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From: ed@mtxinu.UUCP
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet noise problem?
Message-ID: <322@mtxinu.UUCP>
Date: Thu, 26-Mar-87 16:32:26 EST
Article-I.D.: mtxinu.322
Posted: Thu Mar 26 16:32:26 1987
Date-Received: Sat, 28-Mar-87 08:40:48 EST
References: <7823@utzoo.UUCP>
Reply-To: ed@mtxinu.UUCP (Ed Gould)
Organization: mt Xinu, Berkeley, CA
Lines: 29

>I'm just in the process of activating some long-dormant PDP11 Ethernet
>hardware, and have hit a problem.  The hardware is an Interlan NI1010A
>controller with an Interlan NT100 transceiver and a vampire tap.  (Yes,
>I know this isn't the latest and greatest...)  The controller seems to
>check out properly, but it claims it is unable to send because of excess
>collisions.  It is the only thing on the Ethernet ...
> [description of various static tests to determine integrity deleted]

Interlan transceivers, unlike some others, are sensitive to the
specification that the tap not be placed within 3m of the end of the
cable.  I suspect that their transceivers are sufficiently sensitive
that they detect collisions with themselves from the small but
inevitable reflections from the terminated end of the cable.  I've
never noticed this sort of problem with TCL transceivers; 3Com
transceivers routinely work very well even when connected together with
nothing longer than a barrel connector and terminated right at the
transceiver.

Be sure to locate the tap at one of the marks on the cable (to avoid
standing-wave problems with other taps added later).  I also try to
avoid the mark nearest to the end of the cable, but that may just be
paranoia - the same reason I type "sync" three times even on recent BSD
systems that do it themselves when I shut down the system.

-- 
Ed Gould                    mt Xinu, 2560 Ninth St., Berkeley, CA  94710  USA
{ucbvax,decvax}!mtxinu!ed   +1 415 644 0146

"A man of quality is not threatened by a woman of equality."