PROCESS IMAGE
As on any other computer, the automation programs are processed on the automation
systems (AS) in the central processing unit (CPU). The control program is processed
cyclically. If signals from the process are processed also, the program does not access the
connected input and output modules directly. Instead, a process image is generated where,
at the beginning of the cycle, all signals are read in and stored at once.
This has two reasons: First, accessing the process image requires appreciably less time
since it is stored in the CPU’s internal memory. Second, this ensures that the input
information was collected within a defined time window -regardless of the execution time of
the control program. This data consistency is attained by inputting, once for each cycle, the
signals of the input modules in the process image of the inputs (PII). The program is then
executed and the results written into the process image of the outputs (PIO).
After the entire program is executed, the data is written from the PIO into the output
modules and only then read out to the process. Then, the process image of the inputs is
updated again