190-01182-02
GTR 225/GNC 255 TSO Installation Manual
Rev. B
Page B-5
B.2
RS-232 NMEA Data Format
B.2.1 Electrical Interface
The I/O signals are compatible with RS-232C. Data is generated at 9600 baud with a word length of 8 bits,
one stop bit, and no parity.
The data format for the serial communication is:
Baud rate
9600
Data bits
8
Stop bits
1
Parity
None
B.2.2 Message Formats
All messages conform to the NMEA 0183 proprietary message format as follows. All characters will be
standard ASCII characters. No binary data characters are used.
“$” .........................Start of message character, ASCII “$” (024h).
“P” ..........................Proprietary message identifier.
“GRM” ...................Garmin company identifier.
c...............................Message class identifier; Identifies a message as either a COMM or NAV
message. The GTR and COM portion of the GNC use “C”, while message for
the NAV portion of the GNC use “V”.
nn ..........................Message identifier, two-digit number in ASCII characters.
d.....d .....................Message data characters defined for each message.
chksum ..................Message checksum, including message identifier (nn) through data characters
(d…..d). The two-digit checksum is generated by adding all values of valid
characters together, ignoring carry (if any). This value is converted into two
encoded hex
1
characters (30h-3Fh).
<CR> .....................ASCII carriage return (0Dh).
<LF> .....................ASCII line feed (0Ah).
The maximum message length, including the start of message character (“$”) and the end of message
<CR><LF> sequence, is 25 bytes.
1.
Encoded hex: each character consists of 4 bits of data placed in the low order 30h. For example, the
8-bit value 5Fh would be encoded as two characters with values of 35h and 3Fh, which map to the ASCII characters
“5” and “?”, respectively.
Summary of Contents for GNC 255
Page 1: ...190 01182 02 March 2013 Revision B GTR 225 GNC 255 TSO Installation Manual ...
Page 2: ......
Page 137: ......
Page 138: ......
