I don't know about all TCAS models, but the Skywatch TAS we have requires other aircraft to have an operating transponder in order to see them, although they can be on any code, not just 1200. I don't know of another way to do it. With no transponder, the only thing that could possibly detect another aircraft is radar, and although we do have weather radar in our mediums, it won't pick up another aircraft well enough to do any good. Some models do use the weather radar for the display, though.