LFU — LIFO Unload

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Description

LFU (LIFO Unload) pops the newest entry off a last-in, first-out stack each time its rung goes from false to true and writes it to Dest. It pairs with the LFL (LIFO Load) that fills the same stack on the same control tag. Use it to step back through recorded positions or undo the most recent change. Do not unload every scan; the instruction is edge-triggered and a rung held true pops exactly one entry.

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Operands

Operand	Type	Format	Valid Range	Required	Description
Control	CONTROL tag	Tag or Tag[n]	Shared with exactly one LFL	Yes	Holds LEN, POS and the status bits.
File	INT or DINT array tag	Tag	Same as the LFL’s	Yes	The stack.
Dest	Writable INT or DINT tag, member or element	Tag	Any	Yes	Where the popped entry is written.
Length	Whole number	4	Same as the LFL’s	Yes	Stack capacity.
Position	Whole number	0	0 to Length	No	Entries at power-up; normally 0.

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Scan Behavior

Prescan

LEN is loaded from Length and POS from Position.

Rung-condition-in is false

EU is cleared. Nothing is popped; Dest holds.

Rung-condition-in is true

On the scan the rung turns true, if POS is above 0 then POS decrements and File[POS] is written to Dest. If POS is 0 nothing happens. EM is set while POS is 0, DN while POS equals LEN.

Postscan

Nothing.

Condition	EU	DN	EM	POS	Dest
Power-up	0	0	1	0	holds
Rung false	0	holds	holds	holds	holds
False → true, POS > 0	1	0	POS = 0 after	−1	File[POS] after the decrement
False → true, POS = 0	1	0	1	holds	holds
RES on the control	0	0	0	0	holds

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Member Bits

The control tag is shared with the LFL; the full table is on the LFL page. From the LFU’s side:

Bit	Mnemonic	Data type	Set by	Cleared by	Operation description
Enable unload	.EU	BOOL	The LFU rung being true	The LFU rung being false, or RES	Mirrors the unload rung and holds its edge memory. .EN belongs to the LFL.
Done	.DN	BOOL	POS reaching LEN	A pop, or RES	The stack was full; one pop clears it.
Empty	.EM	BOOL	POS at 0 after a pop	A push, or RES	Nothing left to pop; Dest keeps its last value.
Position	.POS	INT	Each push (+1)	Each pop (−1), RES (to 0)	Entries on the stack.

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Example

The LFU shares its example with the LFL; the three frames are on the LFL page. In short: Push records Push_Val (9) on Stack, Position 1; then Pop returns it, Pop_Val reads 9, Position 0, EM lit.

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Control Tag Errors and Troubleshooting

Error definitions

.ER cause	Condition that sets it	.POS / status value	Meaning
None	.ER is never set by LadderIDE	—	A File that is not an INT/DINT array, a Length of array size or more, or a third instruction on the control tag is refused at validation.

Troubleshooting hints

Symptom: Dest shows a stale value after a pop. Likely cause: The stack was empty; a pop with POS 0 writes nothing. Fix: Gate the consumer with XIO Stack_Ctl.EM.

Symptom: The popped value is the one pushed first, not last. Likely cause: An FFU was placed instead of an LFU, or the loads went through an FFL. Fix: Both halves must be the LIFO pair on one control tag.

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See Also

• LFL — LIFO Load (the full member table and the example)
• FFU — FIFO Unload (oldest entry first)
• RES — Reset
• Shift/Sequencer Instructions — Category index

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Notes

Popped entries stay in the array. Only POS changes; the element keeps its value until the next push overwrites it.

The block colours neutral. Watch the rung wire, the Position row and the DN/EM indicators.