This paper mainly introduces the CAN bus, and focuses on the can termination resistance in detail.

Can bus

Can is the abbreviation of controller area network (can). It was developed by Bosch Company in Germany, which is famous for R & D and production of automotive electronic products, and finally became an international standard (ISO 11898). It is one of the most widely used fieldbuses in the world. In North America and Western Europe, can bus protocol has become the standard bus for automotive computer control system and embedded industrial control LAN, and has J1939 protocol designed for large trucks and heavy machinery vehicles with can as the underlying protocol.

Can bus is a serial data communication protocol developed by Bosch Company in Germany in the early 1980s to solve the data exchange between many control and test instruments in modern vehicles. It is a multi master bus. The communication medium can be twisted pair, coaxial cable or optical fiber. The communication rate can be up to 1Mbps.

Complete the framing processing of communication data

The CAN bus communication interface integrates the physical layer and data link layer functions of CAN protocol, which can complete the framing processing of communication data, including bit filling, data block coding, cyclic redundancy inspection, priority discrimination and so on.

The number of nodes in the network is not limited in theory

One of the biggest characteristics of CAN protocol is to abolish the traditional station address coding and instead encode the communication data block. The advantage of this method is that the number of nodes in the network is not limited in theory. The identifier of the data block can be composed of 11 bit or 29 bit binary numbers. Therefore, 2 or more different data blocks can be defined. This way of coding according to the data block can also enable different nodes to receive the same data at the same time, which is very useful in the distributed control system. The length of the data segment is up to 8 bytes, which can meet the general requirements of control commands, working status and test data in the general industrial field. At the same time, 8 bytes will not occupy the bus for too long, so as to ensure the real-time communication. Can protocol adopts CRC check and can provide corresponding error handling function to ensure the reliability of data communication. With its excellent characteristics, high reliability and unique design, can is especially suitable for the interconnection of industrial process monitoring equipment. Therefore, it has attracted more and more attention in the industry and has been recognized as one of the most promising fieldbus.

Free communication between nodes can be realized

Can bus adopts multi master competitive bus structure, which has the characteristics of multi master station operation, decentralized arbitration serial bus and broadcast communication. Any node on CAN bus can actively send information to other nodes on the network at any time, regardless of primary and secondary, so it can realize free communication between nodes. Can bus protocol has been certified by the international organization for standardization. The technology is relatively mature. The control chip has been commercialized and has high cost performance. It is especially suitable for data communication between distributed measurement and control systems. The CAN bus card can be arbitrarily inserted into the PC at XT compatible computer to easily form a distributed monitoring system.

Simple structure

Only 2 wires are connected externally, and the error detection and management module is integrated internally.

Transmission distance and rate

Can bus features: (1) data communication is not divided into master and slave. Any node can initiate data communication to any other (one or more) nodes, and the communication order is determined by the information priority of each node. The information of high priority nodes is 134 μ S communication; (2) When multiple nodes initiate communication at the same time, the low priority will avoid the high priority, which will not cause congestion to the communication line; (3) The maximum communication distance can reach 10km (the rate is lower than 5Kbps) and the rate can reach 1Mbps (the communication distance is less than 40m); (4) The transmission medium of CAN bus can be twisted pair and coaxial cable. Can bus is suitable for large amount of data, short-distance communication or long-distance small amount of data, high real-time requirements, multi master and multi slave or equal field use of each node.

Technical introduction

Bit arbitration

In order to process the data in real time, the data must be transmitted quickly, which requires a high speed in the physical transmission path of the data. When several stations need to send data at the same time, it is required to allocate the bus quickly. Real time processing of emergency data exchanged through the network is quite different. A rapidly changing physical quantity, such as automobile engine load, will transmit data more frequently and require shorter delay than a relatively slow changing physical quantity such as automobile engine temperature.

Can bus transmits data in the unit of message. The priority of message is combined in 11 bit identifier, and the identifier with the lowest binary number has the highest priority. Once this priority is established during system design, it can no longer be changed. Conflicts in bus reads can be resolved by bit arbitration. For example, when bit arbitration occurs for identifiers 0111111, 0100100 and 0100111, 0100100 messages will be tracked and other messages will be discarded. The specific process is as follows: when several stations send messages at the same time, the message identifier of station 1 is 0111111, the message identifier of station 2 is 0100110, and the message identifier of station 3 is 0100111. All identifiers have the same two bits 01. Until the third bit is compared, the message of station 1 is discarded because its third bit is high, while the third bit of the message of the other two stations is low. Bits 4, 5 and 6 of station 2 and station 3 messages are the same. The message of station 3 is not discarded until bit 7. Note that the signal in the bus continuously tracks the message of the station that finally obtains the bus reading right. In this example, the message of station 2 is tracked. The advantage of this non-destructive bit arbitration method is that the initial part of the message has been transmitted on the network before the network finally determines which station’s message is transmitted. All stations that have not obtained the bus reading right become the receiving stations with the highest priority message, and will not send the message before the bus is idle again.

