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It is well known, in the CAN community at least, that every CAN and CAN FD network should be terminated with a 120 Ohm resistor at each end of the bus. More information can be found on ‘Why use a resistor?’ and how to maximise signal quality in the whitepaper ‘Using termination to ensure recessive bit transmission’, but what is rarely discussed is how to test if it works correctly. The good news is that, with the right equipment, this is relatively simple!
To check your network’s termination, disconnect the CAN interface’s D-sub 9 pin from the network and measure resistance through the cable by placing a digital multimeter / ohmmeter between pin 2 and 7. Make sure any CAN nodes e.g. a motor controller, are still attached but powered down. If your termination is correct, you should read approximately 60 Ohms (two 120 Ohm resistors in parallel produce a resistance of 60 Ohms). If you read a different value, continue testing as follows:
1. Check that there is 10 kOhm between CANH and the ground, including the power supply. This test is best done without any CAN-communication on the CAN-bus, and will highlight any short cuts between CANH and the surrounding area.
2. Check that there is 10 kOhm between CANL and the ground, including the power supply. Again, do this without communication on the CAN bus.
3. If there’s still a problem, connect an oscilloscope to the CANH signal, which should show a signal level at 2.5 Volt during the idle phase, with the voltage increasing to 4 Volt for the dominant bits and dropping back to 2.5 Volt for the recessive bits. The shape of the bits should be nice and square, without any ringing on the rising and falling edges.
a. If the idle level differs from 2.5 Volt, it could result from bad common ground.
b. If CANH is fixed at a certain level, it may be due to a shortcut to other electrical circuits.
c. If the units do not have a common ground, a voltage offset will result that is proportional to the ground offset.
d. If there is ground noise, it could be impossible to do this measurement. A differential probe is necessary to obtain the voltage on CANH relative to CANL. Please note, CAN could work fine even if the ground noise is more than 40 Volt peak to peak with a frequency from 0 to 500 MHz.
e. All ringing is caused by impedance mismatch, of which the major cause is a drop-line from the main CAN bus to the CAN units. Generally, the unit impedance is 100 kOhm, resulting in a 100% reflection of the energy back to the main CAN bus, delayed by the propagation delay in the drop-line. This energy will return back to the main CAN bus with 0 phase shift. Alternatively, the EMC filter could cause a phase-shift in the reflected energy.
4. The next step is to connect an oscilloscope to the CANL signal, which should also show a signal level of 2.5 Volt during the idle phase, with the voltage decreasing to 1 Volt for the dominant bits and rising back to 2.5 Volt for the recessive bits. The shape of the bits should be nice and square without any ringing on the rising and falling edges. All other results are similar in cause to CANH, as listed above.
5. If there is a lot of ground noise, it will be necessary to use a differential probe to display the signal difference between CANH and CANL. The oscilloscope should show 0 Volt during the idle phase, with a voltage increase to 1 Volt for the dominant bits, falling back to 0 Volt for the recessive bits. The shape of the bits should be nice and square without any ringing on the rising and falling edges. Note that if either CANL or CANH is shorted, it will result in a lower signal level on the dominant amplitude.
The compact Kvaser D-sub 9 pin 120 Ohm termination adapter has a 9-pin male D-sub connector at one end and a 9-pin female D-sub socket at the other, with a 120 ohm terminating resistor between pins 2 (CAN High) and 7 (CAN Low). Compatible with regular high speed ISO 11898-2 CAN and CAN FD, the device slots easily onto the D-sub of any CAN interface such as the Kvaser Leaf, Blackbird, Memorator, Ethercan, Hybrid or USBcan range. It can also be used with any Kvaser PCI Express, PCI or PCIe boards.