[rescue] Cable testers
mouse at Rodents-Montreal.ORG
Sat Apr 23 16:38:27 CDT 2016
>> I'm curious: does happen to anyone know what property testers
>> capable of detecting split pairs are actually testing for?
> In the simplest terms, a conductor goes to the wrong pin. A common
> example I've seen made by the unaware or inattentive: Solid-green
> and solid-blue, or green/white and blue/white, accidentally swapped.
Then perhaps I misunderstood. I took "split pairs" to be a cable that
is correctly wired as far as DC tests go but which, for example, puts
one pair on green and white/blue and another on blue and white/green.
That is, pairs for signaling purposes do not match up with which wires
are twisted together.
As a simple example of one, a friend of mine wired up some cables
thinking the pairs were pins 1/2, 3/4, 5/6, and 7/8 on the 8P8C,
instead of the actual 1/2, 4/5, 3/6, and 7/8. His runs were fairly
short, and they worked at 10Mb, but when he switched to 100Mb they
stopped working, to his great puzzlement until he learnt the pairing.
> (Especially on some cables where you need to have the color vision of
> Vincent van Gogh to tell green/white and blue/white apart.)
I usually deal with that by figuring green/white is the one twisted
with the solid green wire and blue/white is the one twisted with blue.
(In low light I can have trouble telling green from blue even on the
solid wires, but that's a separate issue.)
And I've seen some cables where all the non-solid-colour wires are
solid white and you _have_ to tell them apart by looking at which wire
they're paired with.
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