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

Hello,

I know couple of companies using CAN as a backplane bus. There are no general problems with
such.

The termination is an approach to let the transmission line to see it's line impedance also in
the endpoint of the bus in order to avoid reflections caused by the signal edges.

Steve Corrigan has been published the best approach to understand the minimum distance between
the devices. He made his analysis from impedance point of view, which really makes sense. Thus,
there is no absolute good and bad inter-node distances. Instead, the distance depends on the
entire network structure. Briefly written, if the devices are evenly distributed over the bus
line, their effect on the line impedance is also distributed evenly. But if you have a group of
devices connected into the very same point of the bus, the net impedance of the line together
with the CANHI-to-CANLO -impedance caused by the devices introduce line impedance deviation in
the connection point, causing reflections.

I have met this issue in all too many real applications, where designers have minimized the number
of nodes and tried to install those few nodes in as few tiny groups as possible. And BAM! One of
the important changes has been to increase cable lengths inside the groups in order to "spread
the node impedances" over longer distance of the trunk cable. In CAN high-speed physical layer,
the best signal quality is achieved when the devices are evenly distributed over the network. And
please remind, the more devices are in a network, the smaller residual error probability will be
achieved. But don't exceed fan-out of the transceiver, which typically varies around 100 nodes.

Best regards,

-H

Heikki Saha, CTO
  M.Sc. Automation
  Dr.Tech. Electronics
TK Engineering Oy
Mail address:  P.O. box 810, FIN-65101 VAASA
Visit address: Yrittäjänkatu 17, FIN-65380 VAASA
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http://www.canopen.fi/

----- Original Message -----
From: "Heinz-Jürgen Oertel" <[email protected]>
To: "CANLIST" <[email protected]>
Sent: Sunday, December 7, 2014 1:26:31 PM
Subject: Re: [CANLIST] Why do ISO-11898-2 and ARINC 825 never mention split termination?

Hi,

I know at least two applications of former CANopen customers using CAN/CANopen as back plane bus. But I don't know how they solved the termination.

I'm also not a cable network expert. But I think because you more or less have no cable with a an expansion and wave impedance, it could be enough to have 60 Ohms to satisfy the the load the transmitters need. I might be wrong, sure.  

 Best regards
  best success
 
 Heinz



Am Freitag, 5. Dezember 2014, 22:36:12 schrieb Corey, Richard:
> 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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-- 
mit freundlichen Grüßen
       Heinz-Jürgen Oertel
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