Why the stop button is wired NC but shows NO in ladder
It looks backwards the first time you see it. It is not.
Step 1: the wiring
A stop button is wired normally closed (NC). When nobody touches it, current flows through it to the PLC input. Pressing it opens the circuit.
Why? If the wire breaks or a terminal comes loose, the circuit also opens, and the machine stops. A broken wire fails safe. If the stop were wired NO, a broken wire would mean the stop button silently does nothing.
Step 2: the input bit
Because current flows at rest, the PLC input bit (call it STOP_OK) is 1 when nobody presses stop, and 0 when someone does (or the wire breaks).
Step 3: the ladder contact
A ladder contact does not describe the physical button. It asks a question about the bit:
- NO contact (-| |-): "is the bit 1?" → passes power when true.
- NC contact (-|/|-): "is the bit 0?"
You want the motor rung to pass power while STOP_OK is 1, so you use an NO contact on STOP_OK. Physically NC button, logically NO contact.
START STOP_OK MOTOR --| |--+----| |------------( )-- MOTOR | --| |--+
The common mistake
Drawing an NC contact for the NC button gives a double inversion: the motor runs only while stop is pressed (or the wire is broken). If a ladder "works backwards", check this first.
Real emergency stops go further: they use safety-rated devices and a safety relay or safety PLC, not just a standard input.
Service Call: PLC Trainer walks through this in Basics lesson 2b.