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CAN bus always needs a resistor to shunt it (termination)?
SparkyFX:
--- Quote from: rvalente on August 22, 2020, 01:46:37 pm ---Check this article
https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/07/14/the-importance-of-termination-networks-in-can-transceivers
--- End quote ---
I don't get it, isn't CAN supposed to be immune against common mode noise, given it is a differential signal?
AndyC_772:
"Immune", up to a point. Any transceiver will have a finite CMRR and a finite range of DC values over which it's able to perform the differential subtraction and recover the signal. If either wire goes outside the accepted range, the receiver can no longer cope, and the signal becomes corrupted.
To achieve a signal which is truly immune to CM noise - and I'll well aware that pedants will jump at the chance to pipe up at this point - you'd need transformer isolation. Transformers have really excellent CMRR and no real limitation on the CM voltage range they can support, right up to the dielectric strength of the insulation.
SparkyFX:
--- Quote from: AndyC_772 on August 23, 2020, 03:19:25 pm ---you'd need transformer isolation
--- End quote ---
Sure, as it is done with Ethernet. But it won't play well with recessive/dominant bus states and their error detection or requires separate RX/TX and subsequently hubs/switches that establish the required network topology. Makes everything a lot more complicated.
T3sl4co1l:
Doesn't have to be on the medium side. Isolated RS-485 and CAN transceivers are a common bit of industrial kit. :) There, typically the power is transformer isolated, while the signals are optoisolated.
Of course the question follows -- how isolated is it really, and at what frequencies? CMRR might be very high at DC, sure; DC is trivial. How about at 1MHz, for equipment at the base of an AM transmitter? Or about step or impulse response (with bandwidth beyond 100MHz), for use in a switching inverter, or high voltage spark generator?
Tim
rvalente:
If anyone want's do have a look inside a Industrial Equipment, this is the place:
http://s7detali.narod.ru/
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