As shown in the figure, when the transmission signal output from the communication device to the CAN bus is changed from dominant to recessive, a waveform that oscillates while the signal level (potential difference) on the CAN bus gradually attenuates is generated. In order to connect many communication devices, it is necessary to provide a lot of branch portions in the CAN bus. Impedance mismatches at these branch portions, impedance mismatches of communication devices connected to the CAN bus, etc. This is caused by repeated signal reflection due to factors, and is called ringing.

  When such ringing occurs, each communication device cannot make a dominant / recessive determination until the ringing signal level is attenuated to some extent. The time during which the ringing signal level attenuates varies depending on the length of the branch line that branches the CAN bus. The ringing signal level varies depending on the number of communication devices connected to the CAN bus. For this reason, each communication device needs to make a dominant / recessive decision after waiting for a time period during which ringing is considered to be sufficiently attenuated in consideration of the number of communication devices in the communication system, the length of the branch line, and the like. In addition, the conventional communication system using the CAN protocol has a problem in that speeding up of communication is hindered by the influence of ringing.

  In Patent Document 1, in a transceiver of a communication device, a transistor having an emitter terminal connected to the CAN H line, a collector terminal connected to the CAN L line, and a base terminal connected to the L line via a capacitor is provided. There has been proposed a communication system that can suppress the amplitude of a reflected signal by providing an antireflection circuit.

  In Patent Document 2, in a transceiver of a communication device, a circuit in which resistors are connected in series to a group of diodes connected in series and a group of diodes connected in series in the opposite direction is connected to two lines of a CAN bus. There has been proposed a communication system that can suppress reflection of signals by providing it in between.

ISO 11898-1: 2003 Road vehicles--Controller area network (CAN)-Part1: Datalink layer and physical signaling ISO 11519-1: 1994 Road vehicles--Low-speed serial data communication--Part1: Generaland definitions

JP 2010-200006 A JP 2010-206267 A

  However, the communication systems described in Patent Documents 1 and 2 have a configuration in which a simple circuit for suppressing signal reflection is added between two lines of a CAN bus, and such a configuration is effective in suppressing ringing. Naturally, there was a limit.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a communication system, a communication apparatus, and a communication method capable of reducing the influence of ringing and realizing high-speed communication. There is.

  The communication system according to the present invention includes a plurality of communication devices connected via a common communication line, and each communication device is represented by a binary value of a dominant value or an inferior value via the communication line. In the communication system having communication means for transmitting and receiving a plurality of consecutive bits of information, the communication means has a first time Tp shorter than the information transmission time of 1 bit with respect to the superiority value of the information to be transmitted. After outputting the signal of the first signal level, the signal of the second signal level lower than the first signal level is output over the second time Th shorter than the information transmission time, and the inferior value of the information to be transmitted The signal output to the communication line is not performed.

  In the communication system according to the present invention, the communication device connects / disconnects the other end of the resistor, one end of which is connected to a fixed potential, with respect to the communication line, and the information for one bit. And switching control means for controlling the switching unit to connect the resistor after the first time Tp has elapsed in the transmission time.

  In the communication system according to the present invention, the first time Tp and the second time Th satisfy the conditions of Tp + Th ≦ Tb and Th ≧ 2 × L × A (where Tb is equivalent to one bit) Information transmission time, L is the shortest distance of the communication line interposed from one communication apparatus to another communication apparatus, and A is a signal transmission speed in the communication line. .

Further, the communication device according to the present invention is connected to another device via a common communication line, and each bit is represented by two or more consecutive bits represented by a binary value of a dominant value or a recessive value via the communication line. In a communication apparatus including a communication unit that transmits and receives information, the communication unit outputs a signal of a first signal level over a first time Tp that is shorter than an information transmission time of 1 bit with respect to a superiority value of information to be transmitted. After the output, a signal having a second signal level lower than the first signal level is output for a second time Th shorter than the information transmission time, and a signal to the communication line is determined with respect to an inferior value of information to be transmitted. The output is not performed.

