Path: utzoo!attcan!uunet!husc6!mailrus!uwmcsd1!ig!agate!saturn!eshop
From: eshop@saturn.ucsc.edu (Jim Warner)
Newsgroups: comp.dcom.lans
Subject: Re: Does anyone rent TDR's?
Summary: Low cost TDR
Keywords: TDR Ethernet
Message-ID: <4879@saturn.ucsc.edu>
Date: 17 Sep 88 17:16:56 GMT
References: <425@aoa.UUCP>
Reply-To: eshop@saturn.ucsc.edu (Jim Warner)
Organization: University of California, Santa Cruz
Lines: 55

In article <425@aoa.UUCP> mbr@aoa.UUCP (Mark Rosenthal) writes:
>I would like to rent a Time Domain Reflectometer to check out our backbone
>cable, and I have a few questions....

What is special about the Cabletron TDR is (1) it uses negative going
pulses to probe the cable and (2) it can superimpose a steady negative
voltage onto the cable to force all the stations to defer.

The rational for using negative going pulses is that's what transceivers
normally put on the cable so you minimize the possibility of electrically
stressing your installed transceivers.  I'll buy that.

Cabletron brought one of their TDRs out here for a sales demo.  I
found that it didn't do any more than the setup I was using which
cost a whole lot less money.

I use a conventional pulse generator and an oscilloscope.  The 
requirements for the pulse generator is that it be able to generate
negative going pulses ~50 nS long and that it have a separate output
to send to the trigger input of your oscilloscope.  One pulse
generator that is suitable is B&K model 3300, which sells for about
$400.

Put the pulse generator close to the scope and the end of the E-net
cable.  Run a BNC cable from the generator to a 'T' connector on the 'scope
input.  Another BNC cable goes from the o'scope 'T' to a BNC to N type
adapter on the end of the E-net cable.  Set the pulse generator to
produce 0.5V negative going pulses before you attach it to the E-net
cable.  A good repetition rate is anywhere between 1 and 10 KHz.  A
third BNC cable goes from a spare output on your pulse generator
to external trigger input on your oscilloscope.

You'll be measuring the distance to reflections in time units.  Convert
to distance units by multiplying by 0.77 ft per ns.  You should divide
your observed times by two to convert round trip time to one-way time.

The reason for a triggering the oscilloscope independent of the signal
on the E-net cable is that it then won't trigger on attempts of hosts
to transmit.  This readily serves the same function as Cabletron's
custom negative bias generator.  

I would prefer a packaged portable TDR unit if I had to do TDR in
the field where there was no electricity -- say in a man hole in
the middle of the street.  But I don't climb around in man holes...

I am especially fast to get out my TDR setup whenever (1) we are
dealing with a new contractor and the office wants to know if it's
OK to pay the bill (2) someone wants to convert their network cable
to RG58 with a type N to BNC adapter (3) a network expansion is being
done with cable from a different vendor than the original install.

Of course, if your records are less than perfect, TDR also provides
the authoritative method for determining the length of a cable segment.

jim