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

Hello Rick,

The standards more general, that's why there is only "a resistor" written. Split-terminator behaves
almost as a single resistor at the frequencies close to bit-rate. It's advanced behavior is to filter
higher-frequency components and also improve signal symmetry between CANHI and CANLO.

Split-terminator offers some clear benefits, but unfortunately due to "cost-aware design culture", are
too often restored back to the plain resistor. If you by a cheap product, you'll get a cheap product,
which works as a cheap product... All too many engineers are considering only the purchase cost,
forgetting failure costs and costs caused by various service, troubleshooting and unpredictable
modification actions, caused by wrong design decisions.

Best regards,

-H

Heikki Saha, CTO
  M.Sc. Automation
  Dr.Tech. Electronics
TK Engineering Oy
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----- Original Message -----
From: "Richard Corey" <[email protected]>
To: "[email protected]" <[email protected]>
Sent: Saturday, December 6, 2014 12:36:12 AM
Subject: [CANLIST] Why do ISO-11898-2 and ARINC 825 never mention split termination?


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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