Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 83
Interrupts
Cyclic program processing can be interrupted by the following:
• An interrupt
• A STOP command (mode selector, menu option on the programming device, SFC46 STP,
SFB20 STOP)
• A power outage
• The occurrence of a fault or program error
Scan Cycle Time
The scan cycle time is the time required by the operating system to run the cyclic program and all
the program sections that interrupt the cycle (for example, executing other organization blocks) and
system activities (for example, updating the process image). This time is monitored.
The scan cycle time (TC) is not the same in every cycle. The following figures show different scan
cycle times (TC1 ≠ TC2) for CPUs up to 10/98 and CPUs from 10/98:
In the current cycle, OB1 is interrupted by a time-of-day interrupt.