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What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Author: Apogeeweb Date: 30 Oct 2018  3063

interference

Summary 

This article is mainly to introduce the concept, advantages and anti-jamming elements of CAN bus. Actually, CAN bus is used in all fields in our daily life and already became an international standard. Now, let's get it!

 

 


Catalog   

Ⅰ Definition of the CAN Bus

Ⅱ Advantages of the CAN Bus

  2.1 Real-time Data Communication between Network Nodes

  2.2 Short Development Cycle

  2.3 Formed an International Standard

  2.4 One of the Most Promising Fieldbuses

Ⅲ Six Criteria for Anti-jamming of CAN Bus

  3.1 Isolation and Protection of CAN Bus Interface

  3.2 Improves the Twisting Degree of CAN Bus

  3.3 Ensures Shielding Effect and Correct Grounding

  3.4 Keep CAN Bus Away from Interference Sources

  3.5 Adding Magnetic Loops or Common-mode Inductor

  3.6 Converse CAN Bus to Optical Fiber Transmission

 

 


Definition of the CAN Bus

A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other in applications without a host computer. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles to save on copper, but is also used in many other contexts.

 

Developed by German BOSCH company which is famous for developing and producing automotive electronics products, CAN bus was eventually an international standard (ISO 11898), and became one of the most widely used fieldbuses in the world. In North America and Western Europe, the CAN bus protocol has become the standard bus for automotive computer control systems and embedded industrial control LANs, and there is a J1939 protocol designed for large trucks and heavy machinery vehicles with CAN as the underlying protocol.

 


Ⅱ Advantages of the CAN Bus

CAN belongs to the field bus and is a serial communication network that effectively supports distributed control or real-time control. Compared with many RS-485 distributed control systems based on R-line, the distributed control system based on CAN bus has obvious advantages in the following aspects:

2.1 Real-time Data Communication between Network Nodes

First of all, the CAN controller works in various ways, every network nodes can compete to send data to the bus according to the bus access priority (depending on the message identifier) using a lossless structure of bit-by-bit arbitration. The CAN protocol abolished the website address code and replaced it by encoding the communication data, which can make different nodes receive the same data at the same time. and the CAN protocol is abolished.

 

These characteristics make the data communication between the network nodes formed by the CAN bus real-time, and it is easy to construct redundant structure and improve system reliability and system flexibility. However, using RS-485 can only constitute a master-slave structure system, and the communication method can only be carried out in the manner of polling by the primary station. The real-time performance and reliability of the system are poor. 

2.2 Short Development Cycle  

The CAN bus is connected to the physical bus through two output terminals, CANH and CANL, of the CAN transceiver interface chip 82C250, while the state of the CANH terminal can only be high level or suspended state, the CANL terminal can only be low level or suspended state. This ensures it will not appear in the RS-485 network that when there is an error in the system and multiple nodes send data to the bus at the same time, the bus is short-circuited and thereby some nodes are damaged.

 

Moreover, the CAN node has the function of automatically shutting down the output in case of serious errors, so that the operation of other nodes on the bus will not be affected, ensuring that the bus will not appear a "deadlock" state because of problems with individual nodes in the network. Moreover, the perfect communication protocol of CAN can be realized by the CAN controller chip and its interface chip, which greatly reduces the difficulty of system development and shortens the development cycle, which are unmatched by RS-485 with only electrical protocols. 

2.3 Formed an International Standard

Compared with others, the CAN bus is an international standard fieldbus with many features such as high communication speed, easy implementation, and high price–performance ratio. These are also important reasons why CAN bus is applied in many fields and has strong market competitiveness. 

2.4 One of the Most Promising Fieldbuses

CAN is the LANs of controller, which belongs to the industrial field bus. Compared with the general communication bus, CAN bus has features of outstanding reliability, real-time and flexibility in data communication. Because of its good performance and unique design, CAN bus has been paid more and more attention. CAN bus is widely used in the automobile field so that some famous automobile manufacturers use it to realize the data communication between the internal control system and  agencies for detection and execution.

 

At the same time, due to the characteristics of CAN bus itself, its application scope is no longer limited to the automobile industry, but to automatic control, robot, ect. CAN has become an international standard and has been recognized as one of the most promising fieldbuses. Its typical application protocols are: SAE J1939 / ISO 11783 / CAN Open, CAN aerospacea DeviceNet, NMEA 2000, etc. 

 


Ⅲ Six Criteria for Anti-jamming of CAN Bus  

With more and more applications of CAN bus in electric vehicles, charging piles, power electronics, rail transit and other electromagnetic environment, the problem of signal interference has seriously affected the users' trust in CAN bus. How on earth can we resist interference?  This paper shows the 6 key rules of CAN bus anti-jamming.

 

In the gasoline vehicle era, the CAN bus encounters very little interference, even if there are some relays and solenoid valves pulse, it will not make a difference, and a bit of twisting processing can completely achieve zero error frame.

 

But in the era of electric vehicle, the influence of inverter, motor, charger and other high power equipments on CAN is enough to interrupt communication or damage CAN node. Figure1&2 show the inverter interference of the CAN waveform.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.1&2  Before interface & After interface

 

In the face of interference, automobile factories, parts factories, test and diagnostic equipment manufacturers have been studying "magic formula" of anti-jamming to ensure the stable operation of CAN. This paper introduces 6 iron rules for anti-jamming based on the experience of 15 years CAN field troubleshooting in an electronics company. 

3.1 Isolation and Protection of CAN Bus Interface

Interference not only affects the signal, but also causes the circuit board to crash or burn down, so the isolation of interface and power supply is the first iron rule of anti-jamming. The main purpose of isolation is to avoid ground return current to burn circuit boards, to limit the amplitude of interference, and to prevent the controller from crashing.

