CAN bus wiring test
The wiring test sequence raises questions from time to time. Here we explain why the sequence is as it is. There are also a few tips.
For the measurement the tester must be operated with 24V.
The sequence is as follows :
Preparation for step 1 :
- switch off the system (the bus to be measured must be voltage-free)
- The nodes must be disconnected from the bus (the input capacities could lead to errors).
- The terminations must be removed (they are measured later).
- Attach the tester at the beginning of the system.
Measurement 1 : short circuit test
The resistors of the wiring are measured. They should be infinite high because no wire is supposed to be connected to another in any way. For instance, there will be an error if you forgot to disconnect one of the termination resistors.
Short circuits and transfer resistance between wires sometimes occur in badly mounted plugs.
Preparation for step 2 :
- Connect a short circuit plug to the end of the bus. (In a short circuit plug, all pins are connected to each other.)
Measurement 2 : line-break test
The shorting plug connects all wires of the bus at the end of the cable. The loop impedance and the cable length are determined. Values can differ, then you should look for breaks and bad connections. If you did not disconnect some nodes, the determined length can vary. To measure short cables is not easy. The calculated length is more exact at longer cables.
Preparation for step 3 :
- disconnect the shorting plug
- connect the termination resistor at the end of the bus
Measurement 3 : termination resistor at the end of the bus
Considering the former measured loop resistance, the termination resistor at the end is measured.
Preparation for step 4 :
- connect the termination resistor at the beginning of the bus
Measurement 4 : termination resistor at the beginning of the bus
In the last step, the termination resistor at the beginning of the bus is measured by considering the former measurements of loop resistance and termination resistors at the end of the bus.
- Under some circumstances, the nodes can stay connected to the bus. The calculation of the bus length will be influenced. It is possible that you get errors, for instance when you also measure power supply wires and have a low impedance power supply connected.
- It might happen that you smooth away faults when you move many connectors. If you assume that you have loose connections, it can be better to measure with connected nodes first.
- Starting measurement 1 without doing the preparations will calculate a resistor of about 60 Ohm between CAN_H and CAN_L. This is of course rated as an error because in measurement 1 very high resistors are expected. But to quickly check the 2 termination resistors this procedure is good – even if the device rate this as an error.
It will be not so easy at long trunk lines with higher loop resistance. Here you should compare to former measurements.
- The CAN-Bus Tester 2 could be damaged if you ignore the warnings of the software and start the measurement process with a voltage applied. The CANtouch refuses the measurement as long as there is voltage on the supply lines.