How to measure characteristic impedance
The characteristic impedance of the cable describes the working characteristics of the cable at high frequencies. The multi-purpose uses DC current to measure the resistance, so the multi-meter cannot be used to measure the impedance of the cable. Professional equipment can be used to measure a cable in the case of an open circuit at the far end. Impedance Zoc, and then measure the impedance Zsc in the case of a remote short circuit, and then use the opening of the product of the two to be the characteristic impedance of the wire.
Impedance matching
If there is a mismatch between the source output impedance, the characteristic impedance of the cable and the load input impedance, there will be reflections. When these reflected waves hit the signal generator (source), they are re-transmitted and mixed with the normal signal being transmitted, which is difficult to distinguish Which are the original signals and those are the reflected waves again, so for the high-speed CAN bus, the characteristic impedance of the cable is required to be 120 ohms.