

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

               D A S (DTMF ACCESSORY SQUELCH) APPLICATION NOTE 5

               SPECIAL REMOTE CONTROL APPLICATIONS AND OPERATIONS

                                  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. |
           ----------------------------------------------------------

      *******************************************************************
      *                             NOTES                               *
      *                                                                 *
      *  SPECIAL TYPESETTING FUNCTIONS (I.E., FOOTNOTES, ITALICS,       *
      *  ETC.) ARE MARKED WITHIN THE TEXT USING A DOUBLE SLASH (LIKE    *
      *  THIS //).                                                      *
      *                                                                 *
      *******************************************************************



      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 5, "Special Remote Control
        Applications and Operations", describes how you can make use of



      DAS-AN5: SPECIAL REMOTE CONTROL -2-                     951128/Iss-1


        DAS for special remote control applications.  One particular DAS
        application of interest to ham radio operators may be using DAS to
        control a remote radio transceiver.


      GENERAL PURPOSE REMOTE CONTROL
      ------------------------------------
        General purpose remote control can be handled by means of the
        PER-PLUS commands.  These commands allow you to set either of the
        two relays on DAS to either an "energized" or "de-energized"
        state.  You can also use PER-PLUS commands to set any of the six
        open collector outputs of DAS to either a "floating" or "current
        sink to ground" state.  Additionally, you can set three of the
        open collector outputs used to drive the LEDs (LED0/URG, LED1/PER
        and LED2/GRP) to any of the six LED flash codes.  You can not set
        the state of the output used for LED3/STA since that's used to
        display internal DAS status conditions.

        A complete discussion on the use of the PER-PLUS commands is given
        in the in the "DAS Configuration Reference Guide" under the
        heading "General Purpose Controller Functions".


      RESET CONTROLLER FUNCTIONALITY
      ------------------------------------
        DAS provides a capability to serve as a master reset controller.
        It's function is briefly described in the "DAS Configuration
        Reference Guide".  If you haven't done so already, please review
        the "Reset Controller Functions" section in the Configuration
        Reference Guide.

        In general, when DAS is functioning as a Reset Controller, it
        looks for the special sequence followed by a configured PER ID
        sequence.  When DAS has detected both sequences it then closes K1
        for two seconds which can be used to reset all devices at a site.
        It's expected that K1 on DAS will control additional relays that
        are mounted physically within each piece of equipment at the
        mountain top (or other remote location) that is to be reset.
        Assuming that a small reed relay, similar to Radio Shack 275-233,
        is used within each device to be reset, then the coils of up to 50
        relays can be safely paralleled together and driven by K1.  Each
        relay should include a clamp diode across the coil.

        Relays are added within each piece of equipment because the
        "on-board" reset signal for each device should not leave the
        housing of that device.  If a high impedance reset signal was
        routed outside the housing it may become susceptible to electrical
        noise in general and, electro-magnetic fields from radio
        transmitters specifically.  As discussed above, a relay should be
        added within every device that is to be remotely reset and that
        relay should be within inches of the existing reset switch or
        within inches of the circuit board where the switch is wired.  The
        contacts of that relay should be connected to the reset circuit of
        that device and the low impedance coil windings are brought out of



      DAS-AN5: SPECIAL REMOTE CONTROL -3-                     951128/Iss-1


        the box, with suitable RFI filtering as may be needed, and are
        wired in parallel with any other similar purpose relays.  These
        paralleled relay coils would then be connected to a series circuit
        consisting of the paralleled relays, K1's normally open contacts
        and a 12 volt DC power source.


      REMOTE RADIO CONTROL
      ------------------------------------
        DAS provides a capability to control a remote radio transceiver.
        The DAS feature is called "K2 Control Mode".  It's function is
        briefly described in the "DAS Configuration Reference Guide".  If
        you haven't done so already, please review the "K2 Control Mode"
        section in the Configuration Reference Guide.

        In the remote radio control application (like a remote base) it's
        assumed that there is an audio path between the user and the
        remote radio.  That audio path might be implemented with a
        "4-wire" radio link, a 4-wire audio link or a 2-wire audio
        link//2.  A dial up telephone line is an example of a 2-wire audio
        link.

        Once the user has established a communications link to the remote
        transceiver, he can access that transceiver by entering a
        multi-digit access code (PER+33 in this case).  When the PER+33
        command is detected by DAS it will activate K1.  The remote
        transceiver system should be configured such that when K1 is
        active the receiver audio will be coupled onto the communications
        link back to the user.  When the user wants to activate the
        transmitter on that transceiver he sends a momentary DTMF 0 signal
        to DAS.  When DAS detects this DTMF 0 and then sees that the DTMF
        0 has been removed from the audio path, K2 will be activated for
        30 seconds or until DAS detects DTMF *, which ever comes first.
        K2 would be wired to actuate the transmitter's PTT circuit.  With
        PTT active, any audio on the link will be transmitted by that
        remote radio transmitter.  Normally the user will end the
        transmission with a momentary DTMF * to deactivate the transmitter
        and return to the receive mode.  The 30 second timer provides a
        watchdog function so that if the user fails to send the momentary
        DTMF * the remote transmitter will not be on the air for more than
        30 seconds.  To deselect that remote transceiver unit the user
        would send DTMF * * * (3 stars).

