Siemens SIMATIC S5-90U System Manual page 248

Integrated Blocks and Their Functions
Prerequisites for OB13 Execution
Time-controlled program execution using OB13 is possible only if the following requirements are
met:
• OB13 is programmed.
• The OB13 call interval set in DB1 10.
• Interrupt processing is not disabled by the STEP 5 operation "IA" (see Section 8.2.8).
• The PLC is in the "POWER ON" state and switched to "RUN".
Interrupting OB13 Execution
Time-controlled program execution of OB13 can be suspended by the interrupt-driven program
execution programmed in OB3 after processing the current STEP 5 operation.
After completing interrupt-driven program execution, time-controlled processing is resumed at the
point of interruption.
OB13 is first processed to the end before it can be initiated again.
OB13 can interrupt cyclic program execution in OB1 after any STEP 5 operation.
Note
Even for time-controlled program execution, the maximum of 16 nesting levels must not
be exceeded in general.
OB1
Cyclic program
Figure 9-1. Time-Controlled Program Execution - Possible Interrupts
9-4
Timed
interrupt
PB1
Time-controlled program
OB13
Process
interrupt
Interrupt-driven program
EWA 4NEB 812 6115-02b
S5-90U/S5-95U
OB3