The pair of signal wires (CAN_H and CAN_L) constitutes a transmission line. If the transmission line is not terminated, each signal change on the line causes reflections that may cause communication failures.
Because communication flows both ways on the CAN bus, CAN requires that both ends of the cable be terminated. However, this requirement does not mean that every device should have a termination resistor. If multiple devices are placed along the cable, only the devices on the ends of the cable should have termination resistors. Refer to the following figure for an example of where termination resistors should be placed in a system with more than two devices.
The termination resistors on a cable should match the nominal impedance of the cable. ISO 11898 requires a cable with a nominal impedance of 120 Ω, so you should use a 120 Ω resistor at each end of the cable. Each termination resistor should be capable of dissipating 0.25 W of power.
NI high-speed CAN devices feature software-selectable bus termination for High-Speed CAN transceivers. On the USB-8502 (in high-speed mode) and on CAN HS/FD and CAN XS Transceiver Cables, you can enable 120 Ω termination resistors between CAN_H and CAN_L through an API call.