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

Hello Corey,

when We started with CANopen early 2000, we were using split terminators 
also for the mentioned reason.

Problem was, that to keep the M12-terminator in a decent size, it needed a 
3D-soldering assy inside the mould and and we saw too many failures in 
them and decided to use a simple single 
resistor instead.

Have You ever tried to debug a system (of some 20-30 nodes and 50m cable) 
where a split terminator fails (short circuit) on CAN frequenzies but 
works on DC, when temperature is getting below zero ?

Later we also moved to a 7W version of the resistor (which would make 
split version HUGE) because we managed once to create a small bonfire in a 
terminator due to wrong connection...
Wrong connections should naturally not exist, but when some screw 
terminals are needed along the bus, they are inevitable at some point.

isto




"Corey, Richard"




From:   "Corey, Richard" <[email protected]>
To:     "'[email protected]'" <[email protected]>
Date:   06.12.2014 00:37
Subject:        [CANLIST] Why do ISO-11898-2 and ARINC 825 never mention 
split termination?
Sent by:        [email protected]




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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