Differential pairs CAN_H – CAN_L and RS485_A – RS485_B require matching. It is necessary to install terminal resistance equal to wave resistance of the differential pair at the cable end opposite to the interface module.
Wave resistance does not depend on the line length. It depends on:
- cable type;
- environment of the cable;
- amount and quality of the connections;
- cable twisting pitch;
- cable kink or similar mechanical defect.
Hence, the terminal resistance is selected individually for any particular network. Thus, if it is possible to measure wave resitance, one can easily select the terminal resistance. Otherwise one have to choose it by trial method (starting from 120 Ohm). Normally (if all the transmission line requirements are observed), the terminal resistance is within the range of 100-170 Ohm.
In some cases RS-485 and CAN 2.0 lines can operate without terminal resistances. However, in this case data quality and performance efficiency are not guaranteed due to the possibility of external factors impact.