Russell,

On Mon, 29 Mar 2004 16:28:52 +1200, Russell McMahon wrote:

> Best of all was including a short length of wrong impedance cable in a coaxial ethernet installation.Guess
how I know :-(

Ah, memories!  I remember getting a call from a secretary at our office, who said "Get here and fix this
(expletive deleted) thing before I throw it out of the window!"

Never being one to underestimate a threat like this, I headed back (from a client's) and found that
Wordperfect would take 5 minutes to load, then another 5 to open a file, etc.  It was a network wide problem
so I set to with the time-domain-reflectometer (praising the accounts department that I'd been allowed to buy
one!) and it showed some odd readings - usually you open the termination at the one end end of the 10-base-2
(50-ohm coax) and connect the TDR at the other end and it gives the length of cable to the open end of the
network segment, then you work along and as you exclude a length of cable the remaining length shown reduces
accordingly.  This didn't!  It intermitently showed a much shorter length, and it wasn't consistent from the
two ends,  so I did a "binary chop" to locate where the problem was.  I found that someone had inserted a
piece of coax they'd found in a cupboard, to add their machine to the network, and it was (I think) IBM
terminal cable: 75-ohm instead of 50.  I replaced it with some of the right stuff, and harmomy was restored.
I'd like to say that the secretary was eternally grateful and demonstrated it... but sadly not  :-(

I'm rather glad that 10-base-T (twisted pair) caught on and replaced coax - it's much easier and more reliable
to use all round.

I wonder who got custody of the TDR?

Cheers,

Howard Winter
St.Albans, England

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