Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
102 Manual, 05/2010, A5E02789666-01
4.2.4.2 Time-Delay Interrupt Organization Blocks (OB20 to OB23)
The S7 CPUs provide time delay OBs with which you can program the delayed execution of parts
of your user program.
Rules for Time-Delay Interrupts
Time delay interrupts can only be executed when the corresponding organization block exists in the
CPU program. If this is not the case, an error message is entered in the diagnostic buffer and
asynchronous error handling is executed (OB80, see Error Handling Organization Blocks (OB70 to
OB87 / OB121 to OB122)).
Time delay interrupt OBs that were deselected by the parameter assignment cannot be started.
The CPU recognizes a programming error and changes to STOP mode.
Time delay interrupts are triggered when the delay time specified in SFC32 SRT_DINT has
expired.
Starting the Time-Delay Interrupt
To start a time delay interrupt, you must specify the delay time in SFC32 after which the
corresponding time delay interrupt OB is called. Refer to the "S7-300 Programmable Controller,
Hardware and Installation Manual" and "S7-400, M7-400 Programmable Controllers Module
Specifications Reference Manual" for the maximum permitted length of the delay time.
Priority of the Time-Delay Interrupt OBs
The default priority for the time-delay interrupt OBs is priority class 3 to 6. You can assign
parameters to change the priority classes.