Can has high efficiency because the bus is only used by those stations whose request bus is pending. These requests are processed in order according to the importance of messages in the whole system. This method has many advantages when the network load is heavy, because the priority of bus reading has been placed in each message in order, which can ensure a lower individual latency in the real-time system.

For the reliability of the master station, since the CAN protocol implements decentralized bus control, all main communications, including bus reading (permission) control, are completed in several times in the system. This is the only way to realize a communication system with high reliability.

Comparison between can and other communication schemes

In practice, there are two important bus allocation methods: schedule allocation and demand allocation. In the first method, each node is allocated according to the maximum period regardless of whether each node applies for bus or not. Thus, the bus can be assigned to each station and is a unique station, whether it is bus access immediately or bus access at a specific time. This will ensure that there is a clear bus allocation when accessing the bus. In the second method, the bus is allocated to a station according to the basic requirements of transmitting data, and the bus system is allocated according to the transmission desired by the station (e.g. Ethernet CSMA / CD). Therefore, when multiple stations request bus access at the same time, the bus will terminate the requests of all stations, and no station will obtain bus allocation. In order to allocate buses, more than one bus access is necessary.

The method of CAN bus allocation can ensure that bus allocation can be carried out clearly when different stations apply for bus access. This bit arbitration method can solve the collision problem when two stations send data at the same time. Different from the message arbitration of Ethernet network, the non-destructive method of can to solve the bus access conflict ensures that the bus is not occupied when no useful message is transmitted. Even when the bus is under heavy load, the bus access with message content first has proved to be an effective system. Although the transmission capacity of the bus is insufficient, all unresolved transmission requests are processed in order of importance. In networks such as CSMA / CD, such as Ethernet, the system often crashes due to overload, which will not happen in can.

Message format of can

The message transmitted in the bus consists of 7 parts per frame. Can protocol supports two message formats. The only difference is that the length of identifier (ID) is different. The standard format is 11 bits and the extended format is 29 bits.

In the standard format, the start bit of the message is called frame start (SOF), followed by an arbitration field composed of 11 bit identifier and remote transmission request bit (RTR). The RTR bit indicates whether it is a data frame or a request frame. There are no data bytes in the request frame.

The control field includes an identifier extension (IDE) that indicates whether it is a standard format or an extended format. It also includes a reserved bit (RO) for future expansion. Its last four bits are used to indicate the length of data in the data field (DLC). The data field range is 0 ~ 8 bytes, followed by a cyclic redundancy check (CRC) to detect data errors.

The reply field (ACK) includes a reply bit and a reply separator. The two bits sent by the transmitting station are both hidden level (logic 1). At this time, the receiving station that correctly receives the message sends the master control level (logic 0) to cover it. In this way, the sending station can ensure that at least one station in the network can correctly receive the message.

The end of the message is marked by the end of the frame. There is a short interval bit between two adjacent messages. If no station accesses the bus at this time, the bus will be idle.

Composition of can data frame

Remote frame

The remote frame consists of 6 fields: frame start, arbitration field, control field, CRC field, response field and frame end. There is no data field in the remote frame.

The RTR bit of the remote frame must be hidden.

The data value of DLC is independent. It can be any value from 0 to 8, which is the data length of the corresponding data frame.

Error frame

The error frame is composed of two different fields. The first field is obtained by superimposing the error flags from each station, and the second field is the error delimiter

There are two forms of error flags:

The active error flag consists of 6 consecutive alleles

The passive error flag consists of 6 consecutive hidden bits

The error delimiter includes 8 hidden bits

Overload frame

The overload frame includes two bit fields: overload flag and overload delimiter

Overload condition for sending overload frame:

Request to delay the next data frame or remote frame

A epitope was detected in the intermittent field

The overload flag consists of 6 alleles

The overload delimiter consists of 8 hidden bits

Data error detection

Unlike other buses, can protocol cannot use reply information. In fact, it can signal any error that occurs. Can protocol can use five methods to check errors, of which the first three are based on message content.

