I thought I understood your diagram but apparently not :/ So let's break it apart step by step:

The CN1 is the BOB's (CN == "Common 1" or "Input 1" ? + ground). You've also got a ground from each switch to the star point.

Now, how I understand its functioning:

  1. In the initial state, current flows through SW1 to SW4 and then goes back to the BOB. The circuit is NC and Mach3 knows that limit switches are set to NC. Mach3 also is told that there is an override switch in Normally Open circuit.
  2. Imagine SW1 triggers. The circuit opens, Mach3 notices and stops the machine.
  3. Mach3 won't let you move the machine until you use the limits override. So, you press on SW4, which opens the switches circuit and closes the buzzer circuit. The buzzer starts beeeeeeeeeeeeeeping.
  4. You move your gantry or whatever triggered, or home your machine to be safe and reset Mach3. This releases the triggered switch.
  5. You move SW4 to its original position and the limits are now back into their original state. Mach3 notices and is now happy :)


I might be wrong on #4 and #5. I don't understand why you have a 12V buzzer. Doesn't the BOB send 5V through the limits circuit ? Therefore, having a 5V buzzer would be enough ? It seems to me like your diagram causes the BOB to take 12V in when you press the override switch. I'm not sure the BOB can take that much on the input pins (thought I know it can on the spindle control interface).

Now if you look at my diagram, it acts similarly to yours, I think :/ ? When a limit is triggers, it opens the circuit and you have to put the override switch in position 2, which closes the buzzer circuit. The buzzer is now powered with the BOB 5V (unless I'm wrong about the BOB sending 5V through those pins).

Pleas forgive my lack of understanding :)