Page 6© 2009 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.
This fundamental problem with the NMEA system can be
easily overcome, and multiple Talkers connected together,
using the Actisense NMEA Data Multiplexer “NDC” series
of instruments. If the required system has multiple NMEA
Talker device of the same type (e.g. two GPS units), the
highest priority device can be automatically switched using
the Actisense Autoswitch (NSW-1).
Please visit the Actisense website for full details on these
and other Actisense products.
The output drive capability of many instruments means
that often their NMEA outputs can only drive one or two
Listeners. In this case, the output of the Talker can be
boosted to ‘talk’ to six listeners simultaneously using the
Actisense NMEA Buffer.
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 specication has slowly evolved over the
years, so connecting one device to another is not always a
straight forward 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
specication. 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 oating.
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 oating, then
data corruption / errors may occur.
The Actisense NBF-2 is equipped with ISO-Drive outputs
which removes all of these problems. Because the ISO-
Drive output “oats”, this means that it will automatically
adjust to the voltage of the line it is connected to. Thus, it
simplies installation work, as you no longer need to check
on the type of input used on the equipment that the NBF-2
is connected to. You can connect the “B/-” connection of
the NBF-2 to ground or to a differential “B/-” input without
worry - the voltage levels will simply adjust themselves to
be correct.
Please refer to the Output Connections section for
example of these connection methods.
