TX and red means that the FT-817 is in transmit mode. Otherwise there ist a green RX icon. The PTT state is detected by PTT GND signal from FT-817.
Low state of CTS or DSR or DCD or RI from TTL/USB-Converter will be indicated as TX.
Partial success today. I tried a third TTL/USB converter and finally the FT817 tx frequency changes when I change the rx frequency on the raspi!
The only issue now is that the icon does not turn red on transmit. I tried connecting TX GND to CTS, DSR, and DCD, none made any difference, icon stays green on TX. I even tried directly grounding CTR, DSR, and DCD, still no red icon. But this issue does not seem very important. The only thing that changes when the raspi detects that the FT817 is transmitting is that the speaker is muted or the volume is lowered, correct? I can live without that.
Odd that the icon always stayed red with one converter and now always stays green with the other.