pictures of CAN communication?

Those aren't due to the resistor values. 80 Ohms is not perfect, but close enough. Your logic analyzer will quite certainly not be able to spot the the backlash from such slightly off termination, anyway.

Approximation is bad news. Being off by 20/500 = 4% between CAN controllers' t_q clocks means pretty much guaranteed failure on CAN bus. Depending on your bus timing parameters, you'll want to be off by less than 1% --- and the longer the bus, the closer the match has to be. If remotely possible design all CAN t_q lengths on the bus to be the same. It can be necessary to use a separate CAN controller with its own quartz to this end, if you don't have a choice for the CPU's main clock.

This really would be a good time for you to step back from the hardware for a while; sit down with the CAN spec or a CAN textbook, and learn your way through it. Don't just read it, work it. You could start with your oscilloscope trace and actually parse it by hand. Find out what each and every bit of that signal means. If you don't invest in this learning, you risk getting yourself even deeper into a situation where you're creating problems faster than you solve them.

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Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.
Reply to
Hans-Bernhard Broeker
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J1939 says the stub can be up to 1 meter. Is this true for an CAN buses?

Based on my non-EE reading of that spec He could be OK with 1 resistor on each his left most and right most connector. No?

Reply to
Neil

I'll assume you meant "for all CAN buses"...

Not really. J1939 is not about all CAN buses, but only about those up to 250 kb/s. For faster buses, 1 meter stubs would be trouble, especially if they're really just that: stubs, cables directly coupled the bus line, without any impedance compensation.

It's called CAN "bus", as opposed to a ring, star, tree or network, for a reason. Cabling it otherwise is possible, but you have to be quite careful to avoid nasty problems with signal reflections at the T joints.

Probably. His bus is so short that at 500 kb/s he may even get away with using only *one* terminator, anywhere on the bus. That's because his entire bus cabling combined appears to be shorter than 1 meter.

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Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.
Reply to
Hans-Bernhard Broeker

Thanks. After my current project my 1st J1939 is next. I hope I have picked up enough here to avoid the easy mistakes.

Reply to
Neil

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