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I am thinking of making a CAN "hub" PCB. The PCB will look like exactly like how can buses are depicted in diagrams and there will be connectors for each of the nodes (ESP32s with a transceiver).

Will it be fine for the CAN high and CAN low lines to PCB traces to be on different layers of the PCB? Is it okat that they are non twisted?

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  • \$\begingroup\$ How long wires you expect from hub to nodes? Will each link be a separate CAN segment, or will it all be a single linear bus in a star shape? \$\endgroup\$
    – Justme
    Dec 5, 2023 at 5:09
  • \$\begingroup\$ "The PCB will look like exactly like how can buses are depicted in diagrams" What does that even mean? There's "star network" topology with a master in the middle and there's "multidrop" topology which is just a long line of nodes with no master. CAN requires the latter form. \$\endgroup\$
    – Lundin
    Dec 5, 2023 at 8:40
  • \$\begingroup\$ It will have a line topology, like CAN is supposed to. The hub will have connectors for nodes (ESP32s with transceivers) to connect to this line topology. \$\endgroup\$
    – cr1tical1
    Dec 6, 2023 at 12:26

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The CAN bus wires can be routed in parallel. Twisted and/or shielded cables are not necessary, but may be recommended depending on EMC requirements.

The main problem with your CAN bus "hub" is that you will most likely introduce an excess of unterminated cable stubs. The stubs should be as short as possible. At 1 Mbps, for example, the stub length shouldn't be longer than 30 cm (see ISO11898-2).

AN96116 (chapter 6.5 Unterminated Cable Drop Length) specifies a rule of thumb for the stub length:

\$L_U < \frac{t_{PROPSEG}}{50\times t_p}\$


Edit

i dont quite understand what you mean by cable stubs. Do you mean the "ports" for the nodes on my pcb that arent connected to anything?

No. Stubs are the unterminated drop cables that connect nodes to the CAN bus (highlighted in red). They are unavoidable. But a CAN bus "hub" probably doesn't help to alleviate the problems that stubs can cause.

enter image description here

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  • \$\begingroup\$ Im sorry i dont quite understand what you mean by cable stubs. Do you mean the "ports" for the nodes on my pcb that arent connected to anything? \$\endgroup\$
    – cr1tical1
    Dec 6, 2023 at 12:33
  • \$\begingroup\$ Thank you, I understand now. I will try to find a good compromise for CAN bps which I believe I can change with sandeepmistrys CAN library (CAN.begin?) since i dont think 30cm is quite enough for my case. \$\endgroup\$
    – cr1tical1
    Dec 7, 2023 at 0:40
  • \$\begingroup\$ Do note that the 30cm constraint only applies for a baudrate of 1 Mbps. The constraints for lower baudrates aren't as strict. \$\endgroup\$
    – Velvet
    Dec 7, 2023 at 7:03

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