PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
85
Process image update and process image partitions
The CPU updates local digital and analog I/O points synchronously with the scan cycle using
an internal memory area called the process image. The process image contains a snapshot
of the physical inputs and outputs (the physical I/O points on the CPU, signal board, and
signal modules).
You can configure I/O points to be updated in the process image every scan cycle or when a
specific event interrupt occurs. You can also configure an I/O point to be excluded from
process image updates. For example, your process might only need certain data values
when an event such as a hardware interrupt occurs. By configuring the process image
update for these I/O points to be associated with a partition that you assign to a hardware
interrupt OB, you avoid having the CPU update data values unnecessarily every scan cycle
when your process does not need a continual update.
For I/O that is updated every scan cycle, the CPU performs the following tasks during each
scan cycle:
● The CPU writes the outputs from the process image output area to the physical outputs.
● The CPU reads the physical inputs just prior to the execution of the user program and
stores the input values in the process image input area. These values thus remain
consistent throughout the execution of the user instructions.
● The CPU executes the logic of the user instructions and updates the output values in the
process image output area instead of writing to the actual physical outputs.
This process provides consistent logic through the execution of the user instructions for a
given cycle and prevents the flickering of physical output points that might change state
multiple times in the process image output area.
For controlling whether your process updates I/O points automatically on every scan cycle,
or upon the triggering of events, the S7-1200 provides five process image partitions. The first
process image partition, PIP0, is designated for I/O that is to be automatically updated every
scan cycle, and is the default assignment. You can use the remaining four partitions, PIP1,
PIP2, PIP3, and PIP4 for assigning I/O process image updates to various interrupt events.
You assign I/O to process image partitions in Device Configuration and you assign process
image partitions to interrupt events when you create interrupt OBs (Page 188) or edit OB
properties (Page 188).
By default, when you insert a module in the device view, STEP 7 sets its I/O process image
update to "Automatic update". For I/O configured for "Automatic update", the CPU handles
the data exchange between the module and the process image area automatically during
every scan cycle.
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