3.4.1 cyclic redundancy check (CRC)

Adding redundancy check bit in a frame of message can ensure the correctness of message. The receiving station can judge whether the message is wrong through CRC.

3.4.2 frame check

This method determines the correctness of the message by checking the format and size of the frame in the bit field, and is used to check the format error.

3.4.3. Response error

As mentioned earlier, the received frame is confirmed by the receiving station through an explicit response. If the sending station does not receive a response, it indicates that the receiving station has found an error in the frame, that is, the ACK field has been damaged or there is no receiving station for the message in the network. Can protocol can also detect errors by bit checking.

3.4.4 bus detection

Sometimes, a node in the can can can monitor its own signal. Therefore, the station sending the message can observe the bus level and detect the difference between the sending bit and the receiving bit.

3.4.5 bit filling

Each bit in a frame of message is represented by non return to zero code, which can ensure the maximum efficiency of bit coding. However, if there are too many bits of the same level in a frame of message, it is possible to lose synchronization. To ensure synchronization, synchronization is generated by using bit filling. After five consecutive equal bits, the transmitting station automatically inserts a complementary complement bit; When receiving, this fill bit is automatically discarded. For example, after five consecutive low-level bits, can automatically inserts a high-level bit. Can checks for errors through this coding rule. If there are 6 identical bits in a frame of message, can knows that an error has occurred.

If at least one station detects one or more errors through the above methods, it will send an error flag to terminate the current transmission. This can prevent other stations from receiving wrong messages and ensure the consistency of messages on the network. When a large number of transmitted data are terminated, the transmitting station will automatically resend the data. As a rule, the transmission is restarted within 23 bit cycles after an error is detected. On special occasions, the recovery time of the system is 31 bit cycles.

However, there is a problem with this method, that is, a wrong station will cause all data to be terminated, including correct data. Therefore, if self-monitoring measures are not taken, the bus system should adopt modular design. Therefore, can protocol provides a way to distinguish accidental errors from permanent errors and local station failures. This method can be realized by determining the error of a station itself through statistical evaluation of the error station and entering an operation method that will not have an adverse impact on other stations, that is, the station can close itself to prevent the normal data from being terminated because it is mistakenly regarded as incorrect data.

Hard synchronization and resynchronization

Hard synchronization is only carried out when the jump edge from invisible bit to dominant bit occurs under the condition of bus idle state, indicating the beginning of message transmission. After hard synchronization, the bit time counter restarts counting with the synchronization period. Hard synchronization forcibly places the jump edge that has occurred within the restarted bit time synchronization period. According to the synchronization rules, if a hard synchronization occurs within a bit time, resynchronization will not occur within that bit time. Resynchronization may cause phase buffer segment 1 to be extended or phase buffer segment 2 to be short. The upper limit of the extension time or reduction time of the two phase buffer segments is given by the resynchronization jump width (SJW).

Analysis and configuration of CAN bus terminal resistance

In the past decades, elevator communication system has developed from the initial parallel communication and RS485 communication to the CAN bus communication widely used at present. Parallel communication means that each control signal needs to occupy a separate line to transmit to the elevator main controller. With the increase of the number of elevator floors, the number of cables also increases exponentially. RS485 bus communication establishes the connection between the main controller and the car roof, control box and external call through the communication mode of command and response; The elevator main controller sends query signals to each position controller regularly, and then each sub controller sends back their respective status. RS485 uses three wires to realize serial communication. Although it simplifies the field wiring, it has some disadvantages such as poor flexibility and reliability, so it is gradually replaced by CAN bus. CAN Fieldbus has the following characteristics: (1) multi master bus, and each node controller can actively send information to other nodes on the network at any time; (2) The non-destructive bus arbitration technology is adopted, and the nodes with high priority give priority to transmitting data, which can meet the real-time requirements; (3) It has the function of point-to-point, point to multipoint and global broadcast data transmission; (4) The error rate of CAN bus data is very low. If a node has a serious error, it can automatically leave the bus, and other operations on the bus will not be affected; (5) The communication distance is long, up to 10km (5KB / s), the communication rate can reach 1MB / S (40m), and the actual number of nodes can reach 110; (6) The CAN bus has only two wires, and the new node can be directly connected to the bus, which is easy to install.

