4
On, Off, and Failsafe Operation
The GMA 340 is powered off when the left small knob (5, 7) is rotated fully CCW into the detent.
To turn the unit on rotate the knob clockwise past the click. The left small knobs also functions as the
pilot or copilot’s ICS volume control on the appropriate unit. A failsafe circuit connects the pilot’s headset
and microphone directly to COM 1 in case the power is interrupted or the unit is turned off.
Lighting
LED Button annunciator and marker beacon lamp intensity are controlled automatically by a built-in
photocell on the front panel. Nomenclature backlighting is controlled by the aircraft dimmer buss.
Transceivers
Audio level is controlled by the selected COM radio volume control.
Selection of either COM 1, COM 2, or COM 3 (13) for both MIC and audio source is accomplished
by pressing either COM 1 MIC, COM 2 MIC, or COM 3 MIC (14). The active com audio is always heard
on the headphones.
Additionally, each audio source can be selected independently by pressing COM 1, COM 2, or COM 3
(13). When selected in this way, they remain active as audio sources regardless of which transceiver has
been selected for microphone use.
When a microphone is keyed, the active transceiver’s MIC button LED blinks approximately once per
second to indicate that the radio is transmitting.
Split COM (Not Available on Dual ADF)
Pressing the COM 1/2 button (15) activates the Split COM function. When this mode is active,
COM 1 is dedicated solely to the pilot for MIC/audio while COM 2 is dedicated to the copilot for MIC/
audio. The pilot and copilot can simultaneously transmit in this mode over separate radios. Both pilots
can still listen to COM 3, NAV 1, NAV 2, DME, ADF, and MKR as selected. The Split COM mode is
cancelled by pressing the COM 1/2 button a second time.
When in the Split COM mode the copilot may make PA announcements while the pilot continues
using COM 1 independently. When the PA button is pressed after the Split COM mode is activated, the
copilot’s mic is output over the cabin speaker when keyed. A second press of the PA button returns the
copilot to normal Split COM operation.
If the COM radios utilize a “transmit interlock” system, the Split COM
function may require that this feature is enabled. Refer to the radio’s
installation manual for guidance. GARMIN makes no expressed or implied
guarantees regarding the suitability of the Split COM feature in a given
installation.
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Summary of Contents for 340
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