RATBOY, looking in the TCAS 2000 manual of our aircraft (B757), it says that it does work closely together with the Mode S transponder!
A definition of Mode S transponder from my manual:
Mode S
A type of Secondary Surveillance Radar transmission that permits direct interrogation of the transponder form the ground or from the air. Each Mode S transponder equipped aircraft has a unique ICAO registered address by which the transponder may be selectively interrogated. The mode also supports limited, two-way data communication between the aircraft and the interrogating station.
Mode S is the means through which one TCAS 2000 equipped aircraft coordinates avoidance strategies with another TCAS 2000 equipped aicraft.
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I am not 100% sure but the way I read it, every plane that's TCAS equipped (be it version I, II or 2000) has a Mode S transponder. You're able to see that on the flightplan right, Gonzo?