Veripos LD6 Operation Manual page 78

Procedure Title: LD6 Operations Manual (B Variant)
File Ref.: AB-V-MA-00589
NMEA-0183 STANDARD
The National Marine Electronics Association (NMEA) has developed a specification
defining the interface between various pieces of marine electronic equipment. The standard
permits marine electronics to send information to computers and to other marine equipment
via a serial interface. A full copy of this standard is available for purchase at their web site
(www.nmea.org). The current version of the standard is 3.01.
GPS receiver communication is defined within this specification. The idea of NMEA is to
send a line of data called a sentence that is totally self-contained and independent from
other sentences. There are standard sentences for each device category and in addition
NMEA permits hardware manufactures to define their own proprietary sentences for
whatever purpose they see fit. All standard sentences have a two letter prefix defining the
device using that sentence type. For GPS receivers the prefix is GP followed by a three
letter sequence defining the sentence contents. All proprietary sentences begin with the
letter P and are followed with 3 letters identifying the manufacturer controlling that
sentence.
NMEA consists of sentences, the first word of which, called a data type, defines the
interpretation of the rest of the sentence. Each data type has its own unique interpretation
and is defined in the NMEA standard. Each sentence begins with a '$' and ends with a
carriage return/line feed sequence no longer than 80 characters of visible text (plus the line
terminators). The data is contained within this single line with data items separated by
commas. The data itself is ASCII text and may extend over multiple sentences in certain
specialized instances but is normally fully contained in one variable length sentence. The
data may vary in the amount of precision contained in the sentence. For example time
might be indicated to decimal parts of a second or location may be shown with 3 or even 5
digits after the decimal point. Programs reading the data should only use the commas to
determine the field boundaries and not depend on column positions. There is a provision
for a checksum at the end of each sentence which may or may not be checked by the unit
reading the data. The checksum field consists of a '*' and two hex digits representing the
exclusive OR of all characters between, but not including, the '$' and '*'. A checksum is
required on some sentences.
There have been several changes to the standard but for GPS use the only ones that are
likely to be encountered are 1.5 and 2.0 through 2.3. Version 2.3 added a mode indicator to
several sentences used to indicate the kind of fix the receiver currently has. The value can
be A=autonomous, D=differential, E=Estimated, N=not valid, S=Simulator. Sometimes
there can be a null value as well. Only the A and D values correspond to an active and
reliable sentence. This mode character has been added to the RMC, RMB, VTG, and GLL,
sentences and optionally some others including the BWC and XTE sentences.
The hardware interface for GPS receivers is designed to meet the NMEA requirements.
They are compatible also with most computer serial ports using RS232 protocols, however
strictly speaking the NMEA standard is not RS232. They recommend conformance to EIA-
422. The interface speed generally can be adjusted but the NMEA standard is 4800 baud
with 8 bits of data, no parity, and one stop bit. All GPS receivers supporting NMEA should
support this speed. Note that, at a baud rate of 4800, you can easily send enough data to
more than fill a full second of time.
At 4800 baud 480 characters per second can be sent. As an NMEA sentence can be as
long as 82 characters this can be limited to less than six different sentences. The actual
Rev No: A4 Page 78