Figure 2. Connecting Both Ports of the NI 9852 to CAN Buses
Cable Specifications
Cables should meet the physical medium requirements specified in ISO 11898,
shown in the following table. Belden cable (3084A) meets all these requirements
and should be suitable for most applications.
Characteristic
Value
Length-related resistance
90 mΩ/m nominal
Length-related capacitance: CAN_L and ground, CAN_H and ground, CAN_L
and CAN_H
30 pF/m nominal
Table 3. ISO 11898 Specifications for Characteristics of a CAN_H and CAN_L Pair of Wires
Determining the Necessary Termination Resistance for the Board
Unlike High-Speed CAN, Low-Speed/Fault-Tolerant CAN requires termination at the
Low-Speed/Fault-Tolerant CAN transceiver instead of on the cable itself. Termination
requires two resistors, RTH for CAN_H and RTL for CAN_L. This configuration allows
the Philips Fault-Tolerant CAN transceiver to detect and recover from bus faults. It is
important to determine the overall termination of the existing network, or the
termination of the individual device, before connecting it to a Low-Speed/Fault-
Tolerant port. Philips recommends an overall RTH and RTL termination of 100 to
500 Ω (each) for a properly terminated low-speed network.
Termination on the low-speed/fault-tolerant ports of the NI 9852 is set through the
NI 985x software to either 1 kΩ or 5 kΩ.
Number of CAN Nodes
The maximum number of nodes depends on the electrical characteristics of the
nodes on the network. If all of the nodes meet the requirements of Low-Speed/
Fault-Tolerant CAN, up to 32 nodes may be connected to the bus.
© National Instruments
7
NI-9852 Getting Started
