Quote:
Originally Posted by Al Skierkiewicz
The wire and the terminations really do matter, sorry.
The twist of the wires or at least the constant spacing between wires sets the transmission line impedance. The CAN transceivers need the termination resistors to operate correctly. They help set the signalling voltage level and terminate the transmission line. Without them, the reflections at each end of the line allow errors to build up. This is an effect of the length of the line. The longer the line, the worse the effect. Also, the CAN buss should never be parallel wired. In short systems it may seem to work but, as is typical, this system will fail you when you need it the most.
The twisted wiring allows both wires to be exposed to external noise. Since the intruding noise is in phase as it passes through the wire, the twist will cause the 'in phase' noise to be cancelled at the source. Any noise that remains will also be in phase and that will be cancelled at the differential input to the transceiver chip. The twist of 2-3 turns per inch will produce a shunt capacitance of ~20-30pf per foot.
Murphy is always lurking...
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Given the length of wiring in FRC robots, reflections are not an issue. Large industrial systems are another matter.
As for twisting and external noise, running the wires together is what allows it to be robust and to cancel out the noise with the differential receiver, not the twisting. The twisting creates an inductance in the line, running the wire in parallel (twisted or not) creates capacitance between the two. The inductance added by the twisting helps to cancel this out.