Z zero plate (no replies)

So I want to make a zero plate that I can put on the surface of my material. The plate will be connected to digital pin. Then I would have a lead from the 5v that I would connect to my mill bit. When these two meet the digital pin would go high. My first question is whether or not the ramps 1.4 has an unused digital port that I can use for the signal pin. Second question is if I can access this pin in the marlin firmware and change the default z zero function to my own function?

Thanks