LIFO Unload (LFU)

This table lists the controllers and applications that support this instruction.

Architecture	Standard applications	Safety applications
CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, and GuardLogix 5570 controllers	Yes	No
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers	Yes	Yes

The LFU instruction unloads the value at .POS of the LIFO and stores 0 in that location.

Use the LFU instruction with the LFL instruction to store and retrieve data in a last-in/first-out order.

When enabled, the LFU instruction unloads the value at .POS of the LIFO and places that value in the Destination. The instruction unloads one value and replaces it with 0 each time the instruction is enabled, until the LIFO is empty. If the LIFO is empty, the LFU returns 0 to the Destination.

IMPORTANT: You must test and confirm that the instruction does not change data that you don’t want it to change.

The LFU instruction operates on contiguous memory. The scope of the instruction is constrained by the base tag. The LFL instruction will not write data outside of the base tag but can cross member boundaries. If you specify an array that is a member of a structure, and the length exceeds the size of that array you must test and confirm that the LFL instruction does not change data you do not want changed.

The data is constrained by the specified member.

If the instruction tries to read past the end of an array, the instruction sets the .ER bit and generates a major fault.

Typically, the Source and the LIFO are the same data type. If Source and LIFO data types mismatch, the instruction converts the Source value to the data type of the FIFO tag.

A smaller integer converts to a larger integer by sign-extension.

Available Languages

Ladder Diagram

Operands

There are data conversion rules for mixed data types within an instruction.

Ladder Diagram

Operand	Type	Format	Description
LIFO	SINT INT DINT REAL String type structure	array tag	LIFO to modify Specify the first element of the LIFO Not use CONTROL.POS in the subscript
Destination	SINT INT DINT REAL String type structure	tag	Value unloaded from the LIFO.
Control	CONTROL	tag	Control structure for the operation Typically use the same CONTROL as the associated LFL.
Length	DINT	immediate	Maximum number of elements the LIFO can hold at one time
Position	DINT	immediate	Next location in the LIFO where the instruction unloads data Initial value is typically 0

CONTROL Structure

Mnemonic	Data Type	Description
.EU	BOOL	The enable bit indicates the LFU instruction is enabled.
.DN	BOOL	The done bit is set to indicate that the LIFO is full (.POS = .LEN).
.EM	BOOL	The empty bit indicates the LIFO is empty. If .LEN < or = to 0 or .POS < 0, both the .EM bit and .DN bit are set.
.LEN	DINT	The length specifies the maximum number of elements the LIFO can hold at one time.
.POS	DINT	The position identifies the end of the data that has been loaded into the LIFO.