Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 16)

An Introduction to Programmable
Controllers
Chapter 2
2Ć9
Figure 2.4
A Simplified Example of a Machine with a Programmable Controller
Conveyor
Motor
Limit
Switch
Conveyor
Unit
Controller
Input Output
10144–I
Since the limit switch is wired normally-closed, the conveyor motor will run
until the arriving part opens the switch. At that time, the condition for
energizing the motor will no longer be met. Therefore, the motor will be
de-energized.
When the condition is met, we say it is true. When the condition is not met, we
say it is false. There may be more than one condition that has to be met before
an action can be executed. When all the conditions in that set of conditions are
true, the action is executed and we say the statement is true. When one or more
of the conditions are false, the action is not executed and we say the statement is
false.
Upon power up, the CPU begins the scan sequence (Figure 2.5) with 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.
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 that bit location, indicating that
the output terminal should not be energized.
Scan Sequence