Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!ginosko!uunet!fernwood!vixie!avsd!childers
From: childers@avsd.UUCP (Richard Childers)
Newsgroups: comp.dcom.lans
Subject: Re: Grounding Ethernet coax lines.
Message-ID: <2128@avsd.UUCP>
Date: 5 Oct 89 22:14:16 GMT
References: <3824.25292578@zeus.unl.edu>
Reply-To: childers@avsd.UUCP (Richard Childers)
Organization: Metaprogrammers International
Lines: 106

network@zeus.unl.edu writes:

>I have heard that it is advisable to ground these [ethernet] lines. 

From what I understand, "ground" is used in slightly different ways in
the computer and radio fields.

In radio, "ground" represents a convenient sink for excess electrons,
as well as a gross but adequate reference level for electrical inter-
-actions.

In computers and telecommunications, "ground" represents a carefully
maintained reference voltage level, a median against which machines on
both ends of the cable orient themselves and their interpretation of
signals.

It is entirely separate from the "ground" that is used as a sink for
excess electrons, although conceptually related, and in fact, while the
frames of the machines are assumed to be grounded, usually through the
power socket(s), the actual power that is delivered into the power supply
via power cables is electrically isolated from the ground, even though
they are both in the same cable, and is more oriented towards handling
power supply problems than it is oriented towards defining voltage levels
for telecommunications.

It's easiest to understand if you think of the planet Earth as a giant
capacitor, with an even number of electrons scattered throughout, as a
gigantic reference level for busy humans playing with electrical currents,
wherever they are.

Things get ugly when you're in a plane or a boat, and have to fake it or
make it. I'm not sure how it works in space, I'd assume you need to create
some sort of "sink" electronically. That's why cars and trucks drag strips
of conductive material, to synchronize their electrical level to the planet,
which everyone else is using as a reference. Rubber tires insulate bodies
nicely, and without this, as you get out of your car, touching the car door,
you may get a shock as the excess electrons make their merry way through
your nice, conductive body and down into the ground.

So, to summarize :

	o the "ground" ( or "earth", in Great Britain ) represents a mean
		reference for all voltage measurements on planet Earth

	o it also acts as a sink for excess electrons

	o in telecommunications, while there is a need for a mean reference
		voltage between machines, there is no need for a "sink", as
		there are _no_ excess electrons in microcircuitry

	o such that, while you should ground your machines, you should not
		ground the cables explicitly, other than the normal ground
		that is associated with connecting them normally as per
		specifications.

Now, no doubt I'll get snowed by zillions of corrections by EEs, but that's
about how I understand it and it seems to work.

>1)  Should a line be grounded only on one end, or on both ends?

Both ends.

>    Should it be grounded at all?

Yes. To the machines using the cable, only.

>2)  What about in line taps that go to devices such as a repeater
>    or bridge.  Meter checks show that this makes a solid ground,
>    so does that count as one of the grounds?  In these cases do
>    we need to add any ground at all?

You shouldn't. The outside of the connector is ground.

>3)  BNC Tee connections to a Ethernet card in a PC seem to make a
>    very weak ground or no ground at all.  Should we ground these
>    connections?

They should be adequate, as designed.

>4)  Is it advisable to isolate against all unintentional grounds
>    as with a connection touching a metal rack by wrapping the
>    metal part of the connection?

Preventing accidental grounding can rarely hurt, and almost always helps,
as a preventative measure.

>Suggestions and comments are appreciated.

It wouldn't hurt to go find a book on antenna theory and / or ground loops,
if you're interested in getting into ham radio or are really dedicated to
network management, as I have heard ethernet being explained successfully
as an insulated antenna ...

>: Steven Lendt              /\ Managers and users were all having a fit      :
>: Network Manager           \/ Thinking their systems the most important bit :
>: Department of Computing   /\ I said to each one with very small wit        :
>: Univ of Nebraska @ Omaha  \/ What's not physical net, ain't really nothin' :

-- richard


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