 

As shown in fig. 3, when interface and power supply are not isolated, the ground potential of the two nodes is inconsistent, there is a return current and a common-mode signal is generated. The anti-common-mode interference ability of CAN is - 12~7V, if the common-mode difference exceeds this value, it will break. And if the common-mode difference exceeds±36V, the transceiver or circuit board will be burned down.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.3 The ground return current when not isolated

 

As shown in Fig. 4 and Fig. 5, the circuit adds CTM1051KAT isolation module to isolate the ground return current and limit the interference amplitude.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.4 CTM1051KAT isolation module

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.5 Effect of isolated ground return current

 

After the adding isolation, will everything be perfect? Definitely not, isolation only blocks and if the intensity of interference is high, such as reaching 2KV surges, then isolation can also be broken. Therefore, to achieve a higher level of protection, the surge circuit must be added. Fig.6 is a high-speed bus standard anti-surge circuit.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.6 Signal protection circuit

 

This protection circuit can reach 4KV surge without damage, but it should be noted: to pass the 2500VDC withstand voltage test, GDT and R3 need to be removed to prevent high voltage breakdown them, resulting in failure of the test.

 

3.2 Improves the Twisting Degree of CAN Bus

In order to improve the anti-jamming ability, CAN bus adopts CANH and CANL differential transmission, as shown in fig.7, the effect is keeping the difference value of CANH-CANL unchanged when encountered the interference.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.7 Schematic diagram of differential anti-jamming

 

However, this anti-jamming ability has a prerequisite: CANH and CANL must be closely together, otherwise the interference intensities are different, resulting in differential signal being interfered. Therefore, CANH and CANL should be tightly twisted together. Usually twisted-pair wire is only 33 strands / m, but in the strong interference situation, only if the degree of twisting is more than 55 can the anti-jamming effect be better.

 

In addition, the core cross-sectional area of the cable should be greater than 0.35g~0.5 mm2, and the interline capacitance of CAN_H to CAN_L should be less than 75 pF/m。If the shielded twisted pair wire is used and if the capacitance of CAN_H (or CAN_L) to the shield layer is less than 110 pF/m, the impedance of the cable can be reduced better, thereby reducing the amplitude of the jitter voltage. 

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.8 Twisted pair

3.3 Ensures Shielding Effect and Correct Grounding

The CAN bus with shielding layer can resist the interference of electric field well。The whole shield layer is equivalent to an equipotential body, avoiding the interference of CAN bus. As shown in fig.9, CANH and CANL are wrapped into a standard shielded twisted pair through aluminum foil and oxygen-free copper wire shielding nets. It should be noted that in connection between the bus and the plug-in, a cable less than 25 mm is allowed to be untwisted.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.9 Shielded twisted pair

 

The better CAN shield line has two shielding layers, called double-layer shield line, in which the CAN_GND of the inner layer is connected to the CAN transceiver, and the shield of outer layer is connected to the earth. The inner layer can balance the ground potential of the signal, suppress the common-mode interference and reduce the error frame, but the receiver damage rate will increase when the interference is strong. And the outer layer can release the charge to the earth, as shown in Fig.10.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.10 Double shielded wire

 

Shielding lines will not work before shields are well grounded. So we have to choose a grounding mode. Generally speaking, the single-point grounding can avoid the ground reflux (which is caused by the different ground potential at different locations), and the multi-point grounding can speed up the release of high-frequency interference signals. Therefore, it is necessary to choose the appropriate grounding mode according to the actual situation. 

 

In the application of CAN, because the distance is generally far, most of the shield layers adopt single-point grounding, find a point in the trunk line to directly grounded with wire, the point should be the smallest interference point, and is located near the center of the network.

3.4 Keep CAN Bus Away from Interference Sources

Keep away from the interference source is the simplest anti-jamming method. If the distance of CAN bus and the strong electric interference source are more than the 0.5m, the interference will make no difference. However, in actual wiring, it is often encountered that the space is too small to be mixed the can bus with strong electricity. Fig.11 is a drive system of a new energy vehicle, the CAN bus and the driving line are mixed together, resulting in great interference.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.11 Wiring problem

 

To solve this problem, we can only ensure that strong electricity and weak electricity are strapped separately and far away from each other.

3.5 Adding Magnetic Loops or Common-mode Inductor

Using anti-jamming magnetic loopss is to weaken the effect of interference at specific frequencies. As shown in fig.12, to increase the effect of the magnetic loops, the CAN differential cable can be added with two wires or with a single end. 

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.12 Adding magnetic ring

 

The effect of the magnetic loops can greatly reduce the interference intensity of the specific frequency. Before adding the magnetic loops, it is necessary to test the highest interference frequency by CANScope or oscilloscope FFT, and then customize the magnetic loops of corresponding frequency with manufacturer.  Fig.13 shows the results before and after adding the magnetic loops. It can be seen that the interference intensity is obviously changed. 

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.13 Effect after adding magnetic ring

It should be noted that magnetic loops or common-mode inductors can not be added at will. If the adaptive frequency is not correct, it will affect the normal signal communication.

3.6 Converse CAN Bus to Optical Fiber Transmission

The ultimate measure of anti-jamming is to transform CAN bus into optical fiber, which is a transmission medium which can not be interfered by electromagnetic interference. If the first five anti-jamming methods can not solve the interference problem, CAN bus can be converted into optical fiber to accomplish "impeccable". Fig.14 is the fiber backbone network consisted of CANHub-AF1S1 and CANHub-AF2S2.

What Is CAN Bus: Concepts, Advantages and Anti-jamming Elements

Fig.14 Optical fiber backbone transmission using optical fiber converter

 

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