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 LFU 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 LIFO is full (.POS = .LEN).
The empty bit indicates that the LIFO 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 LIFO can hold at one time.
The position identifies the end of the data that has been loaded into the LIFO.
Use the LFU instruction with the LFL instruction to store and retrieve data in a
last-in/first-out order.
When enabled, the LFU instruction unloads the value at .POS of the LIFO and
places that value in the Destination. The instruction unloads one value and
replaces it with 0 each time the instruction is enabled, until the LIFO is empty. If
the LIFO is empty, the LFU returns 0 to the Destination.
IMPORTANT
Not affected
A major fault will occur if
Length > LIFO 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 LFU 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
429