Subject: Re: 1 of 2 Ethernets suddenly stopped working?
To: Der Beagle <sopwith!>
From: Bernard Gardner <>
List: port-alpha
Date: 11/15/2001 21:03:38
On Mon, Nov 12, 2001 at 03:56:31PM +0000, Der Beagle wrote:
> machine: 164lx with 2 21040 cards running 1.5.1  de driver
> Both Ethernets were working okay until Friday, when one suddenly
> stopped working.  Switched which network is connected to which
> board, problem follows network, so board seems okay.  Swapped
> out AUI cable, transceiver, measured coax (10base-2) with ohmmeter,
> etc. etc. no joy.  Outgoing pings sometimes increase count as shown by

I know, it's obvious, but I figure it's worth mentioning since you
haven't said that you checked for it...

The most common cause of this kind of problem I ever saw back when
I had to look after 10base-2 networks was that someone had pulled
one of the terminators, usually so that they could plug the cable
directly into the back of their machine without a T-piece (I can't
even start to try to explain why). Don't know how big your network
is, but this would definitely be the thing I'd be looking for.

It would probably explain the high collision count too.

Oh, and strangely enough, this always seemed to happen on a Friday...

Do other machines on the network work OK? If not, break the network
in half at your station by removing one side of the network from your
T piece and terminate that side, if that doesn't fix it, swap cable
sides. The problem is on the side that makes the network work when
it's removed. Trace the cable on that side to find the break. If you
know the path of the network, start from what should be the far end
and do a binary chop search back down the cable. Otherwise trace the
cable from your end until you reach the fault.

This of course is assuming that your network is bigger than two
or three machines in the same room (the network I used to look
after covered 5 buildings with a combination of thin, thick and
fiber ethernet and a lot of bridges and repeaters).

As I remember it, resistance (what you probably measured with your
ohmeter) isn't a fantastically useful measure of the health of your
network (unles you're seeing open or short circuit). I think the
terminators are actually providing impedance (but I'm not an electrical
engineer, so I may well be wrong on this).