Originally Posted by
QuangVuong
This is a theory which will work with Channel 3 if that channel is a analogue one. If it's digital it may not work.
I already wrote it. This will work only with support in firmware.
In hardware there is resistor somewhere connected between CH3 button and +5V. Another pin of CH3 button is conneted to GND. And that pin of CH3 button with resistor is connected to microcontroller to ADC pin, so it will be possible to use it in new firmware (after writing code for it).