Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 417
Array (File)/Shift Instructions (BSL, BSR, FFL, FFU, LFL, LFU) Chapter 9
CONTROL Structure
Description: 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.
Arithmetic Status Flags: Not affected
Fault Conditions:
Mnemonic Data Type Description
.EU BOOL The enable unload bit indicates that the FFU instruction is enabled. The .EU bit is set to
preset a false unload when the program scan begins.
.DN BOOL The done bit is set to indicate that the FIFO is full (.POS = .LEN).
.EM BOOL The empty bit indicates that the FIFO is empty. If .LEN ≤ 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.
You must test and confirm that the instruction doesn’t
change data that you don’t want it to change.
The FFU instruction operates on contiguous memory. In
some cases, the instruction unloads data from other
members of the tag. This happens if the length is too big
and the tag is a user-defined data type.
A major fault will occur if Fault type Fault code
Length > FIFO array size 4 20
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