Actisense NMEA 0183 Manual

The NMEA 0183
Information sheet
Everything you wanted to know about
NMEA 0183 (but were afraid to ask)
Issue 1.50
 Understanding the NMEA 0183 specification
 How to connect NMEA 0183 devices together
 How to understand NMEA 0183 sentence formats

Actisense™
Page 2© 2006 Active Research Limited
Contents
Important Notices 3
Notices 3
Feedback! 3
NMEA 0183 Introduction 4
The basics 4
Electrical specification 4
Communication specification 4
Connecting a computer to your NMEA 0183 system 5
Testing the computer connection with ‘HyperTerminal’ 5
Example data using an Actisense talker device 6
Connecting devices together 7
PC OPTO-Isolation cables 7
NMEA Data Multiplexers / Combiners 7
NMEA Buffers 7
NMEA Autoswitches 7
The NMEA 0183 signals 7
The different versions of the NMEA 0183 standard 7
Connecting a differential NMEA talker to other devices 8
NMEA Listener device (1): Standard differential NMEA 0183 v2.0+ device 8
NMEA Interface (2): PC with an opto-isolated input (via Actisense™ PC-OPTO-1) 8
NMEA Listener device (3): Single-ended NMEA interface device 8
Connecting a single-ended NMEA talker to other devices 9
NMEA Listener device (1): Standard differential NMEA 0183 v2.0+ devices 9
NMEA Interface (2): PC with an opto-isolated input (via Actisense™ PC-OPTO-1) 9
NMEA Listener device (3): Single ended NMEA interface devices 9
NMEA 0183 Sentence Format 10
DBT – Depth Below Transducer 11
DPT – Depth 11
MTW – Mean Temperature of Water 11
VHW – Water Speed and Heading 11
VLW – Dual Ground/Water Distance 11
Contact Information 12
Company Information 12

The NMEA 0183 Information sheet
Page 3© 2006 Active Research Limited
Important Notices
This Actisense™ document is for informational purposes
only, and to the best of our knowledge, the information
contained within is accurate and true. Any use of the
information contained within this document is solely at the
user’s own risk.
No responsibility will be accepted for any personal injury
or damage to a boat or its connected equipment resulting
directly or indirectly from information contained within.
Navigational equipment used onboard a boat can be
critical to the safe passage of the vessel, so if in doubt,
contact an expert equipment installer before making any
modifications to your current system.
Unfortunately, we cannot published the entire set of NMEA
0183 sentence formatters here, as it is copyrighted by
NMEA and must be published from the NMEA website -
see the NMEA contact details at end of document.
Notices
When using this document, keep the following in mind:
The information contained in this document and any
specifications thereof may be changed without prior notice.
To obtain up-to-date information, contact Active Research
Limited (refer to the Contact Information section) or visit
the Actisense™ website (www.actisense.com).
Active Research Limited will not be liable for infringement
of copyright, industrial property right, or other rights of a
third party caused by the use of information or drawings
described in this document.
All rights are reserved: The contents of this document may
not be transferred or copied without the expressed written
permission of Active Research Limited.
Feedback!
This document has been produced from some of the
knowledge and experience gained by Actisense from
many years of experience in NMEA interfacing.
Please report any errors, omissions or inaccuracies to
Actisense - we intend this document to be a useful resource
tool to installers new to NMEA 0183 and inevitably we
may have missed out the vital piece of information that
they may have required to achieve a working NMEA 0183
system.

