RE: Minimum stub length

No it doesn’t matter.
There are two issues.  If the stubs become too long the network starts to look more like a star than a bus and you end up with reflections off the end of the stub.  You can't terminate every stub because that would lower the DC parallel resistance of the bus exceeding the drive capability of the bus drivers.
Steve Corrigan also wrote a paper on minimum distance between nodes on the bus.   This would also cause reflections that could impact the bits.  Somewhere  I have scope photos of that phenomena from the Olympic Rings CAN bus because we ran both 1Mbps and 500kbps and you can see that specific reflection on or around the ACK bits.  Our lights were closer than the 17" or so he specified.
The lower the bit rate the less these issues will affect you.   At lower bit rates you can sample 3x and a sample point at 75% tends to avoid the rounded corners of the rising and falling edges due to bus inductance and capacitance.  The reflections due to edges are still the same size but the bits are wider so they have less impact.
Just before he died Steve C. was planning on some testing with a length of the Thick Ethernet Cable we had left over from the rings project.  His idea was to determine how bending the cable affected the signal integrity.  Alas he never finished that.  He is missed by all of us.

John Dammeyer

From: [email protected] [mailto:[email protected]] On Behalf Of YAP
Sent: May-06-15 7:40 AM
To: [email protected]
Subject: [CANLIST] Minimum stub length

Hi all,
I have a system that runs on 125Kbps and I have always heard that one should try to keep stub length at a maximum of 0.3 meters (probably originate from late Steve Corrigan once in a time). But is there a minimum? And how critical is this at 125Kbps. If one goes down to 0,2 meters for instance. Does it mater at all?
Cheers
/Ake


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Ake Hedman
Grodans Paradis AB, http://www.grodansparadis.com

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