Endstop on PS1

[DanielYWoo]

PS1 in the diagram connects to an infrared sensor as an endstop. How does it work? I am kind of confused because I think we need to know the min and max positions on both ends of the opto disk. I see there is a gap in the opto disk, I am not sure, do we use the gap for min/max detection and homing?

[DanielYWoo]

And PS6 connects to a 3-pin mech switch as the endstop, I guess a 2-pin switch works too if we enable the built-in pull-up resistor on the Arduino board, right? That saves a wire.