Siemens S7-1200 System Manual page 80

PLC concepts
4.1 Execution of the user program
4. Optionally select a specific OB from the "Organization block" drop-down list.
5. From the "Process image" drop-down list, change "Automatic update" to "PIP1", "PIP2",
"PIP3", "PIP4" or "None". A selection of "None" means that you can only read from and
write to this I/O using immediate instructions. To add the points back to the process
image automatic update, change this selection back to "Automatic update".
You can immediately read physical input values and immediately write physical output
values when an instruction executes. An immediate read accesses the current state of the
physical input and does not update the process image input area, regardless of whether the
point is configured to be stored in the process image. An immediate write to the physical
output updates both the process image output area (if the point is configured to be stored in
the process image) and the physical output point. Append the suffix ":P" to the I/O address if
you want the program to immediately access I/O data directly from the physical point instead
of using the process image.
Note
Use of process image partitions
If you assign I/O to one of the process image partitions PIP1 - PIP4, and do not assign an
OB to that partition, then the CPU never updates that I/O to or from the process image.
Assigning I/O to a PIP that does not have a corresponding OB assignment, is the same as
assigning the process image to "None". You can read the I/O directly from the physical I/O
with an immediate read instruction, or write to the physical I/O with an immediate write
instruction. The CPU does not update the process image.
The CPU supports distributed I/O for both PROFINET and PROFIBUS networks (Page 509).
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S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG