Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 25)

An Introduction to
Programmable Controllers
Chapter 2
2Ć11
Next, the processor scans the program. It does this statement by statement.
Each statement is scanned in this way:
1. For each condition, the processor checks, or “reads,” the image table to see
if the condition has been met.
2. 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 processor writes a zero into the bit location, indicating that the output
terminal should not be energized.
Here is a simple explanation of the program. If input 02 is on, then turn on
output 02. If input 02 is off, then turn off output 02. The program could be
written this way:
If (condition) Then (action)
Input bit 02 is on Turn output bit 02 on
In this example, the processor reads a 1 at input bit location 02 and knows that
the condition has been met. The processor then carries out the action
instruction by writing a 1 into output bit location 02.
If there were more statements in the program, the processor would continue in
this same manner scanning each statement and executing each instruction until
it reached the end of the program. Statement by statement, the processor would
write a 0 or a 1 into an output bit as directed by the program. Then, the
processor would read specific image table bits to see if the proper set of
conditions were met. After reading and executing all program statements, the
processor scans the output image table and energizes or de-energizes output
terminals. The processor then goes to the input modules to update the input
image table.
Now the entire process is repeated. In fact, it’s repeated over and over again,
many times a minute. Each time, the processor sets or resets output bits. Next,
the processor senses the status of the input terminals. Finally, the processor
scans the program and orders each output terminal on or off according to the
state of its corresponding bit in the output image table.
When forcing is attempted, the processor’s I/O scan slows down to do the
forcing (see chapter 19). When forcing is terminated, the processor
automatically switches back to the faster I/O scan mode.
When this example begins, the processor is energizing output terminal 02
because output bit 02 is on.
When the part is conveyed to the work station, it turns the limit switch off.
When the limit switch is off, there is no voltage at input terminal 02. The
processor scans the input image table, senses no voltage, and responds by
writing a zero into bit 02 in the input image table.