================================================================ 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. | ---------------------------------------------------------- ******************************************************************* * 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 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. ---ooOoo---