Specifications

Actisense
®
Page 6© 2011 Active Research Limited
NMEA 0183 bus at any one time. To combine data buses,
you must use a multiplexer, such as the Actisense NDC.
This safely receives the data from multiple talkers, and
combines it together into a single stream.
Two or more Talkers are simply not possible on the same
data line as they are not synchronised to each other, and
will attempt to ‘talk’ at the same time (over each other).
It is also likely that one Talker will have a stronger drive
circuit, and the other talker will not even get to send
anything.
This will resulting in corruption or total loss 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.
Connecting two talkers together can even lead to one or
both of the Talkers becoming damaged.
A Talker can be connected to multiple Listeners, but often
many talkers used in electronic devices on the market do
not provide enough driver current to talk to more than one
or two listeners at one time, thus special NMEA ampliers
or “buffers” need to be employed.
Listener specication
A listener receiver must be design to operate with a
minimum differential input voltage of 2.0 Volts, and not
take more than 2.0mA from the line at that voltage. The
NMEA standard document shows a circuit that will meet
this requirement. Actisense has designed it’s own OPTO
input circuit which exceeds this requirement, extending the
range of which good data may be successfully received.
A Listener must also be OPTO isolated to prevent ground
loop problems. Again, many manufacturers have not
implemented this and this has caused more problems in
installations, as connecting together two grounds of two
systems can be a major source of faults on installations.
It also causes major issues when connecting a version 2.0
Talker to a version 1.x listener, as connecting the “B” line
to the ground causes current to ow every time the “B” line
tries to go to +5 volts.
Listeners must also be tolerant of the higher voltages used
in NMEA version 1 signalling.
See ‘ISO-Drive’ section for a solution to this problem.
Communication specication
The data communication specication for NMEA version 1,
2 and 3 is essentially the same as RS232 with the settings:
4800 baud, 1 start bit, 1 stop bit and “No Parity”.
In addition, NMEA 0183 version 3.0 added a new baud
rate of 38400 baud. This is known as “NMEA 0183-HS”
This was to allow for the latest ARPA and AIS equipment
which require a higher baud rate link to send the larger
quantity of information they produce in a timely manner.
The data sent is all in printable ASCII form (data bit 7 is
always zero in NMEA 0183 data transmissions, and all
characters should be between ASCII HEX 20 and HEX
7E), so may be viewed directly on a PC “terminal” type
program (like ‘HyperTerminal’) - even though what is seen
may not make much sense.
Note:
NMEA 0183-HS or NMEA 0183 v3 is in no way related to
the new NMEA 2000 standard (as is incorrectly suggested
by some manufacturers), which uses a totally different
electrical connection standard that is not compatible with
NMEA 0183 in any way.
If you require to convert between NMEA 0183 and the new
NMEA 2000 network, the Actisense NMEA 2000 Gateway
will allow full bidirectional conversion between these
two very different standards. Please visit the Actisense
website for full details.