Actisense™
Page 4© 2006 Active Research Limited
NMEA 0183 Introduction
Designed over 20 years ago, the NMEA 0183 slowly
became the common method by which marine electronics
devices could talk to one another. The standard specifies
both the electrical connections that make up an NMEA
system, and the format of the data sentences that carry
the NMEA information.
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 device.
The need for NMEA 0183 evolved from the earlier “NMEA
0180” and “NMEA 0182” standards. Because they differ in
baud rate and transmission parameters from NMEA 0183,
they are completely incompatible with NMEA 0183.
Similarly, the new “NMEA 2000” standard is totally
different, and the two networks cannot be mixed without
using a special gateway device to convert between the
two standards (the new Actisense NMEA 2000 Gateway).
The basics
NMEA data is transmitted from an information source such
as a GPS, depth sounder, or gyro compass etc. These
data sending devices are called “Talkers”.
Equipment receiving this information such as a chart-
plotter, radar, PC or NMEA display is called a “Listener”.
Unfortunately, only one Talker can be connected to a single
NMEA 0183 data line at any one time.
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).
This will 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. It can even lead to one or both of the Talkers
becoming damaged.
Electrical specification
The latest specifications for NMEA 0183 (version 2 and
onwards) should, as a minimum, meet the requirements
of the computer standard “RS422” (Standard EIA-422-A).
They use +5 / 0 volt signalling, which is low voltage and
easy to interface to computer equipment.
However, voltage levels present on an NMEA 0183 bus can
be much greater - up to +/- 15 volt, particularly where older
equipment is used, as the original version 1 specification
used +/- 12-15 volt signalling.
Thus, all NMEA 0183 inputs conforming to v2.0 and higher
should be capable of receiving +/- 15 volt differential
signals without causing damage.
The NMEA 0183 specification also requires that all
receiving equipment must be opto-isolated; this opto-
isolation requirement reduces the chances of interference
and removes the problem of ground loop effects.
Communication specification
The data communication specification 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, 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 manufactuers), 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 bi-directional conversion between these two
very different standards. Please visit the Actisense™
website for full details.

The NMEA 0183 Information sheet
Page 5© 2006 Active Research Limited
Connecting a computer to your
NMEA 0183 system
As the NMEA specification basically uses the same
asynchronous comms specification in accordance with
ANSI standard as used on a standard PC serial port, most
PC’s can read NMEA directly through their Comms port.
Many installations have been performed where the NMEA
0183 data signal of a talker is connected to the computer
RS232 “Rx” line, and the ground of the talker to the
Computer “Ground “ connection.
In our experience, almost 100% of PC’s will show the data
of the NMEA link with this simple connection.
This is dangerous and most definitely not recommended,
however, as high voltage spikes can be present on the
signal cable, by induction from motors, alternators, radar
pulses, high power radios etc. and will damage the Comms
port on a laptop or PC that has only been designed for
domestic use.
This happens all the time, and has even happened to our
lab PCs in controlled conditions. We now always use Opto
isolators on our test benches (see below).
Connecting directly to the computer RS232 port also
breaks the NMEA 0183 requirement for opto-isolation
between Talker and Listener; therefore there may be
warranty conditions to this effect on equipment sold for
marine use.
This opto-isolation requirement was created to prevent
ground loops within an NMEA 0183 system - this can be
a bigger problem than the high voltage spikes, as ground
currents can destroy a PC by allowing amps of current
to flow through the delicate printed circuit boards within.
(See the Actisense ground loop information sheet for more
information).
To safely interface to the NMEA 0183 system on a boat, it
is best to obtain an opto-isolated NMEA to RS232 adapter
cable (called the PC-OPTO-1) which is available from the
Actisense™ website. Simply plug the PC-OPTO-1 in to
the PC port (see Connections section of this document),
and connect the open end wires to the NMEA 0183 lines.
Testing the computer connection
with ‘HyperTerminal’
If you don’t have special navigation software installed yet,
you can test whether data is entering your serial port using
the “HyperTerminal” program included with all windows
systems from Windows 95 onwards.
This can be found in the Windows Start menu under
‘Accessories → Communications’.
Start the HyperTerminal program, and it will ask you for a
new connection. Select an icon and give the connection
a name. Select “COM x” in the port setup dialogue box
(where x is the number of the Comm port which the NMEA
device is connected to).
On the dialogue box (above), select 4800 Bits per second,
8 Data bits, Parity “None”, Stop bits “1”, Flow control
“None”, and then click on “OK” to connect to the port.
You should now see the so-called NMEA “strings” displayed
on the HyperTerminal display.
This data is in the “raw” NMEA 0183 format, and is
“decoded” by navigation programs to provide you with all
your navigational data.
The data looks a little odd at the moment, so how can you
understand what the data is? The answer is simple. The
data is formatted according to the NMEA 0183 standard,
which states how the data should be transmitted, and what
should be transmitted where in each sentence. Refer to the
NMEA 0183 Sentence Format section of this document for
more details.

