Chuck,

Higher voltage provides better immunity to noise.  Industrial machines are usually wired with +12 or +24vdc logic.  Many times, users are retrofitting machines that already have components installed using +12vdc or +24vdc logic.

Why would one bother to use a 24V power supply for the limit switches, instead of just using the board's 5V power supply?

And also, why do we use 20k resistors for the 24V setup, and only a 100 ohm resistor for the 5V? Even with the change in voltage and the resistors in parallel it seems like a huge difference in final current.

http://cnc4pc.com/Tech_Docs/3_C10R10.pdf

http://cnc4pc.com/Tech_Docs/C10%20+%20Mechanic%20Switch.pdf

Thanks!