AFI — Always False Instruction

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Description

AFI (Always False Instruction) is an input instruction that never passes power. Placed in series on a rung it makes everything to its right false: an OTE goes off, a timer resets, a counter stops counting, exactly as if the rung’s own conditions had gone false. Use it to take part of a rung, or a whole rung, out of service without deleting it: to hold an output off while the rest of the program is tested during commissioning, or to park logic you plan to finish later. Do not leave an AFI in finished logic; an AFI nobody remembers is the classic “why won’t this output turn on” fault. AFI replaces the NOP of older platforms, which LadderIDE does not have.

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Operands

Operand	Type	Format	Valid Range	Required	Description
None	—	—	—	—	AFI has no tag and no dialog. Dragging it onto a rung places it at once.

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Scan Behavior

Prescan

Nothing.

Rung-condition-in is false

The AFI is false. Everything to its right is false.

Rung-condition-in is true

The AFI is still false. Everything to its right sees a false rung-in, so the outputs there behave as they do on any false rung: OTE coils turn off, timers and counters take their rung-false action, latches and one-shots hold.

Postscan

Nothing.

Conditions to the left of AFI	AFI	Outputs to the right
False	False	Off, as on a false rung
True	False	Off, as on a false rung

Inside a branch. AFI blocks only the leg it is on. In an input branch the other legs still pass power, so the rung can still be true through them. In an output branch leg it de-energizes that leg’s outputs and leaves the other legs alone.

Position. AFI must come before the outputs it is meant to block. An AFI placed after an output on the main line, or after an output within a branch leg, is a build error: AFI instruction must be moved in front of the output.

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Example

Scenario: A pump’s run rung is finished, but the pump is not wired yet. An AFI keeps the output off while the request logic upstream is tested.

Tags:

• Pump_Req — Pump request, BOOL
• Pump_Run — Pump run output, BOOL

Rung 1:

—|XIC Pump_Req|—|AFI|———(OTE Pump_Run)———

Scan 1 — Pump_Req = 0. The XIC does not conduct. Pump_Run = 0.

Scan 2 — Pump_Req = 1. The XIC conducts and lights, the AFI does not, and the wire beyond it stays dark. Pump_Run = 0.

Values before and after: Pump_Run is 0 in both scans. When the pump is wired, delete the AFI and the rung works as drawn.

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See Also

• JSR — Jump to Subroutine (run or skip a whole routine)
• XIC — Examine If Closed (the condition an AFI usually follows)
• OTE — Output Energize (turns off behind an AFI)
• OTL — Output Latch (holds its bit behind an AFI)
• Program Control Instructions — Category index

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Notes

AFI or Disable Rung? Right-click a rung number and choose Disable Rung to leave a rung out of the build altogether: the controller never writes its outputs, so they keep whatever value they had. An AFI keeps the rung in the scan and actively drives its outputs false every cycle. Use Disable Rung to freeze a rung, AFI to force it off.

Behind an AFI, false means false. A TON behind an AFI holds at zero, an RTO keeps its accumulator, a CTU counts nothing, an OTL keeps its bit set and an OTU leaves its bit alone. Each instruction takes its normal rung-false action; nothing is skipped.

A rung still needs an output. An AFI on its own, with no output instruction after it, is the same build error as any other rung without an output.

Allowed inside an MBS body. A block’s logic may carry an AFI like any other condition.

Find them before shipping. Search the program for AFI before a project is released. An AFI on a rung is easy to see at 150 %, and easy to miss in a routine nobody opens.