At present, can bus has two main applications in elevator. One is the communication control system of single elevator, that is, the communication of car, machine room and each floor of single elevator; The second is the group control elevator communication control system, that is, the communication between elevators. In addition, the remote monitoring system of some elevators can also use can bus communication.

3 signal reflection and impedance matching

3.1 signal reflection

According to the transmission line principle, when the signal encounters impedance discontinuity in propagation (such as entering the load from the transmission line), it will produce reflected wave. The superposition of the reflected signal on the original signal will change the shape of the original signal, resulting in the loss or distortion of the signal, affecting the communication quality and even unable to communicate normally.

Transmission line equation for two lines:


Where: R, l, G and C are resistance, inductance, conductance and capacitance per unit length of transmission line respectively

V (Z, t) and I (Z, t) are the voltage and current at z position and t time respectively

t. Z is the time axis and displacement axis respectively

In the lossless transmission line, there is r = g = 0, which is substituted into equation (1) and decoupled to obtain:


The form of the solution of equation (2) is:


Where ZC is the characteristic impedance of the transmission line

V is the propagation speed of the signal on the transmission line

As can be seen from equation (3), the voltage and current in the transmission line have two components, and the displacement Z of the component is in the same direction as time t, which is a forward traveling wave; The middle displacement Z is opposite to the time t, which is the backward traveling wave, in which the forward traveling wave is the incident wave and the backward traveling wave is the reflected wave. The reflection characteristic of transmission line is usually expressed by reflection coefficient Γ L means:


3.2 impedance matching of elevator can bus

It is obviously troublesome to calculate the reflection coefficient through equation (4). We can also calculate the reflection coefficient through the system boundary conditions. Consider the case where the signal enters the load medium from the transmission line medium, as shown in Fig. 1. At the interface, the current and voltage shall be continuous without step, so there are:


According to Ohm’s Law:


Simultaneous equations (5) and (6) show that:


As can be seen from equation (7), when RL = ZC, Γ= 0, the load does not produce a reflected signal. Therefore, the condition that the signal does not reflect when entering the load from the transmission line is that the load resistance is equal to the characteristic impedance of the transmission line, that is, RL = ZC. This time is called impedance matching.

From the above analysis, it can be seen that the impedance matching condition of elevator can bus is that the load resistance on elevator can bus is equal to the characteristic impedance of transmission line. The characteristic impedance of twisted pair shielded wire used in CAN bus is 60 Ω. Generally, the input impedance of can transceiver is as high as about 20K Ω, which is much larger than the characteristic impedance of transmission line. Therefore, a resistance with a total value of about 60 Ω needs to be incorporated between CAN buses to realize impedance matching.

3.3 short distance transmission of can signal

Considering that the can communication signal is transmitted in the direction of Figure 2, the signal propagates in the direction of Z at the speed V, the propagation distance L, the rising edge slope k and the rising edge time Ts of the signal. In the case of no impedance matching, the time delay of the reflected signal back to the output. If the single time delay is very small, the reflected signal will be covered by the rising edge and will not affect the communication. It is generally believed that when the signal delay is less than 20% of TS, the reflection of the signal is acceptable. Considering that can communication is actually can_ H and can_ L difference, so it requires time delay T 4 elevator can bus topology analysis

4.1 introduction to network topology

The network topology of communication system generally includes the following types:

(1) Star topology: there must be a host in the star topology. Each extension is connected to the host through point-to-point, and the communication between extensions must be transferred through the host. In this topology, the failure of a single extension will not affect the communication between the host and other extensions. However, since the communication between extensions can only be completed through the host, once the host fails, the network communication cannot work normally, so the requirements for the reliability and capacity of the host are very high; In addition, each extension should be connected to the host through point-to-point mode, with a large amount of cables.

(2) Ring topology: there is no host in the ring topology. In the ring topology, each node forms a closed loop through the links between nodes. The transmission of data on the link is unidirectional. Each node receives data from one link and sends it from another link. The data circulates on the network in one direction. The advantage of ring topology is that the required medium length is short; The disadvantage is that the failure of one node will cause the failure of the whole network.

(3) Network topology: each node of the network topology has one or more links connected with other nodes. There are multiple paths between nodes to transmit data. When transmitting data, it is possible to choose a relatively idle channel or bypass the fault point, so the network resources can be fully utilized. The fault of a single node or line has little impact on the network and high network reliability. But its structure is complex and its cost is high.

