[Soekris] Strange problem with net4801

Clemens Zauner czauner at onlineloop.com
Thu Jan 20 01:17:26 UTC 2005


On Wed, 19 Jan 2005, Horst Laschinsky wrote:

> > Do not use cables with shielding on both sides. either unshielded,
> > or shield connected to ground on only one end.
>
> What the f...? That worked! Can anyone explain me why? Up to now,
> I always thought the sole reason for the existance of common grounds
> was to _avoid_ disturbances???

Well, see - thats complicated. Ignoring the fact that Ethernet-Spec
ist for *U*TP (with "U" meaning: unshielded) you can build up funny
effects in AC-electrical Environment.

1) define ground. Look at your DC-Power-supply. Are you sure that
   ground == ground for two of them (The anser is usually "NO")?

2) electrical installions ofter suffer from poor grounding. In many
   Offices/Rooms/...

3) What you have / build when connecting two devices with an R >0 Ohm
   against pyhsical ground, by some rather massive Cable (=Capacity)
   is just a plain Oscilator (R-C Chain).

4) Its easy to construct a "Spannungsteiler" building this way.
   Sorry, I don't know the english verb for that. e.g. here in
   .at Power-Companies run AC with 230V. in about 10% of all instalations
   one can find 115V AC on patchpanels (on poorly grounded equipmet)

And so on ...  additionaly rx/tx on ethernet is usually galvanic seperated
on the in/out leads.

General rule-of-thumb:
When planting ethernet, and you feel like grounding, do in *one*
place. for instance at the patch-rack. Never connect the shield on
the wall-jacks.
Besides avoiding nasty HF-Problems, this can be a real life-safer
for your core-equipement if one of this cheap office-computers
does something very stupid like providing 230V on shield.

Besides: Ethernet-cables ar not for providing "common ground".
For providing common ground there is this green/yellow stuff
with copper in it. Usually with a thickness >= 1.5 mm².

cu
	Clemens.

PS: Avoid *any* cabling company doing incompetent shielding.
    Throw them out.



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