Technical data

Table Of Contents
Organization Blocks
System Software for S7-300/400 System and Standard Functions - Volume 1/2
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A5E00709327-01
1.25 Background Organization Block (OB90)
Description
With STEP 7, you can monitor a maximum scan cycle time and can guarantee a
minimum scan cycle time. If the execution time of OB1 including all the nested
interrupts and system activities is less than the minimum scan cycle time that you
have specified, the operating system reacts as follows:
It calls the background OB (providing it exists on the CPU).
It delays the next OB1 start (if OB90 does not exist on the CPU).
Understanding the Operation of OB90
OB90 has the lowest priority of all OBs. It is interrupted by any system activity and
any interrupt (even by OB1 after the minimum cycle time has elapsed) and is only
resumed if the selected minimum scan cycle time has not yet been reached. The
one exception to this is the execution of SFCs and SFBs that are started in OB90.
These are executed with the priority of OB1 and are therefore not interrupted by
OB1. There is no time monitoring of OB90.
The user program in OB90 is processed starting with the first instruction in the
following situations:
Following a warm, cold, or hot restart
After loading or deleting a block (with STEP 7)
After loading OB90 on the CPU in the RUN mode
After terminating the background cycle
Note
With configurations in which there is no great difference between the minimum
scan cycle time and the cycle monitoring time, SFC and SFB calls in the
background OB can lead to the cycle time being exceeded unexpectedly.