

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

               D A S (DTMF ACCESSORY SQUELCH) APPLICATION NOTE 3

               SCAN-STOP -- HOW TO USE DAS WITH A SCANNING RADIO

                                  951105/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 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 3, "ScanSop -- How to use DAS
        with a Scanning Radio", describes how to use DAS with a radio



      DAS-AN3: SCAN-STOP              -2-                     951105/Iss-1


        that's scanning several channels.  This can be particularly useful
        for those radio amateurs that want to monitor several local
        repeaters and simplex channels for LiTZ calls, as well as their
        own personal ID sequence.


      DETAILS
      ------------------------------------
        DAS provides two special outputs: OT0 and OT1.  OT0 provides a
        "ScanStop" control output while OT1 provides a "Transpond" control
        output.  This note describes only the ScanStop output.

        The ScanStop output on OT0 is used to control channel changes of a
        transceiver or scanner connected to DAS.  This output normally
        floats.  However, whenever a DTMF signal is detected or the
        speaker control relay is active, the ScanStop output sinks current
        to ground.  For those operators that want to have DAS monitor
        several channels, the operator can make use of the Scan-Stop
        output to control a channel stepper circuit for a transceiver or
        scanner.

        The channel stepper includes an oscillator that is used to advance
        the channel of a transceiver (via the UP or DOWN buttons on a
        microphone) or a scanner (via whatever button on the front panel
        of the scanner causes the scanner to advance to the next channel
        -- for most Radio Shack scanners that's the MANUAL button).  A
        typical oscillator rate is 4 Hz (four channels per second).
        Additionally, the channel stepper provides a 3 second
        retriggerable one-shot that's used to stop the oscillator.  The 3
        second one-shot is driven by the Scan-Stop output of DAS.

        In the normal state, Scan-Stop is inactive (floating) and the 3
        second one-shot is also inactive, thus allowing the oscillator to
        provide 4 pulses per second.  The output of the oscillator usually
        drives an opto-isolator that connects to the channel advance
        circuit of the radio in use.  In this example the radio changes
        channel once every 250 ms.

        When DAS detects a DTMF signal the Scan-Stop output goes active
        (sinking current to ground), which causes the 3 second one-shot to
        be active, which causes the oscillator to be inhibited so it can
        not advance the radio to the next channel.  If the DTMF digits
        continue to be detected at least once every 3 seconds the radio
        will stay on the same channel because the 3 second one-shot
        continues to be re-activated.

        If DAS detects a configured DTMF sequence the speaker control
        relay (K1) will go active.  Recall that the Scan-Stop output is
        the "logical OR" of the DTMF detector and the speaker control
        relay.  The 3 second one-shot will continue to be active while the
        speaker control relay is active, thus continuing to hold the radio
        on the same channel.  When the speaker relay finally times-out,
        the one-shot will time-out, the 4 Hz oscillator will be re-enabled
        and the radio will start stepping through channels again.



      DAS-AN3: SCAN-STOP              -3-                     951105/Iss-1


        Note that this scanning system works well for elongated DTMF
        signals (LiTZ or Long-Digit).  However, for the short duration
        digits of Personal and Group sequences, there is significant
        potential for the system to miss the first digits of a sequence
        because the receiver could be listening to channel 3 when the
        first DTMF signal of the Personal or Group sequence was being
        transmitted on channel 1.  One way to greatly reduce this problem
        is to preface the transmission of any Personal or Group ID
        sequence with a 2 second transmission of # (POUND), when you know
        that one of the intended recipients uses a DAS ScanStop system.
        When the user transmits the long # (POUND) it will cause all DAS
        ScanStop systems to lock onto that channel, thus greatly reducing
        the chance of missing the following digit sequence.  It is still
        possible for DAS ScanStop systems to miss DTMF sequences, even
        when all sequences are prefaced with long # (POUND) symbols.
        However, this is statistically unlikely for most situations.


      CONCLUSION
      ------------------------------------
        This document has presented some high level thoughts about how to
        use DAS with scanning radio systems.  I've incorporate such a
        system into a Radio Shack PRO-58 scanner.  If there's sufficient
        interest I may write it up for submission to //QST// as an
        article.  If you have other suggestions about how to use DAS with
        a scanning radio receiver please let me know.  I would like to
        consider folding users ideas into future revisions of this
        document.


      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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