Why can't the length of the CAN bus branch be too long? Solution to the problem of too long branch lines in CAN network
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Why can't the length of the CAN bus branch be too long? Solution to the problem of too long branch lines in CAN network

Posted Date: 2024-01-31

Why can't the length of the CAN bus branch be too long?

The main reasons why the length of the CAN bus branch cannot be too long are as follows:

1. Signal attenuation and distortion: As the length of the CAN bus branch increases, signal attenuation and distortion will gradually increase. Excessively long branch lines will cause the voltage amplitude of the signal to decrease and the rising and falling edge times of the signal to become longer, thus affecting signal identification and communication quality.

2. Communication stability: The CAN bus uses a differential signal transmission method, which is very sensitive to changes in branch line length. Excessively long branch lines will change the reference point of the signal, resulting in a decrease in signal stability, thus affecting the communication stability of the entire CAN network.

3. Electromagnetic interference: As the length of the CAN bus branch increases, electromagnetic interference will gradually increase. Excessively long branch lines will introduce more external interference sources, thus affecting the electromagnetic compatibility of the signal and causing an increase in the bit error rate of the signal.

4. Load effect: Each node on the CAN bus has a certain load effect. A branch line that is too long will increase the load on the bus, thus affecting the communication speed and stability of the bus.

5. Cost issue: Excessively long branch lines will increase the difficulty and cost of wiring, as well as maintenance and debugging.

Therefore, in order to ensure the stability and reliability of CAN bus communication, the length of the CAN bus branch needs to be reasonably controlled. In specific applications, it is necessary to comprehensively consider the actual situation and select the appropriate branch line length and solution.

Five Solutions to Too Long Branch Lines in CAN Networks

1,Use standard "hand-in-hand" line topology

Linear topology is the topology recommended by the high-speed CAN physical layer specification in IOS-11898-2. Use the "hand-in-hand" wiring rule as shown below to keep the branch length to a minimum. To ensure continuous impedance, the transceiver should be placed close to the interface to reduce the length of the branch stub. The distance between the transceiver and the interface should be controlled within 10cm.

2. Limit the longest branch length according to the baud rate used by the bus.

The way to solve the problem at the root of CAN network layout is to reduce the branch length of CAN nodes, thereby reducing signal reflection and ensuring the stability of the bit width.

3. Add resistance at long nodes

A practical method is to add resistors at the ends of long branches to eliminate signal reflections.

4. Adopt transceiver front-end solution

5. Use network equipment such as bridges and repeaters

The simplest solution is to add a CAN repeater close to the bus end of the branch line that is too long to improve the problem of the branch line being too long.


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