

        ================================================================

               D A S (DTMF ACCESSORY SQUELCH) APPLICATION NOTE 4

       TRANSPONDING -- AUTOMATED METHODS OF ACKNOWLEDGING SELCAL MESSAGES

                                  951128/Iss-1
        ================================================================


                               Paul Newland, ad7i
                              Post Office Box 205
                               Holmdel, NJ  07733
                                 ad7i@tapr.org


           ----------------------------------------------------------
           |        Copyright (c) 1995 by Paul Newland, P.E.        |
           |   All Rights Reserved.  This guide may be reproduced   |
           | by radio amateurs for their own personal use, provided |
           | this copyright notice is included in any reproduction. |
           ----------------------------------------------------------

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      *  ETC.) ARE MARKED WITHIN THE TEXT USING A DOUBLE SLASH (LIKE    *
      *  THIS //).                                                      *
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      INTRODUCTION
      ------------------------------------
        DAS (DTMF Accessory Squelch) acts like a switch connected in
        series between the speaker output of your VHF or UHF transceiver
        and an external speaker.  DAS will monitor a radio channel for
        you, with the speaker switch open so the speaker remains silent,
        until someone sends one of the DTMF sequences you have selected.
        When DAS hears your
         personal Touch-Tone ID sequence on the radio channel it will
        light a LED, sound an buzzer and close the series speaker switch
        so that you can hear the audio of the calling station.

        An introductory discussion of DAS was published in //QST//,
        December, 1995, pages 25-31.  The "DAS Configuration Reference
        Guide", available from the TAPR FTP file server as well as the
        ARRL BBS//1, provides a more complete description of each
        programming option, the purpose of the option and how the various
        options may interact.

        This document, DAS Application Note 4, "Transponding -- Automated
        Methods of Acknowledging SELCAL Messages", describes how you can
        have your DAS system transpond to acknowledge SELCAL messages.



      DAS-AN4: TRANSPONDING           -2-                     951128/Iss-1


      TRANSPONDING
      ------------------------------------
        Transpond simply means to transmit in response to some remote
        command.  In the case of DAS, it would transmit in response to
        receiving a configured PER ID sequence.  DAS itself doesn't
        provide a transponder feature.  I wanted to incorporate that
        feature into the system but there just wasn't enough unused code
        space.  However, DAS does provide "hooks" so that some auxiliary
        circuitry can be added to provide a transponding feature.


      TRANSPONDING VIA OT1
      ------------------------------------
        The OT1 output, when DAS is configured as a SELCAL decoder,
        provides a convenient signal for triggering an external
        transponder circuit.  Let's call the OT1 output the "Transpond"
        output in this case.

        The Transpond output is active for 2 seconds whenever a Personal
        ID sequence is detected.  It is not affected by PER-PLUS commands,
        receipt of LiTZ or Long-Digit messages or by Group ID sequences.
        The idle state of the Transpond output is floating, while the
        active state is a current sink to ground.

        The Transpond output, when used, would normally be connected to a
        circuit that would key the transmitter PTT lead and generate a
        distinctive tone signal to modulate the transmitter.  The purpose
        of this distinctive tone signal is to notify the originating
        operator that the DTMF PER ID sequence was detected and that the
        called station's radio is operational.


      SOME DETAILS
      ------------------------------------
        There are several details to consider when implementing a system
        like this.

        The two second timer is derived from the horn timer (2 or 5
        seconds), so if you want a longer duration time you can go to five
        seconds.  Remember that just because you have received a SELCAL
        message it doesn't mean that the person who sent it to you is
        immediately ready to receive your transponding message.  Some
        transmitters (and DTMF microphones) implement a "transmitter hold
        timer" when the DTMF pad is activated.  When using the DTMF
        keypad, some radios will leave the transmitter active until the
        DTMF pad is idle for 2 seconds.  The DAS Transpond timer (speaker
        timer) duration is important since it can be used as a "hold off"
        timer.  If you configure your transponder system to activate your
        transmitter when the Transpond signal makes the active to idle
        state change (instead of idle to active) that will automatically
        give you a two (or five) second delay.






      DAS-AN4: TRANSPONDING           -3-                     951128/Iss-1


        Your tone generator can be as simple as a 555 timer.  You can get
        different frequency tones from a 555 by adding resistors from the
        Control Voltage lead (pin 5) to ground through the
        collector/emitter junction of a transistor.  Try resistors in the
        3K to 30K range.  If you give the base of the transistor 100 uA of
        current that will be enough to turn on the transistor and enable
        that resistor shunt.  The smaller the resistor value the higher
        the audio frequency, so adjust the basic 555 timer components to
        produce the lowest frequency you want.  Any additional resistance
        from pin 5 to ground will raise the audio frequency.  To ensure
        that you get something close to a 50% duty cycle output from the
        555, use a 1K for the +V to pin 7 resistor and something more than
        20K for the resistor from pins 2/6 to pin 7.

        Realize also that you must provide a means for the transponding
        station to meet FCC station identification requirements.  That
        probably will mean some sort of CW ID device.  You can purchase
        ready made CW ID devices for $50 to $100, or you can make your own
        out of CD4000 logic and diodes, an EPROM and a counter IC, or a
        small microcomputer (like the one used to control DAS).  It
        doesn't matter how you do it, but you must come up with a way to
        provide station identification by the transponding station.


      NOTES
      ------------------------------------
        1.   TAPR FTP File Server ftp.tapr.org, look in /tapr/das.  TAPR
             on the Web at "http://www.tapr.org".  ARRL BBS, +1 860 594
             0306, 8-N-1, look in the FILES area, search on DAS.


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