LOA,
TCAS I was around before Mode S, and works today on certain aircraft fits equipped with Mode A/C only transponders - it's fitted to a number of state aircraft types that haven't yet gone Mode S (e.g RAF Tucano fleet). TCAS I is universally discouraged due to it's impact on 1030/1090MHz and limited capability.
However, the original question from I39 was on TCAS II which DOES require a Mode S transponder to be fitted in tandem with the TCAS II equipment.
RS