Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 245)

Shift Register Instructions
Chapter 13
13Ć2
The FIFO Load and FIFO Unload output instructions that are always used
together to construct an asynchronous word shift register (Figure 13.2) up
to 999 words long. Upon false-true transition of rung decision, the contents
of the input word will be transferred into the stack (FIFO Load); or the
contents of the word designated by the unload pointer will be transferred to
the output word (FIFO Unload).
NOTE: This section assumes the reader has read Chapter 12, file and data
monitor mode, and is familiar with the concepts introduced in that section.
Figure 13.2
FIFO (First In First Out) Operation for a 64 Word FIFO Stack Starting at
Address 400
8
400
477
64 words allocated
for FIFO stack
130
FIFO Load enters
data into stack
Input Addr
040
FIFO Unoad removes
data from stack
Output Addr
This instruction can be used as a synchronous word shift register. When the
rung goes true, the data from a specified input word is shifted into the first
word of the file (Figure 13.1a), the data in the file is shifted up one word
(toward higher number addresses) and the data of the last word in the file
is shifted into the specified output word.
The instruction can operate in either complete or distributed complete
mode. In complete mode the input word data is shifted out in one scan. In
distributed complete mode, it will take a number is scans to shift in one
13.1
Shift File Up