        In summary, use PER+33 to select a particular DAS unit controlling
        a particular remote transceiver.  When a particular DAS unit has
        been selected its K1 remains active until that DAS unit is
        deselected.  Use DTMF 0 to enable K2 and DTMF * to disable K2.
        Use DTMF * * * to disable both K1 and K2 and to deselect that DAS
        unit.

        Please note the following.  When DAS is in the "K2 Control Mode"
        DAS doesn't change output states until the appropriate DTMF signal
        is REMOVED from the audio input of DAS.  Thus, the DTMF 0 used to
        enable K2 (and the transmitter) does not get transmitted by the



      DAS-AN5: SPECIAL REMOTE CONTROL -4-                     951128/Iss-1


        remote transmitter.  Additionally, the DTMF * used to disable K2
        (and the transmitter) ALWAYS gets transmitted by the remote
        transmitter.  Thus, to send a DTMF LiTZ signal via a remotely
        controlled transmitter the user will need to send a momentary DTMF
        0 to activate the transmitter and then follow with the actual LiTZ
        signal.

        Note also that if a user makes brief transmissions on a radio
        channel via a remotely controlled transmitter that receivers of
        that signal will demodulate a DTMF * at the end of each
        transmission.  If a receiver is connected to a DAS unit with the
        STAR IS WILD CARD CHARACTER option set to active, that DAS unit
        may respond to what appears to be a series of Wild Card
        characters.  The two second "end of sequence" timer keeps this
        from being a problem.  As long as the transmissions do not occur
        at a rate faster than one every 2 seconds, those DAS decoders
        should not respond to the DTMF * used to deactivate the remote
        transmitter.

        If the user has a need to transmit a DTMF * as part of a SELCAL
        sequence, the user will need to actually send DTMF * 0 on the link
        for each DTMF * actually transmitted by the remote transmitter.
        The DTMF * will be transmitted by the remote transmitter but it
        will also cause the remote transmitter to become deactivated.  The
        DTMF 0 will reactivate the transmitter, but will not be
        transmitted over the air by the remote transmitter.  For example,
        to send the DTMF sequence 1*34# via a remote transmitter the user
        would send the following DTMF sequence:

                              0  1  *  0  3  4  #  *

        The first DTMF 0 enables the transmitter without that DTMF 0
        signal being transmitted by the transmitter.  The first DTMF *
        will be transmitted but it will also deactivate the transmitter at
        the end of the DTMF *.  The second DTMF 0 will not be transmitted
        but it will reactivate the transmitter.  The second DTMF * will be
        transmitted (and should have no effect on monitoring DAS units)
        and will deactivate the transmitter.


      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.

        2.   The terms "4-wire" and "2-wire" are telephone system terms.
             They are derived from the number of "wires" needed to
             implement a particular circuit.  The important considerations
             of these circuits are as follows.  In a 4-wire circuit, the
             transmit and receive audio travels on totally independent
             circuits.  The only cross-talk that exists in the 4-wire case
             is from the small capacitive and inductive coupling that may
             exist between the transmit and receive signal wires.



      DAS-AN5: SPECIAL REMOTE CONTROL -5-                     951128/Iss-1


             Cross-talk is typically non-existent in 4-wire audio
             circuits, for most practical purposes.  In a 2-wire circuit,
             like the circuit between the telephones in your home and the
             telephone company's central office (the source of dial tone),
             the transmit and receive signals are carried on a 2-wire
             path.  The transmit and receive signals travel on the same
             signal path -- that one pair of wires.  The telephone
             instrument and the central office switching ports each
             include balance networks that attempt to cancel out, or
             balance out, their own transmit signals from their own
             receive signal ports.  In practice, the balance circuits
             don't work well because of the unknown characteristics of the
             wire pairs (which usually include inductors called loading
             coils).  The important consideration for 2-wire circuits is
             that the lack of balance from transmit port to receive port
             can cause problems in the case of a remotely controlled radio
             transceiver.  For example, DTMF signals demodulated by a
             remote radio transceiver might be detected by the remote DTMF
             controller (DAS) that's used to control the transmitter
             associated with that transceiver.  Thus, if a "K2 Control
             Mode" DAS unit was controlling a remote transceiver that used
             a 2-wire audio link, and that transceiver demodulated DTMF 0
             from the radio channel, and that DTMF 0, perhaps because of
             poor balance, was coupled into the transmitter audio input
             port and was detected by the DAS unit monitoring that audio
             input, that remote transceiver will become active -- and
             that's a real world problem.  The only practical way to avoid
             this problem is by using a 4-wire audio link.  Fortunately,
             most all radio links have 4-wire TX/RX cross-talk
             characteristics (low cross-talk), although they are often
             half-duplex paths.


                                   ---ooOoo---