Actisense™
Page 6© 2006 Active Research Limited
Example data using an Actisense
talker device
By connecting a so-called “Triducer” from a company like
Airmar (www.airmar.com), or direct from the Actisense™
website, it is possible to measure the four quantities of
depth, speed, log (distance) and temperature in just one
device with a considerable cost saving versus buying each
transducer/device separately. Plus, all this data is supplied
through just one NMEA 0183 port.
The Actisense™ active depth sounder module (DST-2) has
the same Digital Signal Processor unit, but can connect
to any Triducer (Depth, Speed & Temperature), Biducer
(Depth & Temperature), or Depth only transducer. It allows
easy conversion of the analogue transducer signals into
the digital NMEA 0183 sentences.
These two sensor types output the NMEA strings ‘DBT’,
‘DPT’, ‘MTW’, ‘VHW’ and ‘VLW’.
Why do the strings have these names, and what data do
they contain? Refer to the NMEA 0183 Sentence Format
section of this document for more details.

The NMEA 0183 Information sheet
Page 7© 2006 Active Research Limited
Connecting devices together
Actisense™ produces a full range of products to solve
all NMEA interfacing and interconnection requirements,
including:
PC OPTO-Isolation cables
The Actisense™ PC-OPTO-1 & 2 allow a PC to safely
receive data from an NMEA 0183 system, protecting
against high voltage spikes and preventing dangerous
ground loops. The cable converts neatly between the
differential system used on the NMEA 0183 and the
“single-ended” RS232 system to provide the best quality
of data for the PC port.
NMEA Data Multiplexers / Combiners
The Actisense™ NDC-2 & NDC-3 can allow multiple
“Talkers” to easily share their data with other NMEA 0183
“Listener” devices.
The NDC-2 also allows for filtering of the NMEA 0183 data
to remove any unwanted data, and has a wide range of
input / output Baud rates to match any NMEA device.
NMEA Buffers
The Actisense™ NBF-1 can allow many more NMEA
0183 devices to receive the data from one source by
buffering and amplifying the signal. If you require more
than one source (“Talker”) of data, an Actisense™ NDC
can easily be used before the NBF.
NMEA Autoswitches
The Actisense™ NSW-1 can enable automatic switching
of two or more NMEA 0183 source (“Talker”) devices to
select the highest priority device with valid data - in effect
- automatic backup protection.
This can be used to select the highest priority GPS unit,
and automatically switch to the next highest priority GPS
device if the first fails for one of a number of reasons.
Please visit the Actisense™ website for full details on
these and other Actisense™ interfacing, Depth sounding
and Sonar product ranges.
The NMEA 0183 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 versions of the
NMEA 0183 standard
As the NMEA 0183 specification has slowly evolved over
the years, connecting one device to another is not always a
simple matter. The earlier versions of NMEA 0183 (before
v2.0, as detailed above), used slightly different connection
methods and signal levels: devices had just one “NMEA”
data line (‘Tx’ or ‘Out’), and used the ground as the other
line - similar to a computer serial port. 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 talker to an old type
single-ended listener, connect the NMEA “+/A” output
from the differential driver of the talker to the single-ended
NMEA “Rx” or “In” input of the listener. 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 talker to a new type
differential listener, connect the NMEA ”Tx” or “Out” output
from the single-ended talker to the differential NMEA “+/
A” input of the listener. 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.

Actisense™
Page 8© 2006 Active Research Limited
Connecting a differential NMEA talker to other devices
Figure 1 – Differential NMEA 0183 outputs
NMEA Listener device (1): Standard
differential NMEA 0183 v2.0+ device
This device is a differential device conforming in full to
the NMEA 0183 v2.0 (or higher) standard, and connects
directly to the pins on the differential device.
NMEA Interface (2): PC with an opto-isolated
input (via Actisense™ PC-OPTO-1)
Most PC’s do not have sufficient protection to connect
straight to an NMEA 0183 device. An opto-isolated port
is recommended for use on boats, to protect the PC from
large voltage spikes that can occur when engines switch
on and off - outboard motors can be particularly hazardous
to the quality of electrical signals onboard a boat.
The diagram details how an opto-isolated NMEA interface
(Actisense™ PC-OPTO-1) can be used to allow connection
to the PC. For full detail of the Actisense™ PC-OPTO-1
adapter, please visit the Actisense website.
NMEA Listener device (3): Single-ended
NMEA interface device
To connect the device to the differential output, connect
‘NMEA +/A’ to the device’s ‘NMEA’ input and ‘Ground’ to
the ‘Ground’ on the single ended device. It should be able
to receive the NMEA data correctly.
Never connect the ‘NMEA -/B’ to the ground of a single
ended receiving instrument. The resulting extra load will
at best increase the current consumption of the driver, at
worst it could cause serious damage to it.
NMEA
Listener 3


