CAN Troubleshooting Guide
7. CAN Troubleshooting Guide
The CAN Troubleshooting Guide is a guide to find and eliminate the most frequent hardware-error
causes in the wiring of CAN-networks.
Figure. 8:
Simplified diagram of a CAN network
7.1 Termination
The termination is used to match impedance of a node to the impedance of the transmission line
being used. When impedance is mismatched, the transmitted signal is not completely absorbed by
the load and a portion is reflected back into the transmission line. If the source, transmission line
and load impedance are equal these reflections are eliminated. This test measures the series
resistance of the CAN data pair conductors and the attached terminating resistors.
To test it, please
1.
Turn off all power supplies of the attached CAN nodes.
2.
Measure the DC resistance between CAN_H and CAN_L at the ends of
the network
(see figure above) and at the centre of the network (if the network cable
consists of more than one line section).
The measured value should be between 50 Ω and 70 Ω. The measured value should be nearly the
same at each point of the network.
If the value is below 50 Ω, please make sure that:
- there is no short circuit between CAN_H and CAN_L wiring
- there are not more than two terminating resistors
- the nodes do not have faulty transceivers.
If the value is higher than 70 Ω, please make sure that:
- there are no open circuits in CAN_H or CAN_L wiring
- your bus system has two terminating resistors (one at each end) and that they are 120 Ω
each.
CAN-USB/2
Hardware Manual • Doc. No.: C.2066.21 / Rev. 1.4
Page 21 of 25
1
120
CAN_H
CAN_GND
CAN_L
CAN_L
CAN_H
CAN_GND
V
120
2
3
1
V
1
