Telephone : Checking for "RTS"
- From: "R.Wieser" <address@xxxxxxxxxxxxx>
- Date: Fri, 14 Apr 2006 14:25:53 +0200
Hello All,
I'm currently trying to build a Caller-ID (in the Netherlands, meaning
the phone-company uses DTMF to send the CLID). Currently the detecting of
the DTMF is reliable.
What I'm looking for is a method to check for the polarity-swap between the
A and B lines, which preceedes the CLID-sequence (and closes a(n attempted)
connection).
I can just link an opamp to the A-B lines, but that would cause a direct
connection between my experiment (and the computer that is linked to it) and
the phone-line. Next to that not being allowed by our national
phone-company (no possible path to ground should exist), it allso endangers
my electronics, as they are connected to ground (and phone-lines could be
floating at several tens-of-volts above it).
Another problem is that the resistance of the whole voltage-measuring
circuit should be over a Meg, preferrably even larger (again, mandated by
the phone-companies specs).
As I see it I have little choice : It looks like I have to create a seperate
power-supply, not connected to anything, but for the opamp monitoring the
phone-lines. The result should than be transferred to the rest of the
circuit by an opto-coupler.
In short, what I need :
A method to detect a polarity-change between the A and B lines, with no
electrical connection to the main-circuit. The time between the change of
polarity and the actual signalling of it should be no more than about 200
mSec (leaves me 50 mSec to respond to it :-) ). An added plus would be if
I could ask for the current polarity of the line.
Any hints, tips, tricks or other suggestions are appriciated. An URL to
some allready existing schematics that solve the problem would be great.
Regards,
Rudy Wieser
.
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