Solution
In order to ensure that reflection does not cause communication failure, the transmission line must be terminated. The method of terminating your CAN hardware varies depending on the physical layer of your hardware: High-Speed, Low-Speed, Single-Wire, or Software-Selectable.
High-Speed CAN
For High-Speed CAN, both ends of the pair of signal wires (CAN_H and CAN_L) must be terminated. This is because communication flows both ways on the CAN bus. CAN_L is pin 2, and CAN_H is pin 7 on the standard 9-Pin D-Sub connector. 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 ohms, and therefore 120 ohm resistors should be used for termination. If multiple devices are placed along the cable, only the devices on the ends of the cable need termination resistors. The diagram below gives an example of how to terminate a high-speed network.
Low-Speed CAN
For Low-Speed CAN, each device on the network needs a termination resistor for each data line: R(RTH) for CAN_H and R(RTL) for CAN_L. Unlike the High-Speed CAN termination, Low-Speed CAN requires termination to be on the transceiver rather than on the cable. The resistance of each resistor is calculated through several formulas, and this process is detailed on pages 4-10 through 4-12 of the NI-CAN manual. Here is a diagram detailing where the termination resistors should be placed on a low-speed network.
Note: If you are using a PCMCIA transceiver dongle, you will also need to use termination resistors for CAN_H and CAN_L. These termination resistors are not implemented internally. RTH and RTL appear on the outside of the transceiver dongle as shown in
Figure 4-6. PCMCIA-CAN/LS Cable of the
NI-CAN Hardware and Software manual.
Single-Wire CAN
The NI Single-Wire CAN hardware includes a built-in 9.09 kohm load resistor. This is all the required load resistance for the network, and no additional resistors are needed.
Software-Selectable CAN
The Software-Selectable CAN hardware can be configured to operate either as a High-Speed, Low-Speed, or Single-Wire interface. The required termination is dependent upon which of the physical layers it is configured to use.