NO vs NC

[JAZZCNC]

Damnit!
I still dont understand it.. I think its backwards!

Doesn't matter really what you think the fact is only works one way.

Always think in terms of NO power and open or closed states. So when says NO (normally open) the switch is open when no power is applied. Soon as power is applied it flips to opposite state.!

Ok I think might figured whats confussing you.?

Are you trying to figure out swithces or Relays.? Both work on same principle but do confuse people.

When talking limits or E-stop best to think in terms of completing a Circuit. For the Circuit to be closed NC switches are required then in which case if switch is opened or wire borken the circuit goes open and controller knows about. For this to work you would set the Input active state to be Active low ie: if circuit goes low/open ie no voltage is seen the control goes into E-stop/fault mode.

The opposite can be done with NO switch and Active high state. The input state will then go active when it see's voltage. Down side being if wire breaks the control never knows about it.
NO sw are mostly used on things like Home Sw or ATC etc when you want to inform control something as happened or completed.

Relays however while working the same do confuse people because they flip state only when power is applied so really you work kind of in reverse.
Where you would use a relay is say with fault signal from the drives.
In this case you would run say E-stop wire thru NC contacts on relay who's coil is connected to drive fault signal output. Then if the drive goes into fault it would turn ON the relay which in turn OPEN the NC contact and E-stops the machine.

End of the day it's just Binary Logic states. ON/OFF OPEN/CLOSED LOW/HIGH all the same.