  Further, the communication method according to the present invention is a communication method for transmitting and receiving information of a plurality of consecutive bits each represented by a binary value of a dominant value or a recessive value via a common communication line. The first signal level is output over a first time Tp shorter than the information transmission time for one bit with respect to the dominant value, and then the first signal over a second time Th shorter than the information transmission time. A signal having a second signal level lower than the level is output, and signal output to the communication line is not performed for the inferior value of information to be transmitted.

In the conventional CAN, ringing as shown in FIG. 11 occurs when the transmission of the communication apparatus is changed from dominant to recessive. The communication device outputs a high level signal (a signal having a predetermined potential difference) to the communication line as a dominant transmission, and then becomes recessive and enters a high impedance state. The signal change from dominant to recessive by one communication device is reflected by another communication device in recessive (high impedance state), and this reflection is further reflected by the one communication device that has transitioned to recessive. Reflection occurs and ringing occurs.
Therefore, in the present invention, when the information to be transmitted is a dominant (dominant value), the communication apparatus outputs a signal of the first signal level over a first time shorter than this time with respect to the information transmission time of 1 bit. After the output, a signal of the second signal level (<first signal level) over the second time is output to the communication line. That is, the communication apparatus outputs a pulse-like signal that changes from the first signal level to the second signal level in response to the dominant. When the information to be transmitted is recessive (inferior value), the communication device does not output a signal to the communication line (becomes a high impedance state).
As a result, when dominant transmission and recessive transmission exist simultaneously, a pulse-like signal changing from the first signal level to the second signal level is observed on the communication line. By detecting a signal on the communication line, the device can detect the presence of the communication device that has transmitted the dominant. In addition, the communication apparatus which transmitted recessive can suppress generation | occurrence | production of a reflected signal by performing impedance matching in the area of a 2nd signal level after the time of a 1st signal level passes.
When the information transmitted by the communication apparatus is changed from dominant to recessive, the signal level is lowered from the first signal level to the second signal level, and then the state is shifted to the high impedance state. For this reason, the amplitude of the signal reflected by the communication line can be reduced, and furthermore, by setting the second signal level to approximately 0V, the amplitude of the reflected signal can be approximately 0V. Can be suppressed.
As a result, the influence of ringing on the next bit to which the communication apparatus has transmitted a dominant can be reduced, so that dominant / recessive determination can be performed at an early timing for each bit. Therefore, high-speed communication can be realized.

  In a system in which a plurality of communication devices communicate via a common communication line, a main communication line (main line) is branched to provide a branch line, and the communication apparatus is connected to the end of the main line or the branch line. In the present invention, a switching unit such as a switch for connecting / cutting off a resistor connected to a fixed potential such as a ground potential is provided for a communication line to which each communication device is connected. Each communication device controls the switching unit to connect the resistor after the first time in the 1-bit transmission time. As a result, each communication device in the communication system can connect the resistor to the communication line after the first time has elapsed in the 1-bit transmission time, and perform impedance matching using this resistor. As a result, impedance mismatching, which is a cause of ringing, can be eliminated, and ringing can be suppressed.

Since ringing occurs when a signal output from one communication device is reflected by another communication device, the ringing is the closest to one communication device (the distance of the communication line through which the signal propagates is the shortest). The communication device has a great influence on the occurrence of ringing.
Therefore, in the present invention, when the shortest distance of the communication line interposed between two communication devices included in the communication system is L and the transmission speed of the communication line is A, the second time Th is Th ≧ 2 × L × A
Set to satisfy the conditions. Note that 2 × L is a round trip distance between two communication devices, and 2 × L × A is a time until a signal output from one communication device is reflected by another communication device and returned to the one communication device. It is. By setting the second time Th to be equal to or longer than this time, the impedance matching by the resistor is performed before the reflected wave returns to the one communication device, so that the one communication device does not shift to the high impedance state. The occurrence of ringing can be reliably suppressed.
Further, the first time Tp is, when the information transmission time for 1 bit is Tb,
Tp + Th ≦ Tb
It is necessary to set so as to satisfy the conditions.

