When I initially connected my IC-7300 to my Elecraft KAT-500 Antenna Tuner, I was unable to get the Tuner to start Tuning the KAT-500 from the TUNE button on the front panel. Here is what I discovered:
I was using separate 12 volt power from my Station power supply for the Tuner. Only the Tune Data connection was made from the AH-4 Tuner Connector to the KAT-500 (along with RF connections, of course). This seemed to work fine if I use the TUNE button on the KAT-500, but the IC-7300 behavior was as if it id not realize there was an external Tuner connected.
In discussions with several others about this, Fred WB4QOC suggested that maybe I need to use 12 volts from the IC-7300 for the Tuner power. It turns out that this solved my problem. I did that, and now the TUNE button on the IC-7300 acts as desired, tuning the KAT-500.
Why did this happen? My guess is that the IC-7300 must see current draw on the Tuner (AH-4) port 12-volt line, to know that an external Tuner is in use. Note that this is NOT the case with the IC-7600. I think I have heard that this is also NOT the case with the 7610, but have no experience there.
In the end, it all works great IC-7300, KAT-500 and KPA-500 Amp.