Siemens Simatic S7-1500 Function Manual page 15

Program execution
2.1 Principle of operation
Using process image partitions
If you have distributed a program over various OBs, for example, due to different response
time requirements, it is advisable and often necessary to assign the update of the used I/O
data directly to these OBs. You can use process image partitions for this purpose.
You group the input and output data in a process image partition according to their use in the
program and assign the data to the OB.
A process image partition of the inputs (PIPI) permits the associated input data for an OB
program to be updated immediately before the OB program starts.
A process image partition of the outputs (PIPQ) permits the output data associated with an
OB program to become effective on the outputs immediately after the OB program runs.
You have 32 (0 ... 31) process image partitions at your disposal. The I/O is assigned to the
process image partition 0 by default (setting: "Automatic update"). Process image partition 0
is permanently assigned to cyclic execution.
You have to configure the "system-side update of process image partitions". You can find
additional information on configuration of process image partitions in the online help for
STEP 7 under the keyword "Assign process image/process image partition".
Interruptibility of program execution
Each organization block is processed according to the priority it has been assigned. You can
adapt the priority according to the response time requirements for most organization blocks.
All cycle OBs always have the lowest priority of 1. The highest priority is 26.
Communication tasks always have priority 15. If necessary, you can change the priority of
your blocks and select a higher priority than the communication.
Organization blocks or system activities with higher priority interrupt organization blocks or
system activities with lower priority. Organization blocks or system activities with higher
priority interrupt thus extend the runtime of the interrupted organization blocks or system
activities. If two pending tasks have the same priority, these tasks are processed in the order
in which the tasks occurred.
Note
Higher priority OBs
Communication functionality is strongly influenced by too many and/or runtime-intensive OBs
with a priority > 15.
When using OBs with a priority > 15, you should therefore consider the runtime load that
they cause.
14
Cycle and response times
Function Manual, 10/2018, A5E03461504-AD