LADDER LOGICResources / Ladder logic symbolsUpdated 29 Oct 2026

Ladder logic contact symbols: NO, NC and edge contacts

Contacts are the conditions of a rung. Each one reads a bit, an input, an output or an internal flag, and passes power along the rung when its condition is true.

SymbolIEC 61131-3What it is

Normally open contactPasses power along the rung when its bit is 1 (true).

Normally closed contactPasses power when its bit is 0 (false).

Positive transition contactPasses power for one scan when its bit changes from 0 to 1.

Negative transition contactPasses power for one scan when its bit changes from 1 to 0.

One shot (ONS)The Allen-Bradley one shot: an instruction placed on the rung that passes power for one scan when the rung before it goes true.

Contacts

A normally open contact (Allen-Bradley XIC, examine if closed) passes power when its bit is 1. A normally closed contact (XIO, examine if open) passes power when its bit is 0. That is about the bit in memory, not the wiring of the field device: a stop button wired normally closed is usually programmed with a normally open contact.

Edge contacts, marked P and N, pass power for one scan only when their bit changes from 0 to 1 or from 1 to 0.

Frequently asked questions

Which contacts appear on a ladder diagram?

This legend covers normally open contact, normally closed contact, positive transition contact, negative transition contact, one shot (ONS). Project legends may add more.

Can I download these symbols?

Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.

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Contacts

Frequently asked questions

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