NorthStar MX500, Operator'S & Installation Manual

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Installation Notes
Version 3.0
217
Installation Manual
MX500 NMEA 0183 Sentences
The NMEA Standard provides for asynchronous transmission, with a
single
Talker and multiple Listeners per line. Typical use includes infor-
mation transfer from electronic positioning and navigational devices to
autopilots, plotters, terminals, printers, etc.
The NMEA 0183 Standard uses an 8 bit ASCII block oriented protocol,
that is not compatible with the NMEA 0180 simple format or the NMEA
0182 complex format due to differences in data format, baud rate and
parity bits.
Data Format
Data are transmitted in different sentences, each containing one or more
data fields. A field consists of a string of characters immediately pre-
ceded by a “,” (comma) character, except for the first (address) field
which is preceded by “$”, indicating the beginning of a new record. Hex
0D 0A <CR>< LF> - end of sentence - must follow the last field in each
sentence.
The data fields are identified only by their position within the sentence
as determined by the field delimiters (comma). Numerical data fields
within a sentence may vary in length from one sentence to another,
depending on the precision available.
Data field position is therefore determined only by counting “,” (com-
mas) rather than counting the total number of characters from the
beginning of the sentence.
The last data field is not followed by a comma delimiter. When a
checksum is used, the last data field is followed by an asterisk “*”,
indicating that the checksum follows. The checksum is the absolute
value calculated by
exclusive -OR’ing the 8 data bits (no start bits or stop bits) of each
character in the sentence between, but excluding “$” and “*”. The
hexadecimal value of the most significant and least significant 4 bits
of the result are converted to two ASCII characters (0- 9, A- F) for
transmission. The most significant character is transmitted first.