  In the case of the present invention, the communication apparatus performs a signal output of the second signal level over the second time after the signal output of the first signal level over the first time for the dominant information transmission. Therefore, the occurrence of ringing can be suppressed within one bit of the transmission data, so that dominant / recessive judgment can be made at each bit at an early timing, and communication that performs arbitration like the CAN protocol is accelerated. can do.

It is a schematic diagram which shows one structural example of a communication system. It is a block diagram which shows the structure of a communication apparatus. It is a schematic diagram for demonstrating the signal which each ECU transmits / receives in the communication system which concerns on this invention. It is a schematic diagram for demonstrating the impedance matching by a CAN communication control part. It is a schematic diagram for demonstrating 1st time and 2nd time which concern on data transmission. It is a flowchart which shows the procedure of the transmission process by a CAN communication control part. It is a flowchart which shows the procedure of the transmission process by a CAN communication control part. It is a flowchart which shows the procedure of the reception process by a CAN communication control part. It is a flowchart which shows the procedure of the reception process by a CAN communication control part. It is a schematic diagram for demonstrating the effect of the communication system which concerns on this invention. It is a schematic diagram which shows an example of the signal waveform in the conventional communication system.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof. FIG. 1 is a schematic diagram illustrating a configuration example of a communication system. The communication system according to the present embodiment includes, for example, a plurality of ECUs (Electronic Control Units) 1 mounted on a vehicle (not shown) as a communication device, and the plurality of ECUs 1 are connected via a common communication line 5. is there. In FIG. 1, the plurality of ECUs 1 are distinguished from each other by being denoted by reference numerals 1 a to 1 e, and the communication line 5 is distinguished into a trunk line 5 a and branch lines 5 b to 5 d. In other words, the illustrated communication system is configured such that ECUs 1a and 1e are connected via a trunk line 5a, and ECUs 1b to 1d are connected to three branch lines 5b to 5d branched from the trunk line 5a of the communication line 5, respectively.

  For example, when a signal is output to the branch line 5b of the communication line 5 by the ECU 1b, this signal passes from the branch line 5b to the ECU 1c via the trunk line 5a and the branch line 5c, and the reflected wave reflected by the terminal portion of the ECU 1c or the like is reflected on the branch line 5c. Then, it reaches the ECU 1b via the trunk line 5a and the branch line 5b (see the broken arrow in FIG. 1). Although not shown in FIG. 1, a similar reflected wave is also generated in the ECU 1d. Repeating such signal reflection results in ringing as shown in FIG. As the distance between the branch lines 5b to 5d becomes longer, the time until the reflected wave returns to the signal output source becomes longer. Therefore, the ringing period becomes longer, and the time for which the ringing continues (time required for attenuation) becomes longer. Become. Further, when the number of branch lines 5b to 5d of the communication line 5 increases (the number of ECUs 1 increases), the number of occurrences of reflected waves increases, so that the ringing amplitude increases and the time required for the ringing to attenuate is increased. become longer.

  FIG. 2 is a block diagram showing a configuration of the communication apparatus (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 CAN communication control unit 16, and the like. The control unit 11 is configured by using an arithmetic processing device such as a CPU (Central Processing Unit) or MPU (MicroProcessing Unit), and various control processes are performed by reading and executing a control program stored in the ROM 12. It 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 a vehicle, or various switches for operation arranged 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 CAN communication control unit 16 has a terminal connected to the communication line 5, and transmits / receives information to / from another ECU 1 according to the CAN protocol via the communication line 5 connected to the terminal. Is. The CAN communication control unit 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 17. The transmission unit 17 of the CAN communication control unit 16 outputs a signal to the communication line 5 according to the value of each bit of the given transmission data (0 (dominant) or 1 (recessive)). In the CAN protocol, a twist line is used as the communication line 5, and the transmission unit 17 outputs a differential signal to the communication line 5. The transmission unit 17 sequentially processes each bit of transmission data composed of a plurality of bits, and outputs a signal of the first signal level over a predetermined first time when the value of the processing target bit is dominant. After that, a signal of the second signal level (<first signal level) is output over a predetermined second time. In addition, when the value of the processing target bit is recessive, the transmission unit 17 sets the terminal in a high impedance state.

