Re: 2 questions...

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi Richard,

-----Original Message-----
From:	Richard Malcolm-Smith [SMTP:rich@xxxxxxx]

Keith Doxey wrote:

> That shouldnt happen.
> You must use all 4 pairs for the 2 lines.
> DO NOT USE A SPLIT PAIR FOR THE THIRD WIRE OF EACH LINE
> That is the wire that carries the ringing circuit and if you use a
both
> wires of the pair one for each bell wire you WILL get overhearing as
you
> have created a long transformer winding which effectively puts them in
> contact with each other.

What you describe is the exact reason that phone wiring in NZ is no
longer done with a third wire.

The 3rd wire acts as an antenna for RF noise, and on cat 5 it is not
twisted with the balanced pair. The decoupling cap in the master socket
provides a nice path onto one line of the pair.

You are much better off using RJ45 connectors and getting a PABX adaptor
for any gear that has a requirement for the 3rd wire.

Unfortunately with the way UK phones are wired that wouldnt work very well.
The third wire is used for the bell signal to each of the bells/ringers and
is also used to shunt the bells whilst dialling. Any pulse dial equipment
on the line could cause the other phones to tinkle/chirp in sympathy with
the dialling if you only used 2 wires. Tone dial equipment would be OK in
that respect.

Also each phone would then have its own 1.8uF capacitor connecting its own
bell to the line and this could result in a drop in the speech level.

> Normal house telephone cable has 3 pairs and when people have a second
line
> the 3 unused wires are often pressed into service and you get exactly
the
> problem you have described.

I was under the impression that it was simply 6 wires all twisted
together rather then 3 actual pairs.

No. The UK cable used by BT has twisted pairs although the twist rate is
much less than that of CAT5.

I remember the hassle I used to have with some of the older cable (pre
1981) where the White wire was just that....PLAIN WHITE. They were VERY
loosley twisted and you sometimes had to carefully remove over a metre of
sheath to be able to get the pairs without loosing the pairing. Wires only
has a single colour on them so a 10 pair cable would have
2 Blue
2 Orange
2 Green
2 Brown
2 Slate (Grey)
5 White
5 Red

Needless to say that many faults in those days were due to crossed pairs.
I remember coming across an installation where 5 wires were needed and the
guy that had installed it had used Blue, Orange, Green, Brown.....and the 4
whites twisted together :-)))

Keith

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation



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