Pin
Signal
4
NC
5
SHLD
6
COM
7
CAN_H
8
NC
9
V
SUP
Table 1. Pin Assignments for the NI-9881
CAN Bus Topology and Termination
A CAN bus consists of two or more CAN nodes cabled together. The CAN_H and
CAN_L pins of each node are connected to the main CAN bus cable through a short
connection known as a “stub.” 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 bus causes reflections that may cause communication errors.
Because the CAN bus is bidirectional, both ends of the cable must be terminated.
However, this requirement does not mean that every node on the bus should have a
termination resistor; only the two nodes at the far end of the cable should have
termination resistors.
The following figure shows a simplified diagram of a CAN bus with multiple CAN
nodes and proper termination resistor (R
t
) locations.
Figure 2. CAN Bus Topology and Termination Resistor Locations
Connecting a CAN Bus to the NI-9881
You can connect the NI-9881 port to any location on a CAN bus. The following figure
shows one example of connecting the NI-9881 directly to one CAN node.
ni.com
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NI-9881 Getting Started
