I believe the XPDR is not functioning because I need to connect the Power ON pin (13) to a ground pin to get the transponder to power up. This can simply be a permanent link from pin 13 to one of the ground pins or it can be through a switch on the panel. Be careful here – the Power ON pin is NOT the same as the Controller Power pin. If you connect Controller Power to ground you create a dead short. (All other pins on the DB-25 as per the Trig documents.)