The Elecraft KX3 is a bit unique for interfacing with a transceiver because it has both mic/audio connections as well as an I/Q connection for external panadapters. Tests by early adopters have shown that it's a bad idea to connect the inputs/output directly between the rig and PC as ground hum and noise results. You can homebrew your own connections with isolation transformers to resolve this.
If you happen to have a Rigexpert Standard, the basic interface diagram is now available. This cable not only handles soundcard interface, but rig control / programming, as well. One thing that this diagram omits is the 2.5 mm I/Q connection. To add that, connect a cable to Pin 6, which is the sub receiver output and adjust the gain with the Input 2 knob on the front:
If you happen to have a Rigexpert Standard, the basic interface diagram is now available. This cable not only handles soundcard interface, but rig control / programming, as well. One thing that this diagram omits is the 2.5 mm I/Q connection. To add that, connect a cable to Pin 6, which is the sub receiver output and adjust the gain with the Input 2 knob on the front: