Determining the Necessary Termination Resistance for the Board

Unlike High-Speed CAN, Low-Speed CAN requires termination at the Low-Speed CAN transceiver instead of on the cable. The termination requires two resistors: RTH for AN_H and RTL for CAN_L. This configuration allows the NXP fault-tolerant CAN transceiver to detect and recover from bus faults. You can use the NI-XNET Low-Speed/Fault-Tolerant CAN hardware to connect to a Low-Speed CAN network having from two to 32 nodes as specified by NXP (including the port on the CAN Low-Speed/Fault-Tolerant interface). You also can use the Low-Speed/Fault-Tolerant interface to communicate with individual Low-Speed CAN devices. It is important to determine the overall termination of the existing network, or the individual device termination, before connecting it to a Low-Speed/Fault-Tolerant port.

NXP recommends an overall RTH and RTL termination of 100 Ω to 500 Ω (each) for a properly terminated low-speed network. You can determine the overall network termination as follows:

NXP also recommends an individual device RTH and RTL termination of 500 Ω to –16 KΩ. After determining the existing network or device termination, you can use the following formula to indicate which nearest value the termination property needs to be set to produce the proper overall RTH and RTL termination of 100 Ω to 500 Ω upon connection of the card:

where R_RTH overall should be 100 Ω to 500 Ω.

NI-XNET Low-Speed/Fault-Tolerant CAN hardware features software selectable bus termination resistors, allowing you to adjust the overall network termination through an API call. In general, if the existing network has an overall network termination of 125 Ω or less, you should select the 5 KΩ option for your NI-XNET device. For existing overall network termination above 125 Ω, you should select the 1 KΩ termination option for your NI-XNET device.