NDC comm NDC-3 - Manual (Page 6)

Page 6 © 2005 Active Research Limited
Connecting devices together
The basics
NMEA data is transmitted from an information source such
as GPS, depth sounder, gyro compass etc. These data
sending devices are called “ Talkers”.
Equipment receiving this information such as a chart-
plotter, radar or NMEA display is called a “ Listener”.
Unfortunately, only one Talker can be connected on to a
single NMEA 0183 system at any one time. Two or more
Talkers are simply not possible because they are not
synchronised to each other, and will attempt to ‘talk’ at
the same time (over each other), resulting in corruption of
the NMEA data, and potentially in disaster if valuable data
such as navigation information is lost or corrupted so that
it is incorrect and/or misleading.
Actisense™ produces a full range of products to solve all
NMEA interfacing requirements.
Please visit the Actisense™ website for full details on
these and other Actisense™ interfacing, Depth sounding
and Sonar products.
The NMEA signals
The NMEA 0183 system v2.0 and later uses a ”differential”
signalling scheme, whereby two wires are used to transmit
the NMEA data. These connections will be labelled as
either NMEA “ A” and “B“ or NMEA “+” and “-“ respectively,
depending on the instrument and manufacturer.
When connecting between different manufacturers, there
can be some confusion, but it is simple and easy to
remember: NMEA “ A” connects to NMEA “+” and NMEA
“ B“ connects to NMEA “-“.
The different NMEA standards
The NMEA 0183 specification has slowly evolved over the
years, so connecting one device to another is not always a
straightforward matter. The earlier versions of NMEA 0183
(before v2.0, as detailed above), used slightly different
connection methods and signal levels: the instruments
had just one “NMEA” data line (‘ Tx’ or ‘Out’), and used
the ground as the other line - similar to the way a computer
serial port works. This connection method is referred to as
“single ended” instead of the “differential” method used by
NMEA 0183 v2.0 devices.
The data format is largely the same between both
systems, with v2.0 adding some extra sentence strings,
and removing older (redundant) sentence strings from the
specification. The situation is further complicated, as many
manufacturers still use the old (“single ended”) method of
connection because it is cheaper to implement.
So how can an older type NMEA device be connected to a
newer type device?
Care is needed – it is possible to damage or overload
the output of a newer differential device if it is incorrectly
connected to an older device. This is because the older
devices used ground as the return, whereas the newer
devices actually drive the NMEA “ -/B” line between 5v and
0v. Thus, connecting this output to ground will result in high
currents being drawn by the driver instrument, resulting in
potential overheating and damage to the driver circuits.
To connect a new type differential device to an old type
single-ended system, connect the NMEA “ +/A” output from
the differential driver to the single-ended NMEA “ Rx” or “In”
input of the device. Leave the NMEA “ -/B” output floating.
Connect the ground line of the differential output device to
the ground of the single-ended device. This provides the
required data signal return current path.
To connect an old type single-ended device to a new type
differential device, connect the NMEA ” Tx” or “Out” output
from the single-ended driver to the differential NMEA
“ +/A” input of the device. Connect the ground line of the
single-ended output device to the NMEA “ -/B” input of the
differential device. This provides the data signal return
current path. If the NMEA “ -/B” input is left floating, then
data corruption / errors may occur.
Please refer to the Output Connections section for
example of these connection methods.