Organization Blocks (Obs) - Siemens SIMATIC S5-95F Manual

S5-95F
7.3.1

Organization Blocks (OBs)

Organization blocks form the interface between the operating system and the control program.
Organization blocks are handled in one of the following three ways.
• One organization block (OB1) is called cyclically by the operating system.
• Some organization blocks are event-driven or time-driven.They are called by the operating
system in response to events or at certain times.
- By a switch from STOP to RUN
- By a switch from Power OFF to Power ON (OB21 and OB22)
- By interrupts (OB2, OB3, OB13)
- By battery failure (OB34)
- Error recognition (OB37)
• Some other organization blocks represent operating functions (similar to the integral function
blocks). They can be called by the control program (OB31 and OB251). OBs 31 and 251 are
already programmed and cannot be changed by you. You simply call these OBs in your control
program.
OB No.
OB1 Cyclic program processing
Interrupt-driven program processing
OB2 Interrupt-driven program processing (OB2 interrupts)
OB3 Interrupt-driven program processing (OB3 interrupts)
OB13 Time-controlled program processing
Handling the start-up procedures
OB21 When starting manually (STOP to RUN)
OB22 When power returns
Handling programming errors and device errors
OB34 Battery failure
OB37 Error handling
Special functions
OB31 Scan time triggering (hardware watchdog)
OB251 PID control algorithm
Organization blocks OB2, OB3, OB13, OB21, OB22, OB34 and OB37 are called and processed by
the operating system, even if OB1 has not been programmed.
EWA 4NEB 812 6210-02
Table 7-3. Overview of Organization Blocks
Function
Introduction to STEP 5
7-9