Re: Minimum stub length

Thanks, a good walk through.  It's pretty hard to come up with some general
guidlines for end customers of products like the ones we sell IMO.
"Impedance" will not be taken in as something understandable at all by our
customers. ;-) You never fall into this discussion with Ethernet for
instance. Even when there was coax the problem one use to have was
termination problems.

/Ake

2015-05-07 12:12 GMT+02:00 heikki saha <[email protected]>:

> Hello,
>
> We have two cases, which are commonly mixed, when different cabling
> approaches are
> used:
>
> Stub-length
>
> Stub-length has maximum length, depending on the bit-rate, where the main
> criteria
> comes from the transmission line properties. The end of the stub is
> unterminated,
> causing reflections. The longer stub, the closer to the sample point
> reflections
> appear. So, the shorter stub, the better. The optimum length is zero, but
> in real
> cabling/wiring, we have always non-zero stub length, typically varying some
> millimeters into couple of centimeters. If a stub length is some dozens of
> centimeters, the cabling/wiring approach should be re-designed.
>
> Device-to-device distance
>
> Device-to-device distance or length of daisy-chaining cables has a minimum
> value,
> as proved by Steve Corrigan. The main thing is line impedance, which shall
> remain
> within the specified tolerances. It is hidden into Steve's analysis, that
> the
> minimum daisy-chaining cable length is relative to the length of the
> entire network.
> As long as the devices are evenly distributed along the network, the net
> impedance
> is approximately constant over the network. I have confirmed this in many
> customer
> cases, having very different dimensions. In some applications,
> device-to-device
> distance may be some millimeters and in one case we had to increase
> device-to-device
> distance from ~0.5m into ~1.5m in order to get rid of the problems.
>
> Summary
>
> One professor, lecturing us years ago introduction into electronics,
> always used to
> emphasize, that "there are not conductors, there is only impedance". That
> fits well
> also for solving CAN physical layer problems.
>
> 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
> +358 (0)50 588 6894
> [email protected]
> http://www.tke.fi/
> http://www.canopen.fi/
>
> ----- Original Message -----
> From: "YAP" <[email protected]>
> To: [email protected]
> Sent: Thursday, May 7, 2015 12:33:32 PM
> Subject: Re: [CANLIST] Minimum stub length
>
>
> Yes Steve is dearly missed. He always had time to answer questions from
> everyone. And did so in a very nice way.
>
> Thanks for your replies. No it is not run on clothes hangers ( :)))) ). At
> least not yet. The nodes I have sit on a DIN rail like this
>
>
> https://groups.google.com/group/vscp/attach/9cab5492e7e16710/006.png?part=0.1.1&authuser=0
> or here http://grodansparadis.com/
>
>
> and can be connected together either over a RJ-45 or a pin list. The
> shorted cable that can be used here the better for users of course.
>
> /Ake
>
>
> 2015-05-06 17:28 GMT+02:00 Bram Kerkhof < [email protected] > :
>
>
>
>
>
>
> If you daisy chain CAN nodes, you effectively have a very short stub so I
> don’t see why it wouldn’t work. I think the quality and quantity of the
> connectors to your nodes will have a far bigger impact on signal integrity
> than a small length of cable. Unless you’re using rusty clothes hangers for
> cable (although I do recall someone trying and succeeding in running CAN on
> clothes hangers somewhere…)
>
>
>
> cheers,
>
> Bram
>
>
>
> From: [email protected] [mailto:
> [email protected] ] On Behalf Of YAP
> Sent: woensdag 6 mei 2015 16:40
> 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
>
>
>
>
>
>
>
> --
>
>
>
> ---
> Ake Hedman
> Grodans Paradis AB, http://www.grodansparadis.com
>
>
> --
>
>
> ---
> Ake Hedman
> Grodans Paradis AB, http://www.grodansparadis.com
> --
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-- 
 ---
Ake Hedman
Grodans Paradis AB, http://www.grodansparadis.com