This is nothing special just a standard interface for which circuit details can be found on the web.
It is however fully isolated grounds between the PC and the HF rig, so to prevent ground loops.
Isolation is implemented by audio transformers in the RX TX audio lines and an opto-isolator chip in the PTT connection.
I'm also using a cheapo usb headphone sound 'card' instead of the main inboard soundcard, thus freeing it up for normal used. The usb card also has a nice low output which needs little attenuation.
Software at the moment is Fldigi (linux) which features a bewildering array of modes and features.I think my Morse keying is faster than my typing though :)