Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 174)

Preface MicroLogix 1000 Programmable Controllers User Manual
9–26
FFU Instruction
When rung conditions change from false-to-true, the controller sets the FFU enable
bit (EU). This unloads the contents of the element at stack position 0 into the
Destination, N7:11. All data in the stack is shifted one element toward position
zero, and the highest numbered element is zeroed. The position value then
decrements.
The FFU instruction unloads an element at each false-to-true transition of the rung,
until the stack is empty. The controller then sets the empty bit (EM).
LIFO Load (LFL) and LIFO Unload (LFU)
LFL and LFU instructions are used in pairs. The LFL instruction loads words into a
user-created file called a LIFO stack. The LFU instruction unloads words from the
LIFO stack in the opposite order as they were entered.
Operation
Instruction parameters have been programmed in the LFL – LFU instruction pair
shown below.
(EN)
(DN)
(EM)
LFL
LIFO LOAD
Source
N7:10
LIFO #N7:12
Control R6:0
Length 34
Position 9
(EU)
(DN)
(EM)
LFU
LIFO UNLOAD
LIFO
#N7:12
Dest N7:11
Control R6:0
Length 34
Position 9
LFU instruction unloads data
from stack #N7:12 at position
8.
N7:12 0
N7:13 1
N7:14 2
3
4
5
6
7
8
9
33
34 words are allocated
for LIFO stack starting at
N7:12, ending at N7:45.
Loading and Unloading of Stack #N7:12
N7:10
N7:11
Position
Destination
Source
LFL instruction loads data into
stack #N7:12 at the next
available position, 9 in this
case.
LFL–LFU Instruction Pair
N7:45
73.78+
4.34/word
Execution Times
(
µ
sec) when:
True False
34.90