Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
108 Manual, 05/2010, A5E02789666-01
4.2.4.6 Background Organization Block (OB90)
If you have specified a minimum scan cycle time with STEP 7 and this is longer than the actual
scan cycle time, the CPU still has processing time available at the end of the cyclic program. This
time is used to execute the background OB. If OB90 does not exist on your CPU, the CPU waits
until the specified minimum scan cycle time has elapsed. You can therefore use OB90 to allow
processes where time is not critical to run and thus avoid wait times.
Priority of the Background OB
The background OB has priority class 29, which corresponds to priority 0.29. It is therefore the OB
with the lowest priority. Its priority class cannot be changed by reassigning parameters.
The following figure shows an example of processing the background cycle, the main program
cycle, and OB10 (in CPUs as of 10/98).