Further, the CAN communication control unit 16 determines whether the signal transmitted on the communication line 5 is a signal corresponding to dominant / recessive by detecting the signal level of the communication line 5 (potential difference of the twist line). The receiving unit 18 receives data in which each bit is represented by dominant / recessive. The CAN communication control unit 16 gives the data received by the receiving unit 18 to the control unit 11. Further, the CAN communication control unit 16 receives the data transmitted by the transmission unit 17 at the reception unit 18, and when the transmission data and the reception data do not match (the recessiveness of the transmission data changes to dominant in the reception data). When the transmission is performed by another ECU 1 connected to the communication line 5, an arbitration process 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.

  FIG. 3 is a schematic diagram for explaining a signal transmitted and received by each ECU 1 in the communication system according to the present invention, in which the vertical axis represents the potential difference V between the twist lines of the signal line 5 and the horizontal axis represents time t. It is. The illustrated example is a signal when transmission data changes from dominant to recessive. When transmitting the dominant data, the transmission unit 17 of the ECU 1 outputs the first signal level (V1) signal during the period from the start time to the first time Tp in the 1-bit transmission time Tb, A signal of the second signal level (0 V) is output for 2 hours Th. Note that Tp + Th ≦ Tb may be satisfied. When Tp + Th <Tb, the transmitter 17 does not output a signal and outputs a high impedance terminal until the 1-bit transmission time Tb is reached after the second time Th has elapsed. State. Further, in the case of performing recessive data transmission, the transmission unit 17 sets a terminal in a high impedance state without outputting a signal for the entire transmission time Tb of 1 bit.

  The receiving unit 18 of the ECU 1 samples the signal level of the communication line 5 at a predetermined period from the start time to the first time Tp in the transmission time Tb of each bit. The receiving unit 18 calculates an average value of the signal level from the sampling result over the first time Tp, and performs dominant / recessive determination according to whether or not the calculated average value exceeds a predetermined threshold value.

  The CAN communication control unit 16 has a function of dynamically performing impedance matching of the communication line 5 when performing communication. FIG. 4 is a schematic diagram for explaining impedance matching by the CAN communication control unit 16, and schematically shows a circuit in the CAN communication control unit 16. The CAN communication control unit 16 has two terminals 16a to which twist lines are connected. In the CAN communication control unit 16, for example, two internal wirings connected to the two terminals 16a are laid on a circuit board or the like.

  The two internal wirings are respectively connected to two output terminals of the output differential amplifier of the transmission unit 17 and two input terminals of the input differential amplifier of the reception unit 18. Each internal wiring is connected to one end of a resistor R via a switch SW, and the other end of the resistor R is connected to a ground potential. The resistance value of each resistor R is set to 60Ω, for example (this resistance value is a half value of 120Ω defined as the termination resistance value of the transmission line by the CAN protocol). The switching unit 19 having two switches SW connected to two internal wirings turns on the two switches SW according to a control signal output from a control circuit (not shown) in the CAN communication control unit 16. / Switch off (connect / block) simultaneously.

  The CAN communication control unit 16 turns off the two switches SW of the switching unit 19 during the period from the start of transmission of each bit to the passage of the first time Tp in the 1-bit transmission time Tb shown in FIG. Resistor R is disconnected from internal wiring. The CAN communication control unit 16 turns on the two switches SW of the switching unit 19 to connect the resistor R to the internal wiring during a period from the end of the first time Tp to the end of the 1-bit transmission time Tb. The connection / disconnection of the resistor R by the CAN communication control unit 16 is performed at the same timing regardless of whether the transmission data is dominant / recessive, and is the same timing when data is not transmitted (only reception operation). Done in In addition, the same processing is performed in the CAN communication control unit 16 of all ECUs 1 in the communication system, thereby realizing dynamic impedance matching.

