CAN Bus Termination
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.
Turn off all power supplies of the attached CAN nodes.
Measure the DC resistance between CAN_H and CAN_L at the middle and ends of the
measured value should be between 50 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.
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_H/CAN_L Voltage Verification
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
test for faulty transceivers, please
1. Turn on all supplies.
2. Stop all network communication.
3. Measure the DC voltage between CAN_H and GND
4. Measure the DC voltage between CAN_L and GND
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
a voltage higher than 4.0 V, please check for excessive voltage.
CAN Bus Ground Verification
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:
Disconnect the shield wire (Shield) from the ground.
Measure the DC resistance between Shield and ground.
Connect Shield wire to ground.
resistance should be higher than 1 M Ω. If it is lower, please
search for additional grounding of the shield wires.
CAN Transceiver Resistance Test
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:
Disconnect the node from the network. Leave the node unpowered.
Measure the DC resistance between CAN_H and CAN_GND.
Measure the DC resistance between CAN_L and CAN_GND.
the resistance should be between 1 M Ω and
4 M Ω or higher. If it is lower than this range, the CAN
transceiver is probably faulty.