Most of the information
below is derived from the official NMEA 0183 documentation. The official
document can be obtained from:
NMEA National Office
P.O. Box 3435
New Bern, NC 28546-3435
USA
Short introduction to
NMEA
The NMEA standard is
developed to enable data communication between electronic marine
instruments, navigation equipment and communications equipment in a
standardized way.
The standard is intended to
support one-way serial data transmission from a single TALKER to one or
more LISTENERS. This is data in printable ASCII form and may include
information such as position, speed, depth etc. Typical messages might be
20 to a maximum of 79 characters in length and generally require
transmission with a maximum of one per second.
The NMEA 0183 standard is
not intended to be used for high-bandwidth applications such as radar or
video imagery, or intensive database or file transfer applications.
There is no provision for
guaranteed delivery of messages and only limited error-checking
capability.
A TALKER is any device that
sends data to other devices within the standard. The type of TALKER is
identified by a 2-character mnemonic.
No provision is made for
more than a single TALKER to be connected to the bus.
A LISTENER is any device
that receives data from another device within this standard.
Multiple LISTENERS may be
connected to a single TALKER. The LISTENER receive circuit shall consist
of an opto isolator and should have protective circuits to limit current,
reverse bias and power dissipation.
One TALKER and multiple
LISTENERS may be connected in parallel over an interconnecting wire. The
number of LISTENERS depends on the output capability and input drive
requirements of individual devices.
Interconnection between
devices may be by means of a two-conductor, shielded, twisted-pair wire.
The drive circuits shall
meet at least the requirements of EIA-422-A. Note that this standard is
not the same as RS232C!
Data between devices is
transferred in the form of "sentences". A NMEA sentence always
starts with the dollar sign ($), followed by a 2-character TALKER
identifier, e.g. "GP" for GPS. The next 3 characters are called
the "sentence formatter" to define the type and format of the
data that follows, e.g. "GLL" (geographic position Lat/Lon).
After this the data fields
follow, separated by a comma. "Null" fields are allowed in a
sentence, and most data fields may be variable in length. Numeric variable
fields may also contain a decimal point and leading/trailing zeros.
A sentence should be
concluded by an "*" followed by a valid checksum. The checksum
is the 8-bit XOR of all characters in the sentence, excluding the leading
"$" and "*" character initiating the checksum.
Many NMEA devices do not
validate the checksum (and many manufacturers do not even bother to send
one), and that is why you should never totally rely on the received data.
Some examples of valid NMEA
sentences:
$GPGLL,5300.97914,N,00259.98174,E,125926,A*28
$GPRMC,125926,A,5300.97914,N,00259.98174,E,748.0,180.0,110202,002.1,E*75
$GPVTG,180.0,T,177.9,M,748.0,N,385.3,K*49
$IIMWD,055.0,T,049.7,M,020.0,N,010.3,M*4E
$IIMWV,055.0,R,020.0,N,A*3F
NemaTalker supports NMEA
version 2.20, and as of NemaTalker version 1.1.0 also NMEA version 2.30
and 3.01 for most GPS sentences and for the $VDVBW sentence in the
Velocity instrument.