FIG. 5 is a schematic diagram for explaining the first time Tp and the second time Th related to data transmission. The two ECUs 1b and 1c included in the communication system shown in FIG. 1 and the communication line 5 connecting them. (Main line 5a and branch lines 5b and 5c) are extracted. Since ringing occurs when a signal output from one ECU 1 is reflected by another ECU 1, the other ECU 1 that is closest to one ECU 1 (with the shortest signal propagation distance) causes the ringing to occur. The impact is great. Here, it is assumed that the two ECUs 1b and 1c are closest to each other in the communication system shown in FIG. 1 (that is, the distance between the ECUs 1b and 1c is the shortest in the communication system).

When the length of the branch lines 5b and 5c branched from the trunk line 5a of the communication line 5 is LS and the length between the branch points of the branch lines 5b and 5c branched from the trunk line 5a is LP, the distance between the two ECUs 1b and 1c L is
L = 2 × LS + LP
It is. Therefore, for example, the time T from when the signal output from the ECU 1b is reflected by the ECU 1c to return to the ECU 1b is A when the transmission speed of the signal of the communication line 5 is A
T = 2 × L × A
It is. Note that the transmission speed A is defined as, for example, 5 nsec / m in the CAN protocol.

Therefore, by setting the second time Th to be equal to or longer than the transmission time T, the 0V signal output and the impedance matching are performed during the second time Th, so that ringing can be suppressed. That is, the second time Th is
Th ≧ 2 × L × A
It is necessary to satisfy the conditions. In addition, as described above, the first time Tp and the second time Th are
Tp + Th ≦ Tb
It is necessary to satisfy the conditions.

  6 and 7 are flowcharts showing a procedure of transmission processing by the CAN communication control unit 16. The CAN communication control unit 16 acquires 1-bit information to be transmitted from the data to be transmitted (step S1), and determines whether or not this 1-bit is dominant (step S2).

  If one bit to be transmitted is dominant (S2: YES), the CAN communication control unit 16 outputs a signal at the first signal level to the communication line 5 (step S3), and proceeds to step S4. In addition, when one bit to be transmitted is not dominant and is recessive (S2: NO), the CAN communication control unit 16 advances the processing to step S4 without performing signal output. Next, the CAN communication control unit 16 starts a timer (step S4), and determines whether or not the first time Tp has elapsed since the start of the bit transmission process (step S5). When the first time Tp has not elapsed (S5: NO), the CAN communication control unit 16 continues the time measurement by the timer and waits until the first time Tp has elapsed.

  When the first time Tp has elapsed (S5: YES), the CAN communication control unit 16 outputs a signal having the second signal level (0 V) to the communication line 5 (step S6). Thereafter, the CAN communication control unit 16 determines whether or not the second time Th has further elapsed from the elapse of the first time Tp (step S7), and when the second time Th has not elapsed (S7: NO). Until the second time Th elapses, the signal of the second signal level is output and waits. When the second time Th has elapsed (S7: YES), the CAN communication control unit 16 stops signal output (step S8).

Next, the CAN communication control unit 16 determines whether or not the 1-bit transmission time Tb has elapsed (step S9). If the 1-bit transmission time Tb has not elapsed (S9: NO), the transmission is performed. Wait until time Tb has elapsed. When the 1-bit transmission time Tb has elapsed (S9: YES), the CAN communication control unit 16 stops the timer (step S10), and determines whether or not transmission of all bits of the transmission data has been completed. Then, it is determined whether or not to end the transmission (step S11). If it is determined not to end the transmission (S11: NO), the CAN communication control unit 16 returns the process to step S1, and performs the same process for the next bit of the transmission data. When it determines with complete | finishing transmission (S11: YES), the CAN communication control part 16 complete | finishes a transmission process.

  FIG. 8 and FIG. 9 are flowcharts showing a procedure of reception processing by the CAN communication control unit 16. First, the CAN communication control unit 16 sets a terminal in a high impedance state without performing signal output (step S21), and disconnects the resistor R by turning off the two switches SW of the switching unit 19 (step S22). .

