Just use ground and data lines, as you say.
You can drive 5V in to the handset RX line, but ideally only when that is powered. Add a resistor in series to limit the current in case of a problem.
Try 10K and you don't get good levels, 1K.
Or use eg. a 1K pullup to the handset 5V and a transistor to switch the data line low - that's actually even better, as nothing can be accidentally powered, and you need that inversion anyway, I believe.
As long as the internal grounds/0V are connected together, the power source should not matter.
You can drive 5V in to the handset RX line, but ideally only when that is powered. Add a resistor in series to limit the current in case of a problem.
Try 10K and you don't get good levels, 1K.
Or use eg. a 1K pullup to the handset 5V and a transistor to switch the data line low - that's actually even better, as nothing can be accidentally powered, and you need that inversion anyway, I believe.
As long as the internal grounds/0V are connected together, the power source should not matter.