In article , EDR
wrote:

Sounds like TCAS

TCAS only works with Mode-S transponders.

Who told you that? A Mode C transponder looks the same on a TCAS
screen as Mode S. The only difference is that two Mode-S-equipped
airplanes are able to coordinate a TCAS maneuver (so both airplanes
won't get told "climb" and climb into each other).

TCAS will interact and with Mode C transponders just fine, and will
even show and give a traffic advisory for aircraft with no altitude
encoding at all.

--
Garner R. Miller
ATP/CFII/MEI
Manchester, CT =USA=