Don't nail me on the A330 description, but whe I remember right, it's similar with the A320
The control/indicating function is assumed by the ATC/TCAS control unit.
(1)
In normal operation, you select the ATC 1(2) through the 1/2 selector switch on the control unit in either position 1 or 2.
A switch STBY/AUTO/ON enables the functions given below:
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in STBY position: the two transponders are electrically supplied but do not operate
-
in AUTO position: the selected transponder operates in flight but operates only in mode S on ground
-
in ON position: the selected transponder operates when the aircraft is in flight or on ground.
Rgds