From: jrd@cc.usu.edu
Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Problem with thin wire and terminated T connectors.
Date: 13 Aug 92 09:21:54 MDT
Organization: Utah State University

In article <13AUG199210030803@mivax.mc.duke.edu>, jim@mivax.mc.duke.edu (James D Dryfoos - Systems) writes:
> A long time ago when we started putting in thire wire segments to connect
> groups of terminal servers in one closet we decided it was convenient to leave
> one or two blank T connectors for later connections without disrupting the net. 
> We were concerned about this as we have heard that these T connectors could
> pick up transient signals and maybe cause problems.  It was advised that they
> have a terminator stuck in them.  So, a barrell connector and terminator were
> connected to each empty T connector.  The thin wire worked so it was left that
> way.  I recently was involved in troubleshooting the poor performance of
> terminal servers on these thin wire nets.  I saw the terminators and thought
> that I had not seen that done before, so removed them.  Sure enough, the
> terminal servers began responding flawlessly.  The terminators caused the
> servers to retransmit packets now and then.  I assume some packets were being
> destroyed by a signaling problem that the terminators must have created?
> 
> I would like to hear anyone's thoughts on this.  Also, is leaving unconnected T
> connectors on the wire really a problem?
> 
> Thanks in advance,
> 
> Jim
> 
> *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
> *  James Dryfoos  -- Medical Informatics - Duke University Medical Center     *
> *                                                                             * 
> *  VMS Systems Programmer -- Postmaster - Duke Electronic Medical Post Office * 
> *  dryfo001@mc.duke.edu - dryfo001@dukemc.bitnet                              *
> *  Box 2914 Durham NC, 27710 USA - EARTH (919) 684-6421                       *
> *=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
	Terminators appear at only one place on a transmission line: at the
very ends. They must not appear elsewhere. The concept, stated in English,
is they represent an infinite length of transmission line and so absorb 
signals without reflecting them. Transmission line theory and practice also
says that placing anything in parallel with the line causes a reflection 
unless the aggregate of the paralleled items equals the impedance of the
line (skipping fine points about conjugate matches here). So a terminator
in the middle of a line presents to one end of a Tee a terminator in parallel 
with a line, and that aggregate impedance is half that of the line.   
	wave --->Tee--->line            wave ---> (term /2)
		  |              ==>              <--- reflection, big
		term
It sometimes is useful to think in terms of acoustic waves and organ pipes
or other wind instruments. An opening or obstruction along the pipe is a
source of reflection if the aggregate of this and whatever else is present
does not look exactly like more of the same pipe. If organ pipes were made
of cones rather than cylinders, and hence a smooth succession of "not exactly 
like more of the same pipe", there would be no resonance to speak of and not 
much to hear.
	Partial effects of putting a terminator in the middle of a line 
can be achieved easily by tying thin wire coax into knots like rope. The
interior insulation gets squeezed, the inner wire comes closer to the outer
braid and the impedance changes a lot. So don't treat coax like rope.
	Open Tees in a line are normally not a problem other than adding
very small reflections and of course permitting more connectors to go bad
later on. Barrels are better but the effect is not much different than just
Tees. "Interference" pickup by open Tees is very very small.
        Joe D.
Newsgroups: comp.dcom.lans.ethernet