NigD2, if a mode-C equipped aircraft gets too close to a TCAS-equipped aircraft, the TCAS will not give either party any avoidance instructions.
It will tell the pilot of the TCAS-equipped aircraft that you're there. But, unless both parties have Mode S, neither will receive any instructions. Unless you have Mode S, the other aircraft doesn't know what you're doing, so it's not able to give avoidance advice.
FFF
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