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Unread 20-02-2016, 00:20
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Re: Wiring Multiple PCMs

Quote:
Originally Posted by rich2202 View Post
True, but then you would have to remove the terminating resistor on the PDP (jumper), and then put on a terminating resistor on the last item in the link. I don't know what the specs are for the terminating resistor, hence put the PDP at the other end from the roborio.

On a can bus as short as the FRC robot, you could get away from best practices. But, they make it easy for a team to be compliant, so why not?
At high speed (1mbps), the recommended max bus length for CAN is 40 m. At this length, the actual electrical design is important and two 120 ohm resistors (one at each end) are a must.

CAN also allows for unterminated stubs from the bus, at 1mbps the recommended length without any electrical design thought is 0.3m. With careful design this can be increased, on a full-length bus.

A FRC CAN bus with only a roboRIO, two PCM's and a PDP could be below the recommended length for an unterminated stub. You definitely need at least one resistor to pull the bus to a recessive state, but the entire bus should be below the length where reflections are significant (~2m). It's also likely that the length after the PDP could be less than 0.3m and be within the spec for an unterminated stub.

tl;dr CAN is very tolerant of non-ideal bus layout if the wires are relatively short.
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