Contacts (input instruction)
NOP: No Operation
Does nothing and passes power through. A placeholder for a rung that is deliberately empty.
Also called: no-op, placeholder contact.
NOP()What it does
NOP occupies a cell and lets power straight through. It exists so that a place in a rung can be marked as intentionally empty, rather than looking like something that was forgotten.
The useful case is while you are building. A rung roughed out with a NOP where a permissive will go still runs, still shows power flow, and reads to anyone looking at it as a gap that is known about.
Read the name carefully, because it misleads in the direction that matters. NOP does nothing, but it does not stop anything either: power flows through it. An instruction that does nothing and blocks the rung is a different one, and it is AFI.
A rung you can run
A placeholder, next to the instruction that really does disable a rung
XIC(Start_PB)NOP()OTE(Test_Lamp)
AFI()XIC(Start_PB)OTE(Disabled_Lamp)Both rungs look disabled and only one of them is. Press Start_PB: the first lamp comes on, because NOP passed the power straight through the gap, and the second stays dark, because AFI is always false. That is the difference the two names hide.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | NOP |
| Siemens (TIA Portal) | NOP 0 in STL; in LAD, an empty branch |
| Mitsubishi (GX Works) | NOP |
| CODESYS and IEC 61131-3 | No equivalent is needed |
Common mistakes
- Using NOP to disable a rung. It passes power, so the rung carries on working exactly as before. AFI is the instruction that makes a rung false, and it is the one to use when you want a rung switched off but kept.
- Leaving placeholders in a finished program. A NOP that survives into production is a note to yourself that nobody else can read. Either finish the condition or delete the rung.
Practise it
Interactive exercises that use NOP. Each one checks your ladder against a real scan.