Hi there,
I wonder if there is any free pin available (using all 3 parallels) on Electrodragon v2 board I would like to turn on/off the panel power supplies from the rPi and if there is any other pin left, a brightness sensor 
If you are using /32 panels (ABCDE), then most/all pins will be used. There might be a TX pin left, you’d have to check all the pins on the rPi and see if any is left.
If you aren’t using ABCDE but only ABCD, you have the 3 E pins that are free
Thanks for reply, the RX pin I think, even in the rpi4 is not possible to use as GPIO right?
About 3 E Pins, can you help me identify them? Guessing electrodragon V2 board has the same schematics as Active Adapter 3 , I don’t see any E lines at the 40pin header
I think you can probably use the RX/TX pins as GPIO if you turn off the serial port in the kernel, but I’ve never tried.
Electrodragon is the active-3 adapter, correct.
I was wrong about 3 pins, you only get one pin back if you are ABCD See on the left, the header, an dlook for row_E connected to pin10.
Thanks Marc for your help, now I understand that, If don’t connect pin10 (GPIO15) Jumper you have it free to use, it’s the RX pin but as you said disabling serial port give you the chance to use it as regular io
I wonder if using as OUTPUT will interference with A0 pint from U2 chip, perhaps would be a good idea to just cut the trace before test 
I think it works without cutting the trace because I use ABCDE and if I run minicom, I see the panel output on the serial port.