Garmin GTX 320 ATC Maintenance Manual page 20

3.3.3.13 SBY (Stand By)
Set the transponder mode select switch to the SBY position. The "SBY" nomenclature and the four
reply code digits shall be illuminated with a higher intensity than the "OFF", "ON", "ALT" and "TST"
nomenclature. All internal power supplies shall be on but the unit shall not reply to any interrogations.
3.3.3.14 ON
Set the transponder mode select switch to the ON position. The "ON" nomenclature and the four reply
code digits shall be illuminated with a higher intensity than the "OFF", "SBY", "ALT" and "TST"
nomenclature.
3.3.3.15 ALT (Altitude)
Set the transponder mode select switch to the ALT position. The "ALT" nomenclature and the four
reply code digits shall be illuminated with a higher intensity than the "OFF", "SBY", "ON" and "TST"
nomenclature.
3.3.3.16 TST (Test)
1. Decrease the RF level control on the ATC-1400A so that the transponder has a 00% reply rate.
2. Rotate and hold the transponder mode select switch in the TST position. "TST", "OFF", "SBY",
"ON" and "ALT" nomenclature shall all be illuminated at the reduced level. The four reply code digits
shall remain illuminated with no change in intensity. The triangle-shaped reply light must turn on and
remain on while the switch is held in the TST position.
3. Release the mode select switch from the TST position. It shall freely snap back to the ALT position
and the triangle-shaped Reply Light shall turn off.