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I am new to the forum and hope you could put me in the right direction.

When designing a J1939 harness what is the physical layer constraints, if multiple ECU's need to be connected? Is there any documentation to help with this.

I am aware of the following, so please correct or add if I missed anything:

  1. The Backbone of the J1939 bus should not be longer than 40m.
  2. 2x 120ohm resistor need to be used on the bus ends
  3. Stub length from the backbone to ECU should not be longer than 0,3m
  4. There is no official wiring colour that need to be used,although Green(CAN L) and Yellow (CAN H) seem to be the most common used?

If referring to Loop in Loop out each module, what is meant by this (Drawing perhaps)? I understand this is a better way of connecting if it is practically possible.

Thank you in advance for all the comments.

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  • \$\begingroup\$ What drawing are you talking about? What is "loop in loop out"? What do wire colors have to do with physical layer constraints? \$\endgroup\$ – Dmitry Grigoryev Mar 5 '19 at 13:31
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1 and 3) These are roughly the requirements of 1 Mbps. At lower baudrates you can use longer length and longer stubs. See recommendations from CAN in Automation for industry standard recommendations (the physical layer of CANopen and J1939 is the same).

4) Indeed, it isn't standardized. Here's my theory of why green and yellow are de-facto standard:
What is the color code for CAN Bus?

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  • \$\begingroup\$ Thanks for your reply. With the bus being J1939, I will keep to 250kbps. \$\endgroup\$ – SkilliePower Mar 6 '19 at 12:13

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