Thinking of adding limit switches to your GRBL machine? Ideally the switches will have to be filtered before they connect to the Uno, even better is if you can optically isolate them - enter the GRBL buffer board. Optically isolated XYX limits.

They also work perfectly for your Z-probe (either use one board, home only X and Y axis and probe your Z or use two boards, with the second filtering your probe, pause and resume buttons)