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Telephone : Checking for "RTS"

Discussion in 'Electronic Design' started by R.Wieser, Apr 14, 2006.

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  1. R.Wieser

    R.Wieser Guest

    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)

    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.

    Rudy Wieser
  2. Ryan Weihl

    Ryan Weihl Guest

    you could place a .1/600v cap between the A/B lines to your design.
    We use 1200bd signaling here but the signal sould pass in either case.
    And then you can use your existing power arrangement. Just make sure the
    caps are high voltage like 600v or 1kv.

  3. Ryan Weihl

    Ryan Weihl Guest

    oh, and google for "dtmf decoder", shows ideas for dtmf caller id.

  4. John Fields

    John Fields Guest

    I don't know what you mean by the "A and B" lines, but if it's like
    tip and ring is over here, why not connect the IRLEDs from two
    optocouplers in anti-parallel and connect that set in series with
    either A or B?

    That way, one or the other of transistors associated with the IRLEDs
    will be turned on and will give you polarity information while being
    completely electrically isolated from the phone line.
  5. R.Wieser

    R.Wieser Guest

    Allready done. No enlightment came from any of the designs. They just
    receive DTMF, and only signal a "valid" when certain criterea to the
    reception of it have been met. None of them bother to check for

    Rudy Wieser
  6. R.Wieser

    R.Wieser Guest

    John Fields <> schreef in berichtnieuws

    Hello John,
    Yes, the A and B lines are comparable to "tip" and "ring".

    And alas, that method is only good when the phone has been picked up. I
    need to know what happens when the phone is still on-hook (before the phone
    rings for the first time)

    Your suggestion is allready part of my design, so I know when the phone has
    been picked up (and tells my controller to interpret eventually incoming
    DTMF-signals differently).

    Thanks for the suggestion though.

    Rudy Wieser
  7. Ken Taylor

    Ken Taylor Guest

    Something like this? -

  8. R.Wieser

    R.Wieser Guest

    Ken Taylor <> schreef in berichtnieuws

    Hello Ken,
    Hmmm .... Looks like it could just do what I need (am not quite sure about
    the difference between the American phone-system and the Dutch one). Though
    it's a bit big (32 pin) and would mean I have to connect everything
    (including the DTMF-decoder and controller) directly to the phoneline ... I
    would have to think of another method to connect it to my computer (No path
    between the phoneline and ground may exist).

    Thanks for the suggestion.

    Rudy Wieser
  9. Ken Taylor

    Ken Taylor Guest

    A telco isolation transformer at the input would solve your isolation
    requirements. As for the pin-count, yes it's probably excessive but it would
    do the trick and it also points toward other chips of its 'genre' which act
    similarly. There are other telephone IC manufacturers about so it's a matter
    of Googling the right keywords.


  10. Ryan Weihl

    Ryan Weihl Guest

    according to this: and the
    included schematic, the dutch telecomm uses what we call a modem with a
    DAA to detect a loop reversal to get the DTMF signaling of the CID.
    Uses a Mitel/Marlink DTMF decoder. Quite a bit different from Bellcore.
    searched for "dutch caller id"

  11. R.Wieser

    R.Wieser Guest

    Ryan Weihl <> schreef in berichtnieuws

    Hello Ryan,
    Thanks for the URL. And I did find that one myself too (I did spend quite
    some time searching for Dutch caller-ID stuff :) ).

    I'm not at all sure what you mean with "a modem and a DAA", (The only
    Modem-alike bit in the schematic is the max232, and isn't connected to the
    phone-line at all), but if you mean that the opto-coupler (ring-detector)
    could allso be used for detecting the line-reversal ... Well, maybe that
    could work.

    I've thought about using it before, but I'm not at all sure about it's

    Thanks for the hint/reminder though, I focussed so much on a 100% solution
    that I forgot this one :-\

    Rudy Wieser

    A few things I have allways have wondered about when looking at this
    schematic :

    1) What effect do those put into anti-series diodes D1 and D2 have ? Both
    together will not pass current in *any* way .... As far as I can see he (the
    designer) mixed up his zener-diode schematic (using an anti-series
    configuration) with a normal diode schematic (using an anti-parallel

    2) why did the designer de-couple from the Phone-line with a trafo *and*
    capacitors etc. Why did he not just leave-out those capacitors ? Or even
    used the simple one-wire (plus ground) input ?
  12. R.Wieser

    R.Wieser Guest

    Ken Taylor <> schreef in berichtnieuws

    Hello Ken,
    Yes, but it would allso block any direct-current or voltage, which is, I
    think, needed by the chip for the reversion detection.

