FIFO Load (FFL)

The FFL instruction copies the Source value to the FIFO.

Use the FFL instruction with the FFU instruction to store and retrieve data in a first-in/first-out order. When used in pairs, the FFL and FFU instructions establish an asynchronous shift register.

Typically, the Source and the FIFO are the same data type.

When enabled, the FFL instruction loads the Source value into the position in the FIFO identified by the .POS value. The instruction loads one value each time the instruction is enabled, until the FIFO is full.

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

The FFL instruction operates on contiguous memory.

The data is constrained by the specified member.

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

Typically, the Source and the FIFO are the same data type. If Source and FIFO 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

Conversion only occurs if the type of the source operand does not match the type of the FIFO.

Ladder Diagram

Operand	Type	Format	Description
Source	SINT INT DINT REAL String type structure	immediate tag	Data to be stored in the FIFO
FIFO	SINT INT DINT REAL String type structure	array tag	FIFO to modify Specify the first element of the FIFO
Control	CONTROL	tag	Control structure for the operation Typically use the same CONTROL as the associated FFU
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
.EN	BOOL	The enable bit indicates the FFL instruction is enabled.
.DN	BOOL	The done bit is set to indicate that the FIFO is full. The .DN bit inhibits loading the FIFO until .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 word specifies the maximum number of elements in the FIFO.
.POS	DINT	The position word identifies the location in the FIFO where the instruction loads the next value.

Affects Math Status Flags

Major/Minor Faults

A major fault will occur if:	Fault Type	Fault Code
The (starting element + .POS) 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 the FFL Flow Chart (Prescan).
Rung-condition-in is false	See FFL Flow Chart (False)
Rung-condition-in is true	See FFL Flow Chart (True)
Postscan	N/A