The role of the terminal resistor of BMW X5

Q: When measuring the CAN network cable of a BMW X5 test car, it was found that there was a 120Ω resistor on one side of the terminal. After parallel connection, the line resistance was 60Ω. Why do you need a terminal resistor?
A: The terminal resistor is to eliminate the signal reflection in the communication cable. During the communication process, there are two reasons for signal reflection: impedance discontinuity and impedance mismatch. In the case of impedance discontinuity, the signal suddenly encounters a small or even no impedance of the cable at the end of the transmission line, and the signal will cause reflection at this place. The principle of this signal reflection is similar to the reflection caused by light entering another medium from one medium. As for the method of eliminating this reflection, since the signal transmission on the cable is bidirectional, a terminal resistor of the same size as the characteristic impedance of the cable needs to be connected at the end of the cable to make the impedance of the cable continuous.
In order to improve the topological capability of the network node, the CAN bus needs to be connected to a 120Ω terminal resistor at both ends to suppress reflection. It plays a very important role in matching the bus impedance. If this resistor is ignored, the anti-interference and reliability of digital communication will be greatly reduced, and even communication will be impossible.