Differential
NMEA
Device
Differential
NMEA
0183
interface
Personal
Computer


NMEA
Interface 2



NMEA +/A
NMEA - /B
Ground
NMEA
Listener 1



The NMEA 0183 Information sheet
Page 9© 2006 Active Research Limited
NMEA Listener device (1): Standard
differential NMEA 0183 v2.0+ devices
This device is a differential device conforming in full to the
NMEA 0183 v2.0 (or higher) standard. Its “NMEA +/A”
input is connected to the “NMEA Out” output of the single
ended device. It’s “NMEA -/B” input is connected to the
“Ground” of the single ended device.
NMEA Interface (2): PC with an opto-isolated
input (via Actisense™ PC-OPTO-1)
Most PC’s do not have sufficient protection to connect
straight to an NMEA 0183 device. An opto-isolated port
is recommended for use on boats, to protect the PC from
large voltage spikes that can occur when engines switch
on and off - outboard motors can be particularly hazardous
to the quality of electrical signals on board a boat.
The diagram details how an opto-isolated NMEA interface
(Actisense™ PC-OPTO-1) can be used to allow connection
to the PC. For full detail of the Actisense™ PC-OPTO-1
adapter, please visit the Actisense website.
NMEA Listener device (3): Single ended
NMEA interface devices
This device is a single ended device. The device is simply
connected as “NMEA Out” to “(NMEA) In” and “Ground”
to “Ground” of the single ended device
Connecting a single-ended NMEA talker to other devices
Figure 2 – Single-ended NMEA 0183 outputs
NMEA
Listener 3


Single-ended
NMEA
Device
Single-
ended
NMEA
0183
interface
Personal
Computer


NMEA
Interface 2



NMEA Out
Ground
NMEA
Listener 1



Actisense™
Page 10© 2006 Active Research Limited
NMEA 0183 Sentence Format
All NMEA 0183 data is sent in the form of text sentences,
each beginning with a ‘$’ or ‘!’ symbol, and use commas to
separate each part of the sentence. NMEA codes are plain
‘ASCII’ text, and have the following format:
$yyXXX,……… *xx <0D><0A>
The sentence always starts with a ‘$’ or ‘!’ symbol
A 2-digit ‘yy’ code follows, giving the instrument type
(e.g. for a GPS device this should be ‘GP’, and for a
depth sounder this should be ‘SD’)
A 3-digit ‘XXX’ code follows, giving the sentence
data type (e.g. ‘GGA’ is a ‘Global Positioning System
Fix Data’ sentence, and ‘DBT’ is a ‘Depth Below
Transducer’ sentence)
A comma follows, then the contents of the sentence
data, which changes depending on the data type and
the current values of what is being monitored.
The final part is an optional two-digit checksum -
optional, but all good equipment will include this to help
safe-guard the data in the sentence. The checksum
is preceded by the ‘*’ character and is calculated by
taking the 8-bit exclusive-OR of all characters in the
sentence, including ‘,’ delimiters, between but not
including the ‘$’ / ’!’ and ‘*’ delimiters.
The string always ends with a carriage return and line-
feed combination (Hex 0D 0A, ASCII ‘\r\n’)
To better explain the NMEA 0183 format, the next section
details all the NMEA 0183 sentences output by the
Actisense™ DST-2 product. The 3-digit ‘XXX’ is replaced
by the 3-digit codes presented below.
In the data format below, the values DATA_#### represents
the digital data, and can have any number of digits before
and after the decimal point (if included). This makes
the system compatible with all levels of required output
precision, depending on the ability of the sensor.
Some data fields are optional – i.e. can be left out, by
having no data between the comma delimiters.
The checksums shown are for example purposes only and
will only be correct with a particular combination of data.
•
•
•
•
•
•