AppendixC CablingRequirements for PXI-8461 High-Speed CAN
PXI-846xand NI-CAN for Windows NT C-6 ni.com
Number of Devices
The maximumnumber o f devicesthat you can connect to a CAN port
dependson the electrical characteristics of the devices on the network. If all
ofthe devices meet the requirements of ISO 11898, at least 30 devices may
beconnected to the bus. Higher numbers of devices may be connected if the
electricalcharacteristics of the devices do not degrade signal quality below
ISO 11898 signal levelspecifications. If all of the devices on the network
meetthe DeviceNetspecifications, 64 devices may be connected to the
network.
Cable Termination
The pair of signal wires (CAN_H and CAN_L) constitutes a transmission
line.If the transmissionline is not terminated, each signal change on the
linecauses reflections thatmay cause communication failures.
Because communication flowsboth ways on the CAN bus, CAN requires
that both ends of the cable be terminated. However,this requirement does
notmean that every device should have a termination resistor.If multiple
devicesare placed along the cable, only the devices on the ends of the cable
shouldhave termination resistors. See Figure C-5for an example of where
termination resistorsshould be placed in a systemwith more than two
devices.
FigureC-5. Termination Resistor Placement
Theterminati on resistors on a cable should match the nominal impedance
of the cable. ISO 11898 requires a cable with a nominal impedance of
120 Ω;therefore, a120 Ωresistor should be used at each end of the cable.
Eachtermination resistor should be capable of dissipating at least 0.25 W
ofp ower.
CAN
Device
CAN
Device
CAN
Device
CAN
Device
CAN_L
CAN_H