Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 17)

An Introduction to Programmable
Controllers
Chapter 2
2Ć10
Figure 2.5
Scan Sequence
10145–I
Output
Image
Table
Input
Terminals
Input
Image
Table
Output
Terminals
Copy output image table status
into output terminal circuits.
Copy input terminal status into
input image table
Program Statement
Execute each program rung in
sequence, writing into bits in the
data table, including the output
image table.
( )
I/O
Scan
Program
Scan
The program in this example has only one statement, “If there is no voltage at
input terminal 02, then energize output terminal 02.” The condition, “If there is
not voltage at input terminal 02,” is really an instruction to the CPU to examine
bit 02 in the input image table to see if it is off. the action portion of our
program, “then energize output terminal 02,” is really another instruction telling
the CPU to turn on bit location 02 in the output image table if the condition has
been met.