190-01182-02 GTR 225/GNC 255 TSO Installation Manual
Rev. F 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 characters (30h-3Fh). [1]
<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.
Note:
[1] Encoded hex: each character consists of 4 bits of data placed in the low order nibble +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.
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