Garmin GNS 480 Installation Manaul page 149

B.2.1.4 NavComm Data
The definition of the NavComm portion of the MapCom data output is as follows. This data output is
compatible with the SL30 or SL40 for sending frequency data to the COM or nav unit.
The COM data is output using the following format.
$PMRRC<msg_id><msg_data><chksum><cr>
$PMRRV<msg_id><msg_data><chksum><cr>
The checksum is computed by an 8 bit addition of the msg_id and msg_data characters, ignoring carry if
any. The resulting 8 bit checksum is converted to two ASCII characters by taking the upper and lower
nibbles, adding 30h to each, and placing the most significant character first in the data message.
B.2.1.4.1 Airport Ident Output
This message is used to output the selected airport ident.
Message Format
$PMRRC04tiiii<chksum><cr>
04...............message id
t..................list type, outputs a 1
iiii ..............ident, four character ASCII
Example Message
$PMRRC041SLE<space>99<cr>
Output ident of "SLE" for the following frequency information.
B.2.1.4.2 Frequency Data Output
This message is used to output the airport frequency information for the previously output ident.
Message Format
$PMRRC05tfmk<chksum><cr>
05 message id
t list type, input 1
f frequency type:
0 = TWR, tower frequency
1 = GND, ground frequency
2 = ATS, for ATIS
3 = ATF, air traffic frequency
4 = APP, for approach
5 = ARR, for arrival
6 = AWS, automatic weather station
7 = CLR, clearance/delivery
mk frequency:
m = desired frequency in MHz in hexadecimal, where m = desired frequency - 30h , with the
desired frequency in the range of 118 to 136 MHz, or 162 MHz.
k = desired frequency in kHz where k = (desired frequency / 25 kHz) + 30h, with the desired
frequency in the range of 000 to 975 kHz in 25 kHz steps, or 0 to 39.
GNS 480 (CNX80) Installation Manual
Appendix B - Serial Interface Specifications
8 = CTF, common traffic advisory frequency
9 = DEP, departure
: (3Ah) = FSS, flight service station
; (3Bh) = RFS, for remote flight service station
< (3Ch) = UNI, for unicom
= (3Dh) = MF, mandatory frequency
> (3Eh) = CTR (Center)
? (3Fh) = undefined, for other frequency types
560-0982-01 Rev L