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