1. What is CAN bus

CAN stands for Controller Area Network, which means area network controller. CAN bus is a serial data communication protocol developed by Bosch in Germany to solve the data exchange between many control and test instruments in current cars.

  1. Causes of problems with RS485 and CAN bus branch lines

Branch line problems are easily overlooked by users. When we are designing, many strange communication failures occur due to the large number or length of branch lines. When RS485 or CAN signals are transmitted on the bus branch lines, some echo signals will be generated, and the test signals will be deformed. Especially when the branch lines are too long or too many, the phenomenon is particularly serious. Common error branch lines are as follows.

  1. Transmission distance of branch line of RS485 and CAN bus

The transmission distance of RS485 and CAN bus is not only related to the twisted pair used, but also closely related to the baud rate of communication. The lower the baud rate, the longer the transmission distance. Conversely, the higher the baud rate, the shorter the transmission distance. In fact, the length of the branch line is also related to the baud rate of communication. For this reason, some DP plugs have appeared on the market to solve the problem of too long branch lines.

3.1 Solve the problem of too long branch lines

For field buses with DB9 interface form, such as CANOpen, PPI, MPI, PROFIBUS. As long as the investment of these connectors is used, the problem of too long branch lines can be solved. For some circuits with only RS485 and CAN interfaces, two lines can be connected to one terminal in parallel to achieve this.

  1. How to deal with the unavoidable long branch line

In display life, due to the on-site environment or its own reasons, some cable branches need to be connected to other places. At this time, we can install repeaters at the branches, connect a new bus through the repeaters, and the transmission distance meets the 4RS85 standard.

  1. Signal and ground shielding of RS485

RS485 does not have standard positive and negative (±) symbols in regulations. Our common representation is A (positive signal). B (negative signal). So we must connect normally when we usually connect. RS485 and CAN bus usually use shielded twisted pair. The shielding layer needs to be connected to the shell ground of each station. Generally, there are standards (SG, FG) on the shell. SG is the signal ground and FG is the shielding ground. When we use multiple products, we also need to use a wire to connect the SG signal to avoid potential generation, which may cause damage to RS485 or CAN devices.

  1. RS485 and CAN bus connection isolation

When we design circuits, we usually use optocouplers at the interface to isolate the RS485 or CAN interface from the internal circuit, so that there will be no electrical connection between the communication interfaces of each station, thereby generating a ground loop, which plays a good role in resisting common mode interference. Of course, we will also adopt some protection designs on the RS485 and CAN bus interfaces. Generally, we will use ceramic discharge tubes to absorb lightning surges for primary protection, and then use fuses such as TVS for secondary protection. Finally, we use TVS to clamp the signals and between the signals and the ground.