CAN has strictly better noise rejection and robustness over the USB pair.
USB could stand to take some lessons from CAN. :)
Indeed, CAN is better than RS-422/485, which is already pretty robust, while having comparable signaling rates (and build-your-own protocol -- it's just the physical layer; whereas, CAN takes care of a huge load of things at once).
That said, USB (Full Speed, which is as described -- full logic level output) is faster, and can be *much, much* faster (High Speed, etc.).
Noise is part of the reason USB is point-to-point, and all of the reason it must be 100% shielded. USB has no input range beyond the rails, and PHYs, and recommended ESD diodes, all have diodes to clamp to the rails.
You might think, well, can't we improve USB's CMRR at AC, by adding ferrite beads or CMCs? Alas, no -- USB itself is unbalanced, that is, it uses unbalanced symbols (J and K). Also, because it is source terminated, you can hardly afford any loading on the load side, so you don't have anything to create a voltage divider with the CMC. (And, needless to say, ferrite beads will swallow high speed signals whole, so no good there.)
RS-485 and CAN are very easy to filter, because they normally expect to have low impedances around. Both are fully differential, so you can always split the termination resistor or filter capacitor, and get common mode "center tap" that can be loaded with whatever impedance you like. (Recommended is a
+V/2 divider, and some R+C damped filtering, so the CMC + C doesn't ring.)
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
"Warren" wrote in message
news:301a77c9-d8bf-440a-a6ff-75506103f538@googlegroups.com...
> Here is an academic question regarding the differences between CAN bus and
> USB differential _signals_. I'm not interested in the protocol
> differences-- just the signaling.
>
> CAN bus was created around 1983 and uses a differential signal alternating
> between V+/2 and V+ for CAN H, and V+/2 and CAN L for the other end. The
> recessive bit returns back to V+/2.
>
> USB, which came later, OTOH, pulls the signal D+ low, and D- high and
> otherwise returns to the idle state of their respective D+/D- idle levels.
>
> My question is about the differential signal design:
>
> Is there an advantage to the CAN bus approach with the recessive bit at
> V++/2, or is the USB differential signal with its wider differential
> voltage better for noise immunity or other reasons?
>
> Another way to phrase the question might be:
>
> if CAN were to be redesigned today with the lessons learned from USB
> (signalling), would they have still chosen the same V+/2 idle signal that
> they chose to use?
>
> Just curious,
> Warren