(4) Bus topology: bus topology is multi master communication, and each node sends and receives data at the same time. It uses a single bus as the transmission medium. All nodes are connected to the bus through the hardware driver interface. The data sent by any node can be received by other nodes. The data is sent in groups. After receiving the data, each node copies the qualified data from the bus through address identification. The advantages of bus topology are short cable length and easy wiring. The bus is only a transmission channel without any processing function. It has high reliability and convenient expansion. The disadvantage is that the scope of the system is limited.

(5) Tree topology: tree topology evolved from bus topology. Starting from the root of the tree, there can be multiple branches under each node. Many characteristics of tree topology are similar to bus topology, but its faults are easier to isolate and check.

4.2 can bus topology

The can high-speed standard iso11898 adopts the bus structure as the network topology, and a terminal resistance is connected at both ends of the bus. However, in practice, the network topology is not a strict bus structure, and some nodes have a certain branch length. In addition, in some applications, it may be better to adjust the terminal network from the perspective of EMC. The following briefly introduces the characteristics of various bus topologies:

(1) Dual terminal bus. In the standard dual terminal bus network topology, a 120 Ω terminal resistance is connected at both ends of the bus, and the total resistance of the bus is 60 Ω. The topology has the advantages of simple wiring, good reliability and long transmission distance. It is the most commonly used bus topology in elevator at present.

(2) Single terminal bus. The simplest way to match can bus is to connect a 60 Ω terminal resistance on the bus. In this topology, the bus resistance is 60 Ω and the impedance is matched. However, in this topology, many nodes are actually not on the bus, but on the branch line, and their transmission distance is limited. The bus length of this topology is only 50% of that of the standard dual terminal bus connection.

(3) Separate the bus. Separate bus is to divide a single terminal resistance into two resistors with the same resistance value on the basis of double terminal bus, and ground the two resistors through a capacitor, as shown in Figure 3. It can be seen that separating the bus does not change the DC characteristics of the bus. There are two grounding modes for the separated bus: 1) separate the two terminals and ground them separately. This topology can optimize the high frequency performance of communication. However, after grounding both terminal resistors, an interfering loop current may be formed through the ground current. In this case, consider 2) grounding only one terminal resistance, which has better transmission characteristics in the range from intermediate frequency to low frequency. This bus structure has complex wiring and is generally used only in specific cases.

(4) Multi terminal bus. In some applications, if you need to add an additional branch, you need to adopt a topology different from the bus structure. At this time, the topology is similar to the star topology. In this case, the multi terminal connection method can be used. Multi terminal structure is to divide the terminal resistance (60 Ω) into more than two resistors, and the resistance on the bus is still guaranteed to be 60 Ω. Figure 4 shows the star topology of three branches. In this case, each branch can see a terminal with a resistance of 180 Ω.

In this topology, if one of the terminals is removed, the impedance on the bus will no longer match exactly. However, it can still be used normally in the case of short-distance transmission. Therefore, in this topology, the communication distance of CAN bus will be much smaller than that of dual terminal topology.

4.3 elevator can bus topology analysis

Fig. 5 is a circuit diagram in which a matching resistance R1 = 60 Ω is directly incorporated into the elevator group control can bus. After R1 is incorporated into the bus, since the impedance of the can transceiver is much greater than R1, the load impedance in the bus is close to 60 Ω, and the CAN bus achieves impedance matching. However, in this topology, if A1 sends a signal to the bus, irim in the circuit and the right part (red) of the matching resistance actually belong to the branch and are not in the bus. Therefore, when its length is “Lmax”, the system will not be able to communicate normally. Further, the can communication is multi master bus, and each node sends and receives signals at the same time. Therefore, in this topology, the length at both ends of the matching resistance must be less than Lmax, which will greatly shorten the maximum transmission distance of CAN bus.

Therefore, in the existing elevator can communication, a 120 Ω terminal resistance is generally connected in parallel at both ends of the bus, as shown in Figure 6. In this topology, the total bus resistance is also close to 60 Ω, and the bus impedance is matched. The connection distance between each node and the bus is small, and the reflection can be ignored. However, the length from the node to the bus shall be minimized, and its length shall be less than Lmax.

In the matching of terminal resistance, the terminal resistance must be placed at the farthest two ends. If one of them is placed in the middle, the structure is shown in Figure 7. In this topology, the can transceiver A1 outside the terminal resistance is on the branch, which will greatly increase the signal reflection of the node and affect the bus communication.

