I had wondered why no one produced a solid state stand alone ADSB transponder that could be fitted to an aircraft with not much beyond a radio in the way of electronics.
Mode S requires input from a C146a GPS unit (ie WAAS, which for GA is currently only Garmin GNS 430, GTN650 or GTN 750). I presume it also requires new altitude encoder with digital output for the GPS unit (gray scale??). WAAS units also require different antennae & coax cabling than non WAAS units.
This will be an expensive conversion for those who require it.