CAN bus wiring test
The sequence of the bus wiring test occasionally raises questions. We want to explain here, why the sequence is how it is. Beside that there are some hints.
The sequence is as following :
Preparation for step 1 :
- stop and switch off your plant
- disconnect the nodes from the bus
- disconnect or switch off termination resistors
- connect tester to the beginning of the bus
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 the supplied shortening plug at the end of the bus
(If you don’t have an original shorting plug : You can simply make your own shorting plug by prepare a standard plug with shorting bridges from every pin to every pin.)
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 or switch on the termination resitor 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 or switch on the termination resitor 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 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 error.
It will be not so easy at long trunk lines with higher loop resistance. Here you should compare to former measurements.