It should be noted that in the above analysis, the influence of line resistance and node impedance on bus resistance is not considered. In practical application, the terminal resistance can be fine tuned according to the line length and the number of nodes to make the total resistance of CAN bus close to 60 Ω as much as possible.

5 Conclusion

Through the analysis of transmission line signal reflection and can bus structure, this paper preliminarily discusses the basic principle of terminal resistance configuration of elevator can bus. The configuration of CAN bus terminal resistance shall follow the following principles:

(1) The configuration of terminal resistance shall meet the impedance matching of CAN bus, and the resistance between buses shall be equal to the characteristic impedance of transmission line. At present, 120 Ω resistors are generally added at both ends of the CAN bus. If the requirements are higher, the resistance value can be fine tuned according to the bus length and the number of nodes to make the resistance value between buses close to 60 Ω as much as possible; (2) The two terminal resistances shall be equivalently configured at the two nodes farthest from the line; (3) The distance between the node and the bus shall be as short as possible, and the specific allowable length will be different due to the influence of signal frequency, line resistance and other factors. In particular, pay more attention to this when connecting temporary equipment nodes such as diagnostic instrument.

In short, the configuration of elevator can communication terminal resistance is essential. Increasing or reducing the resistance will cause communication instability or even failure of normal communication. The position of terminal resistance shall make the total resistance between CAN buses close to the characteristic impedance of transmission line as much as possible, and the length of unmatched part of line shall be as short as possible.

Interference wave of CAN bus

Can bus is used in modern cars. It is a form of “twisted pair”, which is distributed among different computers. It is called “controller area network bus” technology. Compared with the traditional wiring method, the CAN bus system greatly simplifies the line layout, and the data transmission speed is higher, smarter and more accurate.

Can bus transmits high-speed information and data flow, just like waves, wave by wave transmission. It is found that when the information data stream reaches the destination or terminal, it will be reflected at its terminal, resulting in interference wave of information and affecting the normal transmission of information. Just as when the waves hit the coast of the breakwater, they will be blocked by hard rocks, which will superimpose reflected waves on the waves, resulting in greater impact of the waves and destroying the original transmitted information.

How to reduce the reflection of information data flow on the computer terminal? When people observe the waves, they find that when the waves hit the beach, the soft beach will absorb the waves instead of forming reflected waves. What items can absorb the reflected wave of high-speed information data flow on CAN bus? Through a large number of experiments, it is found that at the farthest end of the bus, as long as two resistors are connected in parallel.

  Number of can termination resistors and position of CAN bus terminal resistance

Figure 1. Terminal resistors on CAN bus are connected in parallel

The terminal resistance on the CAN bus is connected in parallel on the bus. Placing the terminal resistance at a distant computer or separately can effectively absorb the reflected wave. The resistance values of the two terminals in Figure 1 are 120 Ω, which are connected in parallel at both ends of the can twisted pair bus, so the resistance between the high and low can wires should be 60 Ω. If it is not 60 Ω but 120 Ω, it means that one terminal resistance is open circuit. If the resistance between two can lines is large, it means that both terminal resistors are open circuit. If the resistance between the two can lines is zero, it means that the two high and low buses are connected together and short circuited. Once the terminal resistance is open, the interference wave can not be effectively absorbed, which will cause the signal transmitted by the bus to be unstable and directly affect the normal operation of relevant computers.

  Number of can termination resistors and position of CAN bus terminal resistance

Figure 2. Terminal resistors are connected in parallel at both ends of can twisted pair bus

For example, the terminal resistance of a BMW car was separately installed under the headlights of the car head. A rear end collision accident caused the connection of the terminal resistance to open circuit, and the maintenance master did not know its purpose, so he discarded it. As a result, the BMW lost an important safety function, that is, the anti-skid function of the vehicle was lost. The anti-skid warning light on the instrument panel also lights up, causing the owner to be very worried. After repeated and careful search, the failure of anti-skid function of the vehicle was eliminated.

It can be seen that the terminal resistance on the CAN bus is very important. In fact, measuring and judging this specific terminal power is not troublesome for every maintenance master and even car riders. It can be easily mastered.


That’s all for the introduction of CAN bus. If there are any deficiencies, please correct them.

Related reading recommendation: read the characteristics, advantages and disadvantages of CAN bus

Related reading recommendation: typical circuit schematic diagram of CAN bus communication

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