National Instruments NI-DNET - Manual (Page 67)

Appendix B Cabling Requirements
© National Instruments Corporation B-7 NI-DNET User Manual
Cable Termination
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 DeviceNet bus, DeviceNet
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 Figure B-7
for an example of where termination resistors should be placed in a system
with more than two devices.
Figure B-7. Termination Resistor Placement
The termination resistors on a cable should match the nominal impedance
of the cable. DeviceNet requires a cable with a nominal impedance of
120
Ω; therefore, a 120 Ω resistor should be used at each end of the cable.
Each termination resistor should each be capable of dissipating at least
0.25 W of power.
120 Ω 120 Ω
CAN_H
CAN_L
DeviceNet
Device
DeviceNet
Device
DeviceNet
Device
DeviceNet
Device