Why CAN bus signals should be isolated

When you use CAN or RS-485 bus for debugging, have you ever encountered occasional communication errors? Or the data cannot be received? A bus that has been in normal use suddenly has a wide range of errors, or the node is damaged? Are you still at a loss for these problems? Using bus isolation may help you solve the problem easily. In the actual bus application, have you ever encountered the following problems:

1. Equipment and personal safety - potential high voltage hazard

The use environment of CAN and RS-485 bus is very complex, and there will be high voltage in some harsh use occasions. It is very easy to generate electric shock hazard, endangering personal or equipment safety.

2. The remote end cannot receive data - the ground potential difference exists

In many practical applications, the communication distance can reach several thousand meters, and the distance between nodes is very long. Designers often directly connect the reference ground of each node to the local ground as the return ground of the signal. It seems normal and reliable, but there are great hidden dangers! Even if the system is debugged normally, various problems may occur after a period of use.

The problem that is often ignored is that there may be a large potential difference between two nodes!!! The actual earth is not an ideal "0" potential. The earth is also a conductor and has impedance. When a large current flows through the earth, there will also be a potential difference between the two ends of the earth. As shown in Figure 1.

If the communication nodes far away are directly connected to their respective local earth, the ground potential difference will be superimposed on the output terminal of the bus transmitter in the form of common mode voltage. The superimposed signal may be far beyond the common mode input voltage range that the receiver can bear, so that the signal can not be received normally, and the transceiver will be damaged seriously. The common mode input range of common CAN and RS-485 transceivers is small. For example, the SN65HVD251 and SP3085 transceivers only support the - 7~+12V common mode input range. The ground flows through the large current injected by various large equipment, and the resulting ground potential difference can be as high as several volts, tens of volts or even hundreds of volts, far beyond the voltage range that the transceiver can withstand.

3. No sign of data error, or component damage - ground loop impact

Since the earth between the nodes has potential difference, why not connect the earth of the two nodes directly with a wire? absolutely wrong! This will only make the situation more serious. This long wire will form a huge ground loop with the earth!

I believe you know from your school days that a closed coil will generate current in a changing magnetic field. 50Hz AC power lines, large motors, etc. are all sources of AC magnetic fields. If the bus is close to or passes through these places, the ground loop will generate currents up to several amperes or even hundreds of amperes. The common mode voltage generated by the current flowing through the ground loop will affect the normal communication of the bus.

In addition to stable magnetic field sources, transient disturbances such as surges, lightning strikes and high-frequency noise of some power lines may be picked up by this giant "ring antenna" and cause communication abnormalities.