Siemens SIMATIC S5 - Page 83

S5-115F Manual Introduction to STEP 5
Figure 2-4 shows how to set up a structured control program. It also illustrates the significance of
organization blocks.
Figure 2-4. Example of Organization Block Use
FB 200FB 2
OB 1
OB 21
System Program
PB 1 SB 1
Control Program
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The functions of the various organization blocks are described on the following pages.
OB 1: Cyclical program scanning
Control program structure is established in OB 1. Using a number of block calls in OB 1, you can
specify the PB or FB processing sequence. Calls can be conditional or unconditional.
OB 2: CPU 942F interrupt handling
CPU 942F implements ”interrupt-driven” processing. Interrupt-driven processing occurs when a
signal from the process causes the CPU in the PLC to interrupt cyclic scanning and to execute a
specific program. After the CPU has executed the special program, it returns to the point of inter-
ruption in the cyclical program and continues scanning from there. The interrupt is triggered by
digital input modules with process interrupt capability.
EWA 4NEB 811 6149-02 2-9