FIFO Unload (FFU)

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 FFU instruction unloads the value from position 0 (first position) of the FIFO and stores that value in the Destination. The remaining data in the FIFO shifts down one position.

Use the FFU instruction with the FFL instruction to store and retrieve data in a first-in/first-out order.

When enabled, the FFU instruction unloads data from the first element of the FIFO and places that value in the Destination. The instruction unloads one value each time the instruction is enabled, until the FIFO is empty. If the FIFO is empty, the FFU returns 0 to the Destination.

Typically, the destination and the FIFO are the same data type. If the types differ, the instruction converts the unloaded value to the type of the destination 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
FIFO	SINT INT DINT REAL String type structure	array tag	FIFO to modify Specify the first element of the FIFO Do Not use CONTROL.POS in the subscript
Destination	SINT INT DINT REAL String type structure	tag	Value unloaded from the FIFO.
Control	CONTROL	tag	Control structure for the operation typically use the same CONTROL as the associated FFL
Length	DINT	immediate	Maximum number of elements the FIFO can hold at one time
Position	DINT	immediate	Next location in the FIFO where the instruction loads data initial value is typically 0

CONTROL Structure

Mnemonic	Data Type	Description
.EU	BOOL	The enable unload bit indicates the FFU instruction is enabled. The .EU bit is set to prevent a false unload when the prescan begins.
.DN	BOOL	The done bit is set to indicate that the FIFO is full (.POS = .LEN).
.EM	BOOL	The empty bit indicates the FIFO is empty. If .LEN is , or = to 0 or .POS < 0, the .EM bit and .DN bits are set.
.LEN	DINT	The length specifies the maximum number of elements in the FIFO.
.POS	DINT	The position identifies the end of the data that has been loaded into the FIFO.

Affects Math Status Flags

Major/Minor Faults

A major fault will occur if:	Fault Type	Fault Code
The specified Length is past the end of FIFO array	4	20

See Common Attributes for operand-related faults.

Execution

Ladder Diagram

Condition / State	Action Taken
Prescan	See FFU Flow Chart (Prescan).
Rung-condition-in is false	See FFL Flow Chart (False).
Rung-condition-in is true	See FFU Flow Chart (True)
Postscan	N/A