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ring indicator over a serial port

Discussion in 'General Electronics' started by Rob, Apr 8, 2004.

  1. Rob

    Rob Guest

    i need to fake a ring indicator signal over a db9 serial port. i think its
    just a matter of applying voltage to a couple of pins. before i go randomly
    playing, does anyone have any idea of the specifics? thanks!
     
  2. Rob

    Rob Guest

    i figured it out, in case anyone ever searches google for this, its 5 and 9
     
  3. Quack

    Quack Guest

    I am pretty sure the voltages are not the same.
    Either it wont do anything, or you may damage your comport, i'd google
    around and look at dtmf circuits etc to see how to interface to a
    phone line before playing around ...

    (i think i remember something about -50v supply for phones - i have
    been in a few t.exchanges and they have huge power thingies, all
    generating -50v)

    good luck :)
     
  4. Quack

    Quack Guest

    It just occured to me that you are probably NOT trying to fake a ring
    on a telephone - but are trying to fake a ring indicator from a modem
    ?

    So you want to 'pretend to be a modem' and give out an RI signal ?

    Okay connect GND and RI to the remote end, from your com port to the
    com port you want to talk to.

    pins on your db9;
    gnd=5
    ri=9

    Now in your software, you want to write a '1' to the right place, so
    your RI pin goes 'high', and the other end should get it.

    base=the commport address, eg: 0x3f8 is normal for COM1, 0x2f8 is
    normal for COM2.

    base+6 = MSR (modem status register)
    In this register, you want to play with the top 4 bites, which are;
    DCD RI DSR CTS - - - -

    so write %01000000 or 64 to 3FE (if you have a normal address and are
    using COM1).

    Never tried it, seems like it should work.

    The other problem is from what i remember of working with modems, they
    also tend to say "RING" in ascii when they get a call, you might want
    to fake that too, depending on what your trying to fool, it may need
    it.
     
  5. Garrett Mace

    Garrett Mace Guest


    He was trying to fake the signals that an external modem sends to a computer
    through an RS232 port in order to indicate an incoming call. Not, in fact,
    trying to generate ring voltage for a telephone line.
     
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