Garmin GNC 255 Installation Manual - page 71
190-01182-02
GTR 225/GNC 255 TSO Installation Manual
Rev. B
Page 6-17
6.6.4 VHF COM
6.6.4.1 Antenna Check
If desired, the antenna VSWR can be checked using an inline wattmeter in the antenna coaxial using
frequencies near both ends of the band. The VSWR should be less than 2:1. A VSWR of 2:1 will cause a
drop in output power of approximately 12%.
6.6.4.2 Receiver/Transmitter Check
Tune the unit to a local VHF frequency and verify the receiver output produces a clear and understandable
audio output. Verify the transmitter functions properly by contacting another station and getting a report of
reliable communications.
6.6.4.3 Aux Audio Input Check
This check is only required for installations with an aux audio source connected to pins P2001-3 and -19.
1. Connect an audio source to the Aux Audio input (P2001-3 and -19).
2. Verify that AUX audio is heard over the pilot headset position.
6.6.4.4 Database Check
Check the Frequency Database to ensure it is current. The database information is displayed during the unit
display start-up sequence. To check the database:
1. Cycle power on the GTR/GNC. The Unit will go through its normal start-up sequence.
2. Press FUNC and select SYS configuration with the larger outer knob. Rotate the smaller inner
knob to select DATABASE INFO, and press the ENT button.
3. Verify that the effectivity (EFCTV) date has not passed for the frequency database.
6.6.4.5 RS-232 Serial Interface Checks
The interfaces to RS-232 equipment such as the GTN 6XX/7XX or GNS 400W/500W series GPS sources
should be checked as follows:
1. Operate the connected GPS source and the GTR/GNC in normal mode. Ensure the aircraft has a
clear view of the sky for this check. This check should not be performed in a hangar.
2. Ensure the connected GPS source has a valid GPS satellite fix.
3. Press the FUNC button and rotate the smaller inner knob to the Nearest Airport menu.
4. Verify that ‘NO GPS’ is not displayed on the screen. If the GTR/GNC displays ‘NO GPS’, verify
the wiring between the GPS source and the GTR/GNC.
The interfaces to RS-232 equipment such as the G500/G600 or other serial interface should be checked as
follows:
1. Operate the connected serial remote tune source and the GTR/GNC in normal mode.
2. Ensure that the remote source is able to display data from the GTR/GNC.