  Next, the CAN communication control unit 16 determines whether or not the signal level of the communication line 5 exceeds a predetermined threshold value (step S23). If the signal level does not exceed the threshold value (S23: NO), the signal level is Wait until the threshold is exceeded. When the signal level exceeds the threshold (S23: YES), the CAN communication control unit 16 samples the signal level of the communication line 5 (step S24), and determines whether or not the first time Tp has elapsed since the start of sampling. Determination is made (step S25). If the first time Tp has not elapsed (S25: NO), the CAN communication control unit 16 returns the process to step S24, and repeatedly performs signal level sampling until the first time Tp has elapsed.

  When the first time Tp has elapsed (S25: YES), the CAN communication control unit 16 connects the resistor R by turning on the two switches SW of the switching unit 19 (step S26).

  Next, the CAN communication control unit 16 determines whether or not the second time Th has further elapsed from the elapse of the first time Tp (step S27). When the second time Th has not elapsed (S27: NO), Wait until the second time Th elapses. When the second time Th has elapsed (S27: YES), the CAN communication control unit 16 disconnects the resistor R by turning off the two switches SW of the switching unit 19 (step S28), stops the signal output, The terminal is set to a high impedance state (step S29).

  Thereafter, the CAN communication control unit 16 determines whether or not the 1-bit transmission time Tb has elapsed (step S30). If the 1-bit transmission time Tb has not elapsed (S30: NO), Wait until time has passed. When the transmission time Tb of 1 bit has elapsed (S30: YES), the CAN communication control unit 16 determines whether or not to end the reception by determining whether or not reception of all bits of the reception data has been completed. Determination is made (step S31). When it is determined not to end the reception (S31: NO), the CAN communication control unit 16 returns the process to step S23 to perform the same process for the next bit of the received data, and performs the processes from step S23 to S31. Do. Moreover, when it determines with complete | finishing reception (S31: YES), the CAN communication control part 16 complete | finishes a reception process.

  FIG. 10 is a schematic diagram for explaining the effect of the communication system according to the present invention, in which the vertical axis represents a potential difference between twist lines and the horizontal axis represents time. The illustrated example is a waveform when 1-bit information transmission time Tb = 1000 ns, dominant transmission is performed from 0 ns to 1000 ns, and recessive transmission is performed from 1000 ns to 2000 ns. In the dominant transmission, signal output is performed for the first time Tp = 800 ns, the first signal level = 2V, the second time Th = 200 ns, and the second signal level = 0V.

From the signal waveform shown in FIG. 10, it can be seen that the ringing signal level (amplitude) is reduced in the communication system of the present invention compared to the waveform of the conventional communication system shown in FIG. In the conventional communication system, the maximum ringing signal level exceeds 4V, whereas in the communication system of the present invention, the maximum ringing signal level is about 0.8V. Therefore, in the communication system of the present invention, it is possible to determine dominant / recessive by determining the signal level on the communication line 5 with a threshold value such as 0.9V or 1.0V.

  In the communication system configured as described above, the transmission unit 17 of the ECU 1 outputs a signal of the first signal level to the communication line 5 over the first time Tp when the transmission data is dominant, and then the second second When the signal of the second signal level (0 V) is output over the time Th, the transmission data is recessive and the signal level of the communication line 5 is recessive, the communication line over the first time Tp and the second time Th. When the transmission data is recessive and the signal level of the communication line 5 is dominant and no signal is output to the communication line 5 for the first time Tp, the signal is not output to the communication line 5. Thereafter, the signal of the second signal level (0 V) is output over the second time Th. The CAN communication control unit 16 of the ECU 1 is configured to be able to connect / disconnect the resistor R to the communication line 5 by switching connection / disconnection of the switch SW of the switching unit 19. The resistor R is connected after the elapse of the first time Tp at the time Tb. With these configurations, the occurrence of ringing in the communication line 5 in the communication system can be suppressed, so that high-speed communication can be realized.

