Garmin GNS 480, Installation Manaul

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Appendix B - Serial Interface Specifications
GNS 480 (CNX80) Installation Manual 560-0982-01 Rev L
Figure B-1. Altitude Data Input
B.1.1.2 Shadin Altitude Format
The Shadin altitude format is 9600 baud with a message length of 17 characters, as defined below:
RMS<sp><+/->12345T<+/->12ul<CR>
where:
RMS ASCII characters
<sp>
space (0x20)
<+/->
sign indicator (0x2b["+"] or 0x2d["-"])
12345 altitude in feet
T
ASCII character
<+/-> sign indicator
12
sensor temperature
ul checksum of bytes 1 through 14 in hex ASCII (e.g., "FA")
<CR>
carriage return (0x0d)
Note: Checksum is calculated by adding each byte in the message (1 through 14).