CAN-Bus Troubleshooting Guide
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1. CAN-Bus Troubleshooting Guide
The CAN-Bus Troubleshooting
Guide is a guide to find and eliminate the most frequent hardware-error causes
in the wiring of CAN-networks.
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:
off all power supplies of the attached CAN nodes.
the DC resistance between CAN_H and CAN_L at the middle and ends of the network
(1) (see figure above).
The measured value should
be between 50 and 70 O
The measured value should be nearly the same at each point of the network.
If the value is below 50 O
, please make sure that:
is no short circuit between CAN_H and CAN_L wiring
are not more than two terminating resistors
nodes do not have faulty transceivers.
If the value is higher than
, please make sure that:
are no open circuits in CAN_H or CAN_L wiring
bus system has two terminating resistors (one at each end) and that they
are 120 O
1.2 CAN_H/CAN_L Voltage
Each node contains a CAN
transceiver that outputs differential signals. When the network communication
is idle the CAN_H and CAN_L voltages are approximately 2.5 volts. Faulty transceivers
can cause the idle voltages to vary and disrupt network communication.
To test for faulty
on all supplies.
all network communication.
the DC voltage between CAN_H and GND (2)
(see figure above).
the DC voltage between CAN_L and GND (3) (see figure above).
Normally the voltage should
be between 2.0 V and 4.0 V. If it is lower than 2.0 V
or higher than 4.0 V, it is possible that one or more nodes have faulty transceivers.
For a voltage lower than 2.0 V please check CAN_H and CAN_L conductors for continuity.
For a voltage higher than 4.0 V, please check for excessive voltage.
the node with a faulty transceiver please test the CAN transceiver resistance
(see section 1.4).
The shield of the CAN
network has to be grounded at only one location. This test will indicate if the
shielding is grounded in several places. To test it, please:
the shield wire (Shield) from the ground.
the DC resistance between Shield and ground (see picture on the right hand).
Shield wire to ground.
The resistance should be
higher than 1 M O
it is lower, please search for additional grounding of the shield wires.
1.4 CAN Transceiver Resistance Test
CAN transceivers have one
circuit that controls CAN_H and another circuit that controls CAN_L.. Experience
has shown that electrical damage to one or both of the circuits may increase
the leakage current in these circuits.
To measure the current
leakage through the CAN circuits, please use an resistance measuring device and:
Normally the resistance
should be between 1 M W
and 4 M W or higher. If it is lower than this
range, the CAN transceiver is probably faulty.
the node from the network. Leave the node unpowered (4) (see figure below).
the DC resistance between CAN_H and CAN_GND (5) (see figure below).
the DC resistance between CAN_L and CAN_GND (6) (see figure below).
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