Page 294 - Programmable Logic Controllers, Fifth Edition - Mobile version

• The FIFO file fills from the beginning address of the
FIFO file and indexes to one higher address for each Vertical storage area
1
false-to-true transition of input A. FIFO stack
In Out In 2 Out
• A false-to-true transition of input B causes all data
in the FIFO file to shift one position toward the 4 3 2 1 1 3 3 3
starting address of the file, with the data from the Horizontal storage area LIFO stack
starting address of the file shifting to the destination Figure 12-37 FIFO and LIFO container stacking
address, N7:11. operations.
The FIFO instruction is often used for inventory con-
trol. One example is where different parts need to be the LIFO instruction pair is shown in Figure 12-38 and the
removed from inventory to be used in production. Each operation of this function can be summarized as follows:
part is assigned a unique code, which is loaded into a
FIFO stack, and parts are removed in the order prescribed • The load and unload of the LIFO stack operates
by the stack. This type of control ensures that the oldest similarly to that of the FIFO stack, except that the
part in the inventory is used first as the first part entered is last word in the LIFO stack is the first word that is
the first part removed. unloaded from the stack.
The opposite principle—where the last data to be • Words can be added to the LIFO stack without dis-
stored are the first to be retrieved—is known as LIFO turbing the words already loaded on the stack.
(Last In, First Out). The LIFO instruction inverts the order • Otherwise, LIFO instructions operate the same as
of the data it receives by outputting the last data received FIFO instructions.
first and the first data received last. A useful analogy is
a pile of work on your desk. As new work arrives you Allen Bradley ControlLogix programming with FIFO
drop it on the top of the stack. If your stack is LIFO, you and LIFO functions operates similarly to the SLC 500
pick your next job from the top of the pile. If your stack instructions except that tags and arrays are used in the
is FIFO, you pick your work from the bottom of the pile. parameter definitions. The program of Figure 12-39 is an
Figure 12-37 shows how the FIFO and LIFO operations example of the use of a ControlLogix FIFO instruction
work for container stacking operations. pair as part of a data stack operation. The operation of the
The difference between FIFO and LIFO stack operation program can be summarized as follows.
is that the LIFO instruction removes data in the reverse of • The thumbwheel switch input is used to set the
the order they are loaded (last in, first out). An example of decimal number.

BSL BSR SQC SQL SQO FFL FFU LFL LFU
File/Misc File Shift / Sequencer Program Control

LFL File #N70:3 Word
LIFO LOAD EN
Source N70:1 3
LIFO #N70:3 DN 4
Control R6:61 5
Length 64 EM 6
Position 0 7
8 64 words allocated for
LFU 9 LIFO stack at #N70:3
LIFO UNLOAD EU 10
LIFO #N70:3 N70:1 Source 11 Destination N70:2
Dest N70:2 DN
Control R6:61
Length 64 EM LIFO load enters 63 LIFO unload
Position 0 data into stack at removes data
next position. from stack in
reverse order.
Transfer of data
Figure 12-38 LIFO instruction pair.

Sequencer and Shift Register Instructions Chapter 12 275

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