When the shortest distance between the two ECUs 1 in the communication system is L and the transmission speed of the communication line 5 is A, the first time Tp and the second time Th are
Tp + Th ≦ Tb
Th ≧ 2 × L × A
Set to satisfy the conditions. As a result, the signal output from one ECU 1 is reflected by the other ECU 1 and the one ECU 1 does not enter the high impedance state before the reflected wave returns to the one ECU 1. It can suppress more reliably.

  In the present embodiment, the communication system is mounted on the vehicle. However, the present invention is not limited to this. The configuration of the communication system shown in FIG. 1 (the number of ECUs 1, the connection form of the ECU 1, etc.) is merely an example, and the present invention is not limited to this. Further, although the twisted line is used as the communication line 5, the present invention is not limited to this, and other structures such as using one cable as the communication line 5 may be used.

  Further, since the first time Tp and the second time Th need only satisfy Tp + Th ≦ Tb, when Tp + Th <Tb, the time from the end of the second time Th to the lapse of 1-bit transmission time Tb Exists. In the present embodiment, the switch SW of the switching unit 19 is turned on during this time. However, the present invention is not limited to this, and the switch SW may be turned off. The flowcharts shown in FIGS. 6 and 7 assume that Tp + Th <Tb. However, the present invention is not limited to this, and Tp + Th = Tb may be used. If the transmission data is dominant, step S12 is performed. And S13 need not be performed. In addition, although the signal level sampling is performed over the first time Tp in the reception process, the present invention is not limited to this, and a configuration may be used in which sampling is performed over a time shorter than the first time Tp.

1, 1a-1e ECU (communication device)
5 communication line 5a trunk line 5b-5d branch line 11 control unit 16 CAN communication control unit (communication means, switching control means)
17 Transmitter 18 Receiver 19 Switcher R Resistor SW Switch

Claims (5)

A plurality of communication devices connected via a common communication line, and each communication device receives information of a plurality of consecutive bits each represented by a binary value of a dominant value or a recessive value via the communication line. In a communication system having communication means for transmitting and receiving,
The communication means includes
A signal having a first signal level is output over a first time Tp shorter than the information transmission time for one bit with respect to the dominant value of the information to be transmitted, and then over a second time Th shorter than the information transmission time. Outputting a signal having a second signal level lower than the first signal level;
A communication system, wherein a signal output to the communication line is not performed for an inferior value of information to be transmitted. The communication device
A switching unit for connecting / blocking the other end of the resistor, one end of which is connected to a fixed potential, with respect to the communication line;
The switching control means for controlling the switching unit to connect the resistor after the first time Tp has elapsed in the information transmission time for one bit. Communications system. The first time Tp and the second time Th are:
Tp + Th ≦ Tb
Th ≧ 2 × L × A
Where Tb is the information transmission time for 1 bit, L is the shortest distance of the communication line interposed from one communication device to another communication device, and A is (The transmission speed of the signal on the communication line.)
The communication system according to claim 1 or 2, characterized in that. A communication device connected to another device via a common communication line, and comprising a communication means for transmitting and receiving information of a plurality of consecutive bits each represented by a binary value of a dominant value or an inferior value via the communication line In
The communication means includes
A signal having a first signal level is output over a first time Tp shorter than the information transmission time for one bit with respect to the dominant value of the information to be transmitted, and then over a second time Th shorter than the information transmission time. Outputting a signal having a second signal level lower than the first signal level;
A communication apparatus characterized by not outputting a signal to the communication line for an inferior value of information to be transmitted. In a communication method of transmitting and receiving information of a plurality of consecutive bits each represented by a binary value of a dominant value or a recessive value via a common communication line,
A signal having a first signal level is output over a first time Tp shorter than the information transmission time for one bit with respect to the dominant value of the information to be transmitted, and then over a second time Th shorter than the information transmission time. Outputting a signal having a second signal level lower than the first signal level;
A communication method characterized by not outputting a signal to the communication line with respect to an inferior value of information to be transmitted.

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