Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 173)

Using Data Handling Instructions
9–25
FIFO Load (FFL) and FIFO Unload (FFU)
FFL and FFU instructions are used in pairs. The FFL instruction loads words into a
user-created file called a FIFO stack. The FFU instruction unloads words from the
FIFO stack in the same order as they were entered.
Operation
Instruction parameters have been programmed in the FFL – FFU instruction pair
shown below.
(EM)
(EN)
(DN)
(EM)
FFL
FIFO LOAD
Source
N7:10
FIFO #N7:12
Control R6:0
Length 34
Position 9
(EU)
FFU
FIFO UNLOAD
FIFO
#N7:12
Dest N7:11
Control R6:0
Length 34
Position 9
FFU instruction unloads data
from stack #N7:12 at position 0,
N7:12.
N7:12 0
N7:13 1
N7:14
2
3
4
5
6
7
8
9
33
34 words are allocated
for FIFO stack starting at
N7:12, ending at N7:45.
FFL–FFU Instruction Pair
Loading and Unloading of Stack #N7:12
N7:10
N7:11
Position Destination
Source
FFL instruction loads data into
stack #N7:12 at the next available
position, 9 in this case.
N7:45
(DN)
FFL Instruction
When rung conditions change from false-to-true, the controller sets the FFL enable
bit (EN). This loads the contents of the Source, N7:10, into the stack structure
indicated by the Position number, 9. The position value then increments.
The FFL instruction loads an element at each false-to-true transition of the rung,
until the stack is filled (34 elements). The controller then sets the done bit (DN),
inhibiting further loading.
Programming
Execution Times
(
µ
sec) when:
True False
61.13 33.67