Well, kinda/sorta, as the bus is pseudo-differential. And there is no galvanic 
isolation that the Ethernet would probably have, so no transformer, because is 
supposed to be a DC-coupled system. Hence the third wire is the 'ground' wire. 
The details and specific requirements will be in the particular bus controller 
IC data sheet. 

As typical, the term 'ground' is abused and should be indicated as the 
equipotential used for signal (CH/CL) reference, or some such exotic phrase. 
CAN is supposed to be tough - must work for a signal wire short to either 12V 
or ground.


-----Original Message-----
From: Ken Javor [mailto:ken.ja...@emccompliance.com] 
Sent: Tuesday, June 12, 2018 4:38 PM
Subject: Re: [PSES] Automotive EMC question

The bus is differential and I don't see transformer coupling to it, so that
means the interface to the bus is differential as well, right?  I don't
understand in that context what is meant by grounding, other than the

Ken Javor
Phone: (256) 650-5261

> From: Brian O'Connell <oconne...@tamuracorp.com>
> Reply-To: Brian O'Connell <oconne...@tamuracorp.com>
> Date: Tue, 12 Jun 2018 22:36:53 +0000
> Conversation: [PSES] Automotive EMC question
> Subject: Re: [PSES] Automotive EMC question
> CAN bus assumes a 'standard' 120 ohm termination for the ends of each CANL/H
> twisted pair. The ISO11898-x series talks about physical layer stuff, to
> include splitting the termination with caps (but have had problems with that).
> LT or TI or ??? has some app notes on this subject. It emphasized that there
> must be only one grounding path for the nodes on the bus.
> Brian
> From: John Woodgate [mailto:j...@woodjohn.uk]
> Sent: Tuesday, June 12, 2018 2:53 PM
> Subject: Re: [PSES] Automotive EMC question
> I hope it also at least recommends that the grounding is at the sending end,
> so that the cable capacitance is charged from the low-impedance source.  I
> guess that, e.g. in the auto environment, the risk of large shield currents is
> too great to allow routine grounding at both ends. But I suppose that
> grounding via a capacitor at the receiving end is not banned. If possible,
> this capacitor should be of the lowest possible inductance, which is not
> difficult with SMD, and if several capacitors are disposed radially around the
> end of the shield and grounded at their outer ends on a metal ring, the
> grounding should be good up to at least 1 GHz.
> John Woodgate OOO-Own Opinions Only
> J M Woodgate and Associates www.woodjohn.uk
> Rayleigh, Essex UK
> On 2018-06-12 22:41, Ken Javor wrote:
> The CAN bus spec says that shield(s) are to be grounded at one end only. How
> does this work vs. meeting stringent rf RE and RS requirements at frequencies
> where cables are electrically long?
> Thank you,
> Ken Javor
> Phone: (256) 650-5261

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