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From: crosson@cam.nist.gov (Bob Crosson)
Newsgroups: comp.dcom.lans
Subject: A Mac and a PC on an Ethernet - Why does one work and not the other?
Message-ID: <1471@fs2.cam.nist.gov>
Date: 30 Oct 89 21:05:18 GMT
Distribution: usa
Organization: National Institute of Standards & Technology, Gaithersburg, MD
Lines: 33


I have a friend who has a 3Com thinwire Ethernet.  His 3Com server is
at one end of the wire.  A secretary's machine is near the middle
of the wire.  A third connection is beyond the secretary's machine 
when viewed from the server.  All connections to the wire are through
transceiver cables and single transceivers.

The secretary's machine works fine.  When my friend connects a Compaq
386 with a 3C503 Ethernet interface and 3Com software into the
transceiver cable at the third location, everything works fine.
When he plugs a Mac II with a 3Com Ethernet interface and software
into the same transceiver cable, it doesn't work.  It fails immediately
when he tries to use the network.

When he puts a BNC-T and two 50 ohm terminators on the transceiver
in place of the thin wire, the Mac says everything is okay, but,
of course, no connections can be made.  The Mac interface is configured
for a transceiver cable connection.

Why do two 3Com interfaces, when plugged into the same transceiver cable
and transceiver, operate differently?  The only thing I can think of
is that perhaps the power supplied to the transceiver from each of the
interfaces is different, causing the transceiver to work differently.
The transceiver and cable are from Cabletron and are probably Ethernet
Version 2/IEEE 802.3 compatible.

Can anyone suggest something to try?  My friend is trying to contact
3Com right now.

Thanks for your assistance.  E-mail can be sent to 

Bob Crosson
crosson@cam.nist.gov