Fundamentals of a
Programmable Controller
Chapter 2
2-10
On power up, the processor begins the scan sequence (Figure 2.2) with a
program pre-scan. This pre-scan is completed as if the entire program lies
within an active MCR zone. Next the processor completes the I/O scan.
During the I/O scan, data from input modules is transferred to the input
image table. Data from output image table is transferred to the
output modules.
Figure 2.2
Scan Sequence
11597
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
Scan Sequence
