Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 59)

Advanced Instruction Set
Chapter 6
6Ć2
Figure 6.1
Scan Sequence
10150-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
Next, the CPU scans the program. It does this statement by statement. Each
statement is scanned in this way:
- First, for each condition, the CPU checks, or “reads,” the image table to
see if the condition has been met.
- Second, if the set of conditions has been met, the CPU writes a one into
the bit location in the output image table corresponding to the output
terminal to be energized. On the other hand, if the set of conditions has
not been met, the CPU writes a zero into the bit location, indicating that
the output terminal should not be energized.