Control Structure - Allen-Bradley Logix5000 Reference Manual

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

CONTROL Structure

Description
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.
The done bit is set to indicate that the FIFO is full (.POS = .LEN).
The empty bit indicates that the FIFO is empty. If .LEN ≤ 0 or .POS < 0, the .EM bit and .DN
bits are set.
The length specifies the maximum number of elements in the FIFO.
The position identifies the end of the data that has been loaded into the FIFO.
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.
IMPORTANT
Not affected
A major fault will occur if
Length > 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 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.
Fault type
4
Chapter 9
Fault code
20
417