The FT817 has two interfaces for which a PC can be used. It has a "CAT" interface for remote control, and an audio data interface for various "digital" modes including PSK31 and RTTY. It is widely recommended that an interface to a PC should provide DC isolation: both to avoid noise problems, and to avoid the PC interface carry "fault" currents under various earth fault conditions.
This interface provides both connections - data audio and PC "CAT" remote control. It is fully electrically isolated. Once implemented, it has allowed me to get into the world of digital keyboard modes.
The audio side of this interface is straight from the KK7UQ design. It provides a path from the radio audio data out to PC sound card "microphone" or "line in" connectors; and it provides the reverse path from PC "speaker" or "line out" to radio data input. It uses 600 ohm audio transformers to isolate the two paths, providing complete isolation. A small buffer amplifier and speaker is provided so you can hear the "transmit" audio signal. It's about as linear as a banana, but for simple audio tones it's fine.
PTT control is provided to allow the PC to put the radio into transmit mode. This is done using control lines on a PC serial port. Most programs seem to be programmable about which lines they use, but RTS and DTR are both common. I've use a jumper so that one of these will drive PTT, and the other provides a 5v power source to the interface. My choice is to use DTR for PTT, and RTS to provide power. An opto isolator provides electrical isolation. A centre-off toggle switch allows choice between auto PTT control, PTT off or transmit.
The "data" side of the interface started life as a copy of another design from the internet. It uses optoisolators for both data paths. The design I started with had "non inverting" logic levels, and didn't work. I also had other non-isolated designs using simple transistor interfaces, which inverted the logic levels. I reconfigured how the optoisolators were used so that the signal levels did invert, and hey presto- connection established. Lesson: don't blindly copy other designs! A little more work was needed to get reliable connection at 38400 baud, but it now works well with Ham Radio Deluxe. You will need to set the radio baud rate using menu option 14.
My unit is constructed on stripboard and is mounted in a small plastic case. I've used a 15 pin D connector to connect out to the radio, with screened cables to the mini-DIN connectors.
So far I've used this on RTTY and PSK31. The audio signal to the PC needs its signal level set so that the sound card doesn't get overloaded; most digital interface programs will provide some kind of an indication of that. On transmit the TX audio level needs to be set so that ALC is not registering on the radio TX meter. For the FT817, menu 26 selects which digital mode is used.
I'm using "Digital Master 780" software which is part of the "Ham Radio Deluxe" suite. I haven't got the hang of what to type in which box yet, so it hasn't always gone well - all my doing!