Maxtrac radio interfacing
Sierra Radio Systems
Applications Note AN3
Interfacing the Series 200 control systems to a
Motorola Maxtrac radio
Version 1.0
April 2006
AN3 – Interfacing to a Motorola Maxtrac radio.
Interfacing to the Motorola Maxtrac
The Motorola Maxtrac is a popular radio because of its small size, good performance and modest price. In a full
duplex environment, two radios are required, one for receive and one for transmit. The Maxtrac easily interfaces with
the Series 200 control system. This app note describes how to interface the Maxtrac to the control system either using
the SRS RIO-II interface board or via direct connection to the control system. This app note does not discuss any
internal modifications required or programming software necessary to configure a particular radio to operate on the
desired frequency. There are many resources on the web that can halp you in getting the radio on the desired
operating frequency and tone.
Maxtrac radios have either a 5 or 16 pin accessory connector on the back. SRS provides a complete interface kit to
connect the control system to a pair of Maxtrac radios. The 16 pin Maxtrac can be configured to deliver all necessary
connections to the connector. The necessary signals are
Receive audio out (unsquelched)
Transmit audio in
COR out (receive signal present indication)
COR & PL/DPL decode out
PTT in (“Push To Talk)
Note: the audio in and out is actually considered flat only in that the radio does not alter what comes in. In other
words, a transmitted FM signal, over the air is pre-emphasized and the rx audio out does not alter this. Therefore, the
audio out of the radio and routed through the control system is pre-emphasized. When the controller drives the
Maxtrac transmitter, the “flat” modulator input does not alter the frequency response so pre-emphasized audio from
the controller is transmitted as pre-emphasized over the air. In this way, the “flat” maxtrac rx and tx just takes the preemphasized audio in, routes it to the transmitters and re-transmits pre-emphasized audio. This means that the original
mobile transmitter pre-emphasizes the audio and the final receiving mobile radio de-emphasizes the audio and
everything is fine.
The other important note is that any device that plugs into the controller that normally uses true flat audio like a
computer for IRLP, EchoLink or other VOIP applications or a remote base radio like an Icom, Kenwood or Yaesu
requires the received audio to be pre-emphasized and the tx audio for that port to be de-emphasized. This can be
achieved either with an external board like the AS-2 or RIO-II or with a few component changes on the RCB.
To pre-emphasize rx audio, replace R24 with a 3.3k and 75k resistor in series with a 0.01 uf cap across the 75k
resistor as it enters the op amp. Add a 4700pf cap to position C9 for transmitter de-emphasis.
In the most simple case, interfacing the Maxtrac radios only requires building a cable that connects from the control
system (DB9-M) on one end and splits into two connectors (AMP) on the other end, one going to the receiver and one
going to the transmitter.
The control system provides the necessary receive audio gating and the control system assumes flat audio in and out
so no external active components are necessary. To make the installation easy, SRS offers a Maxtrac interface kit
that allows almost plug-n-play ease of installation.
AN3 – Interfacing to a Motorola Maxtrac radio.
Do not
Install 7417
Fan control relay
Transmit Connector
Tx audio
Receive Connector
Rx audio
Do not install
MAX232 or C1,
C2, C3, C4
Install Jumpers
The jumper block only needs 4
wire jumpers. No resistors or
capacitors are needed.
AN3 – Interfacing to a Motorola Maxtrac radio.
RIO-II Configuration for the Motorola Maxtrac
Install Op Amp
& pots
AN3 – Interfacing to a Motorola Maxtrac radio.
Recommended Standard 16 Pin Connections
Assign pins in the RSS under “General I/O”
Radio top / Long plastic housing
Connector, with locking tab 16 pin, (AMP 104422-1)
Digikey # 104422-1-ND
Contact, (AMP 1-87309-3) Digikey # A3007-ND
Radio top / Long plastic housing
Radio top / Long plastic housing
Rear view of the Maxtrac radio showing the 16 pin accessory connector.
AN3 – Interfacing to a Motorola Maxtrac radio.
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