I double-checked that ModemManager was turned off on the Rpi system, and tried again. Still no response on the serial port from the QDX.

I then tried on a macOS system, and it's the same result: serial port (as well as audio i/o) appear when the QDX is connected in "normal" mode, but the serial port does not respond.

It's like the QDX powers up and gets ready to work, but then its internal program just stops running.

I'm thinking of starting a different thread for this, since it's no longer about "not loading firmware", and onto some other issues...