Newsgroups: comp.dcom.lans.ethernet
Subject: Re: timedomain reflectometer printout
From: hilbig_t@qis.dofasco.ca
Date: 8 Apr 92 17:56:09 GMT
Organization: Dofasco Inc, Hamilton, Ont. Can.
Nntp-Posting-User: hilbig_t

In article <1992Apr1.091156.13441@crow>, trich@crow.ogi.edu writes:

>   enp0: late collision
>   ie0: no carrier
>   et0: transmit: no carrier
>   ec0: Ethernet cable problem

I wish I had some experience with Sun computers, but the above errors may
be the result of a bad transceiver or transceiver cable.  My guess is that
you would see these errors if the collision detect and/or receive circuit
(echoed back when transmitting) electronics were bad.  However, if all of 
these errors are showing up on all computer don't count on this being your
problem.

> 
> I relaid the ethernet cable (thicknet) and carefully followed the
> specifications.  I am using all Cabletron transceivers.  The number
> of collisions was reduced,  but we still see the above errors which
> seem to appear randomly on any Silicon Graphics or Sun host that
> makes significant use of the net.
>

 
> After much messing around I managed to borrow a timedomain reflectmeter
> which I figured out how to use from the book "Keeping the Link"
> by Martin Nemzow.   The printout shows a phenomena which isn't described
> in this book.   I see the normal negative spike for each transceiver
> every 2.5 meters, but then after a group of transcievers I see a positive
> spike exactly 2.5 meters after the last transciver in the group.
> 
> In fact after the last transceiver in any group I see a positive spike
> 2.5 meters later.  why ?

You may be seeing Ghosts of taps.  A TDR sends a short implulse on the line
which is reflected back by any impedence miss-matches (ie taps).  It's
possible that one reflected signal is being reflected off of other taps.
You would normally never see these because the reflected energy is so
less than the original (incedent) impulse.  Your taps or terminators
would have to be in very bad shape to see this.  You never mentioned the
relative size of the reflected pulses; with your TDR vertical scale set
up to see the full height of the original impulse, you should bearly be 
able to see any tap or terminator reflections (about 1%).

If your reflections are too big, use a DC ohm-meter to make sure your 
50 ohm terminators are in place and your baseband coax resistance isn't
more than 5 ohms (for 500 meters).  If you measure at one end with the
terminator off you should see 50-55 ohms.  If you measure in the middle
you should see 25-28 ohms.  Don't worry about the meter damaging any
devices on the network, it will probably screw up the data however.

Other: Do a walk-thru on a live network shaking your viper-type
transceivers to make sure there are no metal filings in the taps.  Make
sure your baseband is not grouded in more than one location (ground loop
problems).


Hope this helps.

Thomas Hilbig,
Systems Engineer,			Address:    1330 Burlington St East,
Information and Automation Systems,		    Hamilton, Ontario
Dofasco Inc.                            	    Canada   L8N 3J5
hilbig_t@qis.dofasco.ca                 Telephone:  (416) 544-3761
Newsgroups: comp.dcom.lans.ethernet