FFU — FIFO Unload

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Description

FFU (FIFO Unload) takes the oldest entry off the front of a first-in, first-out queue each time its rung goes from false to true, writes it to Dest, and moves the remaining entries down one place. It pairs with the FFL (FIFO Load) that fills the same queue on the same control tag. Use it at the consuming end of a buffer: the packer, the printer, the station that works jobs off in the order they arrived. Do not unload every scan; the instruction is edge-triggered, and a rung held true takes 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 FFL	Yes	Holds LEN, POS and the status bits.
File	INT or DINT array tag	Tag	Same as the FFL’s	Yes	The queue.
Dest	Writable INT or DINT tag, member or element	Tag	Any	Yes	Where the unloaded entry is written.
Length	Whole number	4	Same as the FFL’s	Yes	Queue 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 unloaded; Dest holds.

Rung-condition-in is true

On the scan the rung turns true, if POS is above 0 then File[0] is written to Dest, elements 1 to POS − 1 move down one place, and POS decrements. If POS is 0 nothing happens and Dest holds. 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[0]
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 FFL; the full table is on the FFL page. From the FFU’s side:

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

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Example

The FFU shares its example with the FFL; the three frames are on the FFL page. In short: Load_Job pushes New_Job (7) into Jobs, Position 1; then Unload_Job takes it out, Next_Job reads 7, Position 0, EM lit.

An unload on an empty queue leaves Next_Job at its old value, so test XIO Jobs_Ctl.EM before acting on it.

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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 an unload. Likely cause: The queue was empty; an unload with POS 0 writes nothing. Fix: Gate the consumer with XIO Jobs_Ctl.EM.

Symptom: The same entry comes out twice. Likely cause: Two unload rungs, or an unload rung that toggles twice per event. Fix: One FFU per queue, driven by a clean rising edge.

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See Also

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

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Notes

Order of rungs. Put the FFL rung before the FFU rung when both can fire on the same scan, so a value pushed and pulled in one scan goes through the queue rather than around it.

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