Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
576 Manual, 05/2010, A5E02789666-01
27.2.3.2 Process-Image Input/Output Tables
If the input (I) and output (Q) address areas are accessed in the user program, the program does
not scan the signal states on the digital signal modules but accesses a memory area in the system
memory of the CPU and distributed I/Os. This memory area is known as the process image.
Updating the Process Image
The following figure shows the processing steps within a scan cycle.
One of the internal tasks of the operating system (OS) is to read the status of inputs into the
process image input table (PII). Once this step is complete, the user program is executed with all
blocks that are called in it. The cycle ends with writing the process image output table (PIQ) to the
outputs for the modules. Reading in the process image input table and writing the process image
output table to the outputs for the modules is all independently controlled by the operating system.
One of the internal tasks of the operating system (OS) is to write the process image output table
(PIQ) to the outputs for the modules and to read in the status of inputs into the process image input
table (PII). Once this step is complete, the user program is executed with all blocks that are called