I am still green to the raspberry pi. I am reading a bit on how to add arcade style joysticks to the retro-pi using the GPIO. Simple question. WHen adding buttons and or joystick, the signal being sent from the pi back to the pi via GPIO the 3v+. 5v+ or -. IE when I press a button I am sending a negative or positive signal back to a GPIO pin. I think I have a good reference on how to achive this, but the diagram of GPIO only shows the Positive pins. But it references a common ground to use for all buttons and joysticks. This confused me a bit. If I am sending a negative signal back to the GPIO via the joystick or switch. Which pin should I use?