Headphone port of Pinephone can switched to serial port with dip-switch, I know there is cable for USB-port that make it look to PC as serial port.

But my question what hardware needed to make headphone-port more like standard RS232C serial port (accept +/- 15V input) or midi i/o ports (31,250 baud 5V) or RS-422 (+/- 6V), now port is 3.3V. I think software side is ready for that.