Hi ixo65

You have to reconsider “Stop button” using “NC contact” in the program.

Let´s talk about this using Ladder language, only a “button” as an “input I1” and a lamp as an “output Q1”.

A programed “NC contact” must receive “1” to switch output to “0”. That´s OK.

Physically we will do the connections. Attempting the NC logic in the program, we have to use a “NO button”. When the operator press the physical button, the NC programmed contact will open and the lamp turns off. That’s OK.

But an accident occurs. The wire connection from the voltage line to the “input I1” was brake for some reason. When the operator will press again the button, there is no voltage in “input I1”. The program won’t recognize nothing and Q1 will be turn on.