CAN termination jumper (120 Ω → 60 Ω)
I have a CAN circuit with two TCAN3414DR transceivers on the same PCB, each connected to a different MCU but sharing the same physical CAN bus. This bus then goes via harness to a BMS and a Slave controller.
I need selectable termination:
- 120 Ω normally (without jumper)
- 60 Ω when a jumper is shorted
I’m using (2 × 60.4 Ω) always connected (≈120 Ω), and a second pair (2 × 60.4 Ω) added in parallel via jumper J21 to get ≈60 Ω.
Questions:
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Is this a correct way to implement selectable 120 Ω / 60 Ω CAN termination using a jumper?
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Any signal-integrity or EMC issues with placing the second termination through a jumper?
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Is it OK that both CAN transceivers share the same choke, ESD network, and termination?
Thanks!
2 answers
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I'm assuming you want 60 Ω locally when the remote CAN bus is not connected, and 120 Ω when it is.
A CAN bus with two nodes on the same board isn't very fussy. Yes, what you propose should work fine.
It's more important that the bus look like a properly terminated transmission line when it is long. All the stubs on your board should be "short". That includes the lines to the jumper and resistors it switches in. A few 10s of mm is tolerable depending on the speed you are running the bus at. Lay out that parts carefully and keep all parts of the bus on the board short, and you should be OK.
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Is this a correct way to implement selectable 120 Ω / 60 Ω CAN termination using a jumper?
It is weird to place both terminators on the same board. While this might fix the problem of having an ideal 60Ω between CANH and CANL, it also means that there can be to protection against reflections in the other end of the bus. Which would be wrong for any bus length worth mentioning, unless this use-case scenario is just a minimal stub - then it will be OK.
Jumpers are generally horrible for rugged applications - for such, consider using some connector instead (like a 1x2 JST PH or whatever), where the mating part just shorts two pins on the wire side.
Any signal-integrity or EMC issues with placing the second termination through a jumper?
Nah it is not "high speed" but at most 1Mbps, so you can have any kind of connector there really.
Is it OK that both CAN transceivers share the same choke, ESD network, and termination?
It should be fine, far as I can tell. Though I don't get why you have both ESD diodes and 2 unknown TVS both to chassis.
More common for high integrity applications would be 2 rugged TVS to chassis close to the connector and then 2 smaller TVS like ESD diodes on the inside of the CMC towards signal ground. If you don't have high EMC requirements you can do without any of them and rely on the transceiver's built-in protection diodes.
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