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I have a CAN bus at 125 kbit/s, and the wire length is less than 60" (1.5 m). The impedance on the network is 60 ohms.

I have a requirement to add a module that has its own 120 ohm terminating resistor as an option to my system. The CAN bus seems to work, but is there a problem with a 40 \$\Omega\$ impedance and if so, how can I test this?

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  • \$\begingroup\$ Can you remove the termination from the other nodes and use just the one that has the fixed 120 ohm? \$\endgroup\$ – AlmostDone May 15 '18 at 23:43
  • \$\begingroup\$ It is possible but because the additional node is optional it might be hard to police the impedance if the node is added or if the system is the standard set up. I would rather confirm the operation then the impedance could be between 40-60 ohms depending on nodes added. \$\endgroup\$ – user188950 May 16 '18 at 0:02
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CAN busses have controlled slew rate to minimize external EMI. Examine those edges, at the 8 µs (125 kbaud) bit rate. I'd expect about 1 µs.

The round trip time for 60" will be 20 nanoseconds or less. I'd expect that 20 nanosecond to be invisible on the 1 µs ramp times. Hence the edge fidelity is high.

Your remaining bit error rate risk is the built-in hysteresis of the CAN receivers.

Check the CAN bus specifications, examine the maximum allowed hysteresis, and verify the signal-swing when loaded with the additional 120 ohm load will still produce a signal swing at least 50% higher than the maximum hysteresis.

This 50% margin is to ensure some noise immunity.

You could assist/improve the noise immunity by heavily loading the differential bus with 1,000 pF capacitors. 40 ohms and 1,000 pF capacitors is 40 nanosecond TAU, and the edge ramp will not change shape with 1,000 pF in parallel.

Just some ideas for you to try.

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What you need to do is provide about 120 ohms at each end (physical end) of the bus. A single 60 ohms at a single point will not produce good results, although at 125k and 60 inches you may get away with it.

Adding another module with its own 120 ohms is not a good idea, although at 60 inches, again, you may get away with it.

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You need to place your module with the integrated 120 ohm resistor at the end of the bus, and you can get special connectors that terminate when nothing is plugged in. Or switch it with a relay. You will have a few milliseconds of errors. But the CAN bus should be able to handle that.

Or you must add a bus repeater.

From experience I know you can go up to four termination resistors on one bus, but more will cause immediate problems. It depends on the driving capability of the transceivers. But I have never applied this beyond my desk.

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