    Although, it could just detect the voltage-spikes that are, on the other
    side of the trafo, the result of that line-reversal. Alas, dependability
    of the "has reversed" -signal would than be a bit questionable, as only the
    change-spike can be detected (no "if this volrage holds for at least
    ....mSec" -check can be performed).
    Yep, that is what I did (did not find anything yet though).

    Rudy Wieser
  13. Ryan Weihl

    Ryan Weihl Guest

    1) D1 and D2 are used to protect the ic inputs to 5V since you could
    get external noise spickes from the coupling transformer

    2) this works like a dial-up modem, but instead of using a ringsignal
    it is using a voltage reversal to connect to the phoneline and
    receive the DTMF. The opto is used to detect the line reversal and
    uses the relay contect to present a 600ohm impedance to recv the signal.
    It also presents isolation btwn the unit and phone line. When the DTMF
    signal has been sent the polarity should reverse to the opto and drop
    the relay.
  14. R.Wieser

    R.Wieser Guest

    Ryan Weihl <> schreef in berichtnieuws

    Hello Ryan,
    Could you pass that by me again, but that with a bit more detail ?

    As far as I can see it those diodes *cannot* limit anything to 5V, as they
    are not wired like normal clamping-diodes normally are (and that should take
    4 diodes, 2 for both lines).
    I'm sorry, but I do not follow you here : "is using a voltage reversal" ?
    What part of the schematic might do that ?
    I see. But only when the "phone" is allready Off-hook, and the relay has
    been activated (so the capacitor is bypassed).

    As long as the relay is not activated it could be used as a
    ringing-detector, where it not that the output of that opto-coupler does not
    seem to reach the controller ...

    Now I look more closely at it, I must say that that opto-coupler does not
    actually seem to be able to *do* anything, as it's output, pin 5, is going
    absolutily nowhere ...

    It does however try to directly discharge a 47 uF capacitor, which could
    well lead to it being destroyed on the spot ....

    I don't know what is going on here, but that schematic starts to look very
    funny to me. :-(

    Please enlight me when I'm wrong.

    Rudy Wieser
  15. Ryan Weihl

    Ryan Weihl Guest

    referring to the schematic, Mitel is now Zarlink. I looked at this url and they say the diodes
    should be 15v zeners used as frontend protection. We used 5.6v zeners
    (1n751) in similar arrangements for modem chips. I was involved in
    products like this, thats why I can talk about this. (now retired, but
    still like to keep involved). Can't find anything about the MT8060, but
    should be similar.

    looking at the opto coupler and the loop. there is no current flowing
    thru the opto diode since the 6.8uF cap is in the circuit. But as soon
    as the polarity reverses in the loop you have current thru the opto,
    long enough to turn it on. This should fire the micro which turns on
    the relay. The relay contact shorts C1 (6.8uF) cap and you now have the
    opto turned on, keeping the relay on and the transfomer on the loop.
    Any DTMF tones will now be handled by the decoder. When the loop
    voltage gets reversed by the central office the opto turns off, the
    micro turns the
    relay off and the whole unit gets disconnect from the loop.
    Hope this helps. I know of some modem chips that decode DTMF tones (TDK)
    as well if the Zarlink part is out of range pricewise.
    We use bellcore specs here, and I built a few for us and friends. Had a
    LCD display and also stored the phone nbrs for about a month. The
    program was on a 8051, could retrieve the nbrs from memory, print them,
    etc. Normal caller id's can only display, but on this you could
    retreive and print.

  16. Ryan Weihl

    Ryan Weihl Guest

    I think there is something missing on the schematic.
    Where is the output of the opto connected too?
    There is a 10k pullup on the opto, but no connection shown.
    Maybe we don't have to know everything.

  17. R.Wieser

    R.Wieser Guest

    Ryan Weihl <> schreef in berichtnieuws

    Hello Ryan,
    As mentioned above, I can understand Zeners used that way. :)
    Hmmm .... Yes, I have thought about doing it the same way, but I find it
    bit tricky to rely on a single pulse as the line-reversal signal ... Oh,
    well, just one thing more to stay aware of while creating the controllers
    I'm using the MT8870D myself, and it seems to work o.k. An older "C"
    version refused to work, and it took me some time before I became aware of
    the difference :-\
    Yep. That was what I was, among others, referring to.
    Maybe it should have been connected to INT1 (pin7) on the controller. It's
    (still) free, and it looks logical enough to be able to be true.

    Thanks for the help/explanation.

    Rudy Wieser
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