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Appendix C Cabling Requirements for PXI-8460 Low-Speed CAN
© National Instruments Corporation C-5 PXI-8461 or PXI-8460 and NI-CAN for Windows NT
Low-Speed Termination
Every device on the low-speed CAN network requires a termination
resistor for each CAN data line: R
RTH
for CAN_H and R
RTL
for CAN_L.
Figure C-4 shows termination resistor placement in a low-speed CAN
network.
Figure C-4.
Termination Resistor Placement for Low-Speed CAN
The following sections explain how to determine the correct resistor values
for your PXI-8460, and how to replace those resistors, if necessary.
Determining the Necessary Termination Resistance for Your Board
Unlike high-speed CAN, low-speed CAN requires termination at the
low-speed CAN transceiver instead of on the cable. The termination
requires one resistor for each CAN line. This configuration allows the
Philips fault-tolerant CAN transceiver to detect any of seven network
faults. You can use your PXI-8460 to connect to a low-speed CAN network
having from two to 32 nodes as specified by Philips (including the port on
the PXI-8460 as a node). You can also use the PXI-8460 to communicate
with individual low-speed CAN devices. It is important to determine the
overall termination of your existing network, or the termination of your
individual device, before connecting it to a PXI-8460 port. Philips
recommends an overall RTH and RTL termination of 100 to 500 Ω (each)
for a properly terminated low-speed network. The overall network
termination may be determined as follows:
Low-speed
CAN Device
RTL CAN_L RTH CAN_H
Low-speed
CAN Device
RTL CAN_L RTH CAN_H
Low-speed
CAN Device
RTL CAN_L RTH CAN_H
CAN_H
CAN_L
1
R
RTH overall
†
--------------------------
1
R
RTH node 1
------------------------
1
R
RTH node 2
------------------------
1
R
RTH node 3
------------------------
1
R
RTH or node n...
--------------------------------+++=