How to keep a Garmin GTX330 turned off when powering on avionics? There seems to be an option in the GTX 330 settings that keeps to unit from powering on when receiveing power from the power system. 

I flew in two different airplanes and one immediately powered on the transponder, the other one keeps the transponder turned off, until I press ON. 

Looking thru the Pilot's Guide and Installation Manual didn't give me any clue. 

Can someone help? Seems like an undocumented feature.
If pin number 1 on the back of the GTX330 unit is connected to power (+), the unit will turn on. If it is not, you will have to manually press the on button. If you remove power from pin 1, it will not turn it off.
See the installation manual, which is findable by a simple Google/Bing search, page D-1, figure D-1, Note 3.