Garmin GTR 225 Installation Manual page 45

5.2.4.2 NMEA Format
5.2.4.2.1 Legacy NMEA Support
The GTR/GNC maintains backwards compatibility with legacy SL30/SL40 NMEA commands ($PMMRC
and $PMMRV) as they apply to GTR/GNC functionality. Refer to Appendix B.2.
The following legacy SL30/40 input commands are supported as-is:
 Set active COM frequency (SL30 version – only in 25 KHz spacing mode)
 Set active frequency and transceiver function (SL40 version – only in 25 KHz spacing mode)
 Set standby COM frequency. (SL30 version – only in 25 KHz spacing mode)
 Set standby frequency and transceiver function. (SL40 version – only in 25 KHz spacing mode)

Set Omni-Bearing Select (OBS) value.
 Select squelch test (Squelch Override On/Off)
The following legacy SL30/40 input commands are supported with modifications to fit the GTR/GNC
architecture:
 Set active VOR/LOC frequency: Set monitor mode ignored.
 Set standby VOR/LOC frequency: Set monitor mode ignored.
 Set NAV audio mode: Cannot turn off NAV audio.
 Set Volume Level and Audio Control Parameters: Only headphone, sidetone level, and RF squelch
supported.
 Request data output: Cannot set speed. Only unlisted command supported is Request Reset Status
in both versions.
5.2.4.2.2 NMEA GPS Data
The following NMEA standard GPS messages are used by the GTR/GNC for position source:
 Geographic Position, Latitude/Longitude ($GPGLL)
 Track Made Good and Ground Speed ($GPVTG)
5.2.4.2.3 Input Commands
The following input command messages are supported:
 Request data output.
 Set Active NAV frequency (GNC ONLY)
 Set Standby NAV frequency (GNC ONLY)
 Set Active COM frequency
 Set Standby COM frequency
 Set Squelch Override
 Set COM Volume Level and Audio Control Parameters
 Set COM channel spacing

Set NAV audio mode (GNC ONLY)
 Set Omni-Bearing Select (OBS) value (GNC ONLY)
 Set NAV Volume Level (GNC ONLY)
190-01182-02
Rev. B
GTR 225/GNC 255 TSO Installation Manual
Page 5-7