Sorry, greatwhitehunter, but your understanding is well wide of the mark. Although a Mode S transponder is required for TCAS 2, and although some avionics vendors muddy the water by combining the transponder switch with the TCAS switch, your Mode S transponder operates quite independently of TCAS.
When you are on the ground a Mode S transponder answers to a subset of the possible interrogation types, and reports the fact that it is on the ground. When you are airborne, it answers to all the interrogation types, including legacy Mode A/C interrogations, and reports that it is airborne.
And, to confirm the tongue in cheek replies to rockamora, this behaviour is independent of whether the engines are running, but assumes that the avionics bus has power
Thus, if you are airborne, but the engines are not running, you can squawk 7700 with confidence.
