Hi all,
I’d like a bit of advice on the pre-populated 120 Ohm resistance on the reTerminal.
From what I understand, these resistances must be present only at each end of the bus.
For point-to-point CAN, the pre-populated 120 Ohm resistance is nice because you don’t need to add one yourself.
However, for multipoint (“real”) CAN bus, having each terminal adding 120 Ohm means that the total resistance between CAN-H and CAN-L will be too low.
Now the specifics:
- Is my understanding of the problem correct?
- If so, how do I remove the 2x62 Ohm resistances that are pre-populated between CAN-H and CAN-L in the reTerminal-DM?
- Doesn’t that kill the GND_ISO (not that I need one)?
Thanks for your input,
-J.
H i there,
So, Your CAN understanding is basically correct: for a normal high-speed CAN bus, you want one termination at each physical end only.
The reTerminal DM has a default built-in 120 Ω termination, and if the board is not at the end of the bus, that termination should not be active. The likely reason you are seeing 2 × 62 Ω is that the board is using split termination, which is a normal CAN technique and still represents the end termination.
Removing those parts only makes sense if this node is not an end node. It should not by itself “remove” GND_ISO, since GND_ISO is the isolated CAN-side reference, not the termination resistor itself.
Seeed explicitly describes GND_ISO as the external ground reference for the isolated CAN side. HERE
Seeed’s public wiki confirms the default termination, but I did not find a public board-level instruction that says “remove resistor X and Y.”
HTH
GL 
PJ 
bottom line only remove if terminal is in the middle of the bus of devices.
it may the best would be at the farthest end, maybe disconnect the last sensor if possible and put terminal there. 