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Fundamentals·6 min read·

Normally open and normally closed, in the field and in the code

The most common wiring mistake in automation comes from a contact meaning one thing on a drawing and another in a rung.

Short answer

A normally open contact passes power when its bit is 1; a normally closed one passes power when its bit is 0. The confusion is that the same words describe the physical switch, and a normally closed stop button sends 1 when healthy. So a healthy stop button is read with a normally open instruction, which looks backwards until you separate the device from the instruction.

The one-scan lagRUNG 1LateEarlyRUNG 2StartLateone scan laterrung 1 already ran

Two things share the same two words, and mixing them up is the most common wiring error in the trade.

The device

A normally open pushbutton has no continuity until you press it. A normally closed one has continuity until you press it. That is a fact about the switch on the panel and it has nothing to do with your program.

The instruction

In ladder logic, an examine-if-closed instruction passes rung power when the bit it reads is 1. An examine-if-open instruction passes power when the bit is 0. That is a fact about the code.

Where they collide

A stop button should be wired normally closed, so that a cut wire stops the machine instead of leaving it running. Wired that way, the input is 1 when everything is healthy and 0 when somebody presses stop.

Which means the rung reads it with an examine-if-closed instruction, and to a beginner the ladder appears to say 'run while stop is true'. It does. Stop being true means the stop circuit is intact.

Write the meaning in the tag name. Stop_PB_NC or Stop_Healthy costs nothing and answers the question before it is asked.

The rule worth memorising

Anything whose failure should stop the machine gets wired normally closed: stop buttons, e-stops, guard switches, thermal overloads. Anything whose failure should do nothing gets wired normally open: start buttons, jog, reset.

Common questions

Should a stop button be normally open or normally closed?
Normally closed, always. A normally closed stop button passes current when healthy, so a broken wire or a failed contact stops the machine rather than leaving it running. This is fail-safe wiring and it is the reason the instruction in the rung looks inverted.
Why does my stop button use an XIC instruction?
Because the button is wired normally closed, so the input is 1 when the button is not pressed. Examining for 1 means the rung conducts while the button is healthy. Using XIO there would run the machine only while somebody held the stop button down.

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