Model 865-A Deep Sea Acoustic Release/Transponder M-855-009
Benthos, Inc.
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6.2 Command Theory
The standard Benthos acoustic command consists of two separate frequencies alternating at a specific
shift rate. One frequency, called the "in-band" frequency, is set to the RECEIVE frequency of the
release. The other frequency, called the "out-of-band" frequency, is arbitrarily fixed at 12.5 kHz
(actually the “out-of-band” frequency alternates between 12.5 kHz and 18 kHz on successive
command transmissions) which are otherwise unused frequencies. The detector in the underwater
release will respond "true" for the in-band frequency and "false" for the out-of-band frequency. Its
output, therefore, will be a square wave whose frequency is the shift rate of the command. The
command receive circuit then checks for a specific frequency of this square wave and, if it is correct for
two complete seconds, the release function will be executed.
There are a total of thirteen shift rates, although only eight are supported in the standard release (A-H)
and they are designated by the letter part of the code. The shift rates are:
A: 82.5 Hz B: 110.9 Hz
C: 136.5 Hz D: 100.0 Hz
E: 71.9 F: 77.0
G: 118.8 H: 127.3 Hz
Codes I thru M require special release software and are typically reserved for special applications, they
are:
I: 123.0 Hz J: 131.8
K: 107.2 Hz L: 114.8 Hz
M: 103.5 Hz
The command code is decoded by a microprocessor on the logic board.
6.2.1 Command Release
Upon receipt of the proper command code for RELEASE, a power MOSFET transistor is latched on,
thus allowing the motor to run. Once started, the motor will run until the pin is fully retracted. Once the
pin is fully retracted, the RELEASE LIMIT switch is actuated causing the motor to stop. A backup
timer will disable the motor drive after approximately 40 seconds has elapsed, if the shaft has failed to
reach the release position (it normally takes approximately 20 seconds).
6.3 Transmitter
The transmitter consists of three basic blocks:
1. Transmit frequency synthesizer
2. Pulse control
3. Power amplifier.
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