© Active Research Ltd. Page 4 of 13 21 October 2004
Actisense
Introduction
The Actisense NMEA Depth sounder / Speed log / Temperature sensor (DST) product was
developed to allow the connection of a standard passive (no electronics) depth sounder transducer to
an instrument with a NMEA 0183 digital data input. The Actisense DST interfaces between these two
otherwise incompatible elements.
The NMEA 0183 standard is a purely digital data transmission scheme, using ‘1’s and ‘0’s in a binary
format, to communicate a digital representation of the required information (depth, speed etc.) to a
connected instrument.
A depth sounder transducer is a sensor that must be interfaced to a depth sounder circuit, capable of
both producing the high voltages to drive the depth transducer, yet have very sensitive circuitry to
receive the faint echoes of the sound reflected from the seabed. Such a depth sounder circuit also
needs a powerful software algorithm to track the seabed, and reject false signals that occur
throughout the ocean. For example, an unsophisticated depth sounder device can lock on to a false
seabed due to reflections from nearby targets, other depth sounder devices working on the same
frequency, double depths and even algae.
Actisense has created their current depth sounder circuitry and software over a development period
spanning ten years. The algorithms have been proven in thousands of products throughout the world
that Actisense / Active Research Limited has either manufactured itself, or has played an important
design consultancy role in.
Actisense then added the speed/log and temperature inputs to the system, as the resulting
component cost is small, but it greatly expands the features on offer to the prospective user.
By connecting a so-called “Triducer” from a company like Airmar (www.airmar.com), it is possible to
measure the four quantities of depth, speed, log (distance) and temperature in this one device and so
save the cost of the extra electronic devices. The NMEA 0183 port requirements of the receiving
instrument are also reduced from three to just one.
Technical features
Sounding transducer circuitry – is capable of interfacing to a wide selection of Airmar
(www.airmar.com) transducers (refer to the Compatible transducer list section).
NMEA 0183 full-differential output driver - with a 25mA (maximum) drive capability this can drive
up to 5 typical NMEA 0183 device loads. The full-differential output ensures better quality
communications over longer distances and lower noise emissions on unshielded twisted pair cabling.
Powerful microprocessor - running at a clock frequency of 16MHz, it has enough power to perform
all the processing required to produce the accurate and very fast tracking depth output of the system,
in addition to the speed, distance and temperature calculations.
Flash ROM technology - that supports automatic programming for quick and easy updates, a
minimum of 10,000 erase cycles and over 10-year Data Retention provides for carefree user
configuration.
Low Power Consumption - typically 30mA rms.
