NOP / AFI — No Operation and Always False

Most instructions read or write tags. These two exist for a different reason: they help you build and debug a program. Neither takes any operands — you place them on a rung from the Program tab of the ladder toolbar, and there is nothing to fill in.

NOP — No Operation

Does nothing. Power flows through it unchanged.

Two everyday uses:

• Placeholder — when you know a rung will be needed but haven't written its logic yet, a rung containing just a NOP keeps the routine valid so everything else can compile and run.
• Documentation rung — a NOP rung exists only to carry a rung comment. Programmers use these as headers between sections of a routine: the comment explains what the next rungs do, and the NOP gives the comment a rung to live on without affecting the machine.

NOP()

AFI — Always False

Forces the rest of the rung false, every scan, no matter what comes before it.

Use it to temporarily disable a rung without deleting anything: place an AFI at the start of the rung, and every instruction after it sees no power. Remove it and the rung works exactly as before. This is a standard trick when testing a machine — modeled on the Always False instruction in Logix 5000® controllers.

XIC(StartButton)AFI()OTE(Motor)

Here Motor stays off even while StartButton is pressed, because the AFI blocks the power before it reaches the coil.

Placed inside a branch, an AFI only kills its own path — parallel paths still work:

[AFI(),XIC(Override)]OTE(Motor)

Motor still follows Override, because only the first path is forced false.

AFI is an input instruction — it conditions a rung, it doesn't complete one. A rung containing only an AFI is an error, because every rung still needs an output instruction. If you want a rung that exists only to hold a comment, that's NOP's job.

How They Differ

	Power in → power out	Typical use
NOP	unchanged	stub out a rung, or carry a rung comment as a section header
AFI	always 0	temporarily disable a rung while testing

Common Mistakes

• Leaving an AFI in finished logic — the rung silently never runs. Search your routine for AFI before you call a program done.
• Putting an AFI alone on a rung — it is an input instruction, so the rung still needs an output and reports an error. A comment-only rung is what NOP is for.
• Keeping dead logic disabled with an AFI forever. If the logic is dead, delete it.