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From: wunder@wdl1.UUCP (wunder )
Newsgroups: net.lan
Subject: Re: inter-building ethernet grounding
Message-ID: <389@wdl1.UUCP>
Date: Sun, 26-Aug-84 01:22:14 EDT
Article-I.D.: wdl1.389
Posted: Sun Aug 26 01:22:14 1984
Date-Received: Thu, 30-Aug-84 19:36:55 EDT
Lines: 41
Nf-ID: #R:cornell:-43300:wdl1:8900002:000:1767
Nf-From: wdl1!wunder    Aug 25 18:39:00 1984

We are facing a similar problem here (Ford Aerospace), and don't
have a good soloution either.  I have thought of two things, though.

Thing 1:  IEEE 802.3 requires transceiver cables to be isolated from
the coax.  This means that each 500m Ethernet segment should (must?)
be individually grounded.  This fixes the ground loop problems.
>From the standard (Draft, Rev. D, December 1982):

  "7.4.2 MAU Electrical Characteristics
   7.4.2.1 Electrical Isolation
   The MAU must provide isolation between the AU Interface cable
   and the coaxial trunk cable.  The isolation impedance shall be
   greater than 250kohms, measured between any conductor (including
   shield) of the AU Interface cable and either the center conductor
   or shield of the coaxial cable, at 60 Hz.  The breakdown voltage
   of the isolation means provided shall be at least 250 VAC, rms."

An MAU is a transceiver (Media Access Unit) and an AU Interface cable
is a transceiver cable.


Thing 2:  You should be able to provide lightning protection with
lightning arrestors where the cable enters the building.  Radio receivers
are at least as fragile as Ethernet transceivers, and they have been
protected for years.  I don't have any ready references for lightning
arrestors, but I believe that GE makes them, and Andrews (makers of
antennas and transmitting coax) should be able to help.  With arrestors,
you should be able to run regular coax between your half-repeaters
instead of fiber optics (talking about the runs between buildings, here).
It should be possible to get an arrestor that would go right in the
Ethernet coax -- all you have to do is keep the impedance within specs.


Good luck,

walter underwood

UUCP:  fortune!wdl1!wunder
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