Control Structure - Allen-Bradley Logix5000 Reference Manual

Mnemonic Data Type
.EN BOOL
.DN BOOL
.EM BOOL
.LEN DINT
.POS DINT
Description:
Arithmetic Status Flags:
Fault Conditions:

CONTROL Structure

Description
The enable bit indicates that the FFL instruction is enabled.
The done bit is set to indicate that the FIFO is full (.POS = .LEN). The .DN bit inhibits loading
the FIFO until .POS < .LEN.
The empty bit indicates that the FIFO is empty. If .LEN ≤ 0 or .POS < 0, both the .EM bit and
.DN bit are set.
The length specifies the maximum number of elements the FIFO can hold at one time.
The position identifies the location in the FIFO where the instruction will load the next value.
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
Not affected
A major fault will occur if
(starting element + .POS) > FIFO array size
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Array (File)/Shift Instructions (BSL, BSR, FFL, FFU, LFL, LFU)
You must test and confirm that the instruction doesn't
change data that you don't want it to change.
The FFL instruction operates on contiguous memory. In
some cases, the instruction loads data past the array into
other members of the tag. This happens if the length is too
big and the tag is a user-defined data type.
Fault type
4
Chapter 9
Fault code
20
411