Fwd: Why do ISO-11898-2 and ARINC 825 never mention split termination?

Our document for the physical layer of OpenLCB (model railroad bus)
discusses this, and has some references:
http://www.openlcb.org/trunk/specs/CanPhysicalTN.pdf

David


On Fri, Dec 5, 2014 at 2:36 PM, Corey, Richard <[email protected]>
wrote:

>  ARINC 825 says:
>
> *3.2.3.1 Network Termination*
> A 120 Ω terminating resistor shall be installed on each end of a network
> segment for impedance adaptation and to avoid disturbing reflections.
>
> The ISO specification 11898-2 also refers to termination as “a” resistor.
>
> Why are single resistors suggested or required instead of split
> terminations (two 60 Ohm resistors plus a small capacitor)?
>
> I thought the split termination was usually preferable and rejected some
> high frequency noise from the bus.
>
>
>
> I see that minimum distances are required between the last node and a
> terminator (1.5 meters), and also between each node (0.5 meters).  I’ve
> seen that “minimum distance” suggestion elsewhere but seldom noticed any
> trouble with desktop setups where these distances are inches or less.  (My
> definition of “trouble” is any error frames at all, ever, except during
> power cycles or hot plugging.)
>
> We want to use CANbus to connect three adjacent cards inside a box, but it
> seems crazy to stuff 3.5 meters of cable inside a small box (and make the
> box big enough to fit!).  Does everyone who uses CANbus to connect adjacent
> cards run big loops of cable around and around the cards to meet this
> requirement?
>
> I don’t have any EE training, so all I understand about the reason for
> inter-node distance and node-to-terminator distance is to reduce the risk
> of “reflections”.  Would it be safe to ignore those rules in an unchanging
> network which will never have additional bus segments grafted onto it?
> Couldn’t we adjust the distances until all the wave forms are clean, with
> distances of a few inches at most, and then lock the geometry down and
> never vary it?
>
> Until I read about “minimum distance requirements”, I never worried about
> it in desktop setups.  I only got into trouble once.  I had a 1-inch CAN
> connection between two cards in a cage that “just didn’t work”.  I
> lengthened the loop and then the traces were clean and error frames went
> away, so I stopped worrying.  I forget where the terminators were in that
> rig (it was 10-12 years ago) but they were either within a few inches of
> the two nodes or missing altogether at first.
>
>
>
> Rick Corey  | Software Engineer | Crane Aerospace & Electronics | Desk
> Phone: 425-743-8469 | Cell: 425-345-9954
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