Path: utzoo!utgpu!cunews!software.mitel.com!grayt
From: grayt@Software.Mitel.COM (Tom Gray)
Newsgroups: sci.electronics
Subject: Re: telephone line: which rings?
Message-ID: <7871@nst>
Date: 8 May 91 19:17:18 GMT
References: <1991May7.211057.4572@vax5.cit.cornell.edu>
Distribution: sci
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 39

In article <1991May7.211057.4572@vax5.cit.cornell.edu> reyy@vax5.cit.cornell.edu writes:
>Greetings:
>
>I need some information.  I want to build a device
>that responds to a telephone ring.  Can anyone 
>tell me, or point me to some good lit., as to 
>which one of those wires in the telehpone cable
>corresponds to the ringer, and at what voltage
>does it appear?  Anything I should know about
>when i tap into that line?  (I'm quite an
>amateur at this stuff, so complete explanations
>are the best.  Also, responses via Email will
>prpbably be best.)
>

There are two wires in the loop - tip and ring.
The tip is the green wire and is grounded 
though a resistance at the CO. The ring is the
red wire and is connected to the ringing current
generator at the CO.

There is a telephone maxim that

RED IS RING IS RIGHT.

The red lead is the ring which is punched down
to the right of the green lead (tip). This 
is all you need to know to install a telephone
switch.

As you know, the telephone loop is balanced.
You must ensure that your circuit does not 
imbalance the loop. The differential resistance
of your device must exceed 10K and better yet
30K ohms or there will be significant noise
due to longitudinal currents.