Siemens SIMATIC S5 Manual page 83

S5-115F Manual
Figure 2-4 shows how to set up a structured control program. It also illustrates the significance of
organization blocks.
System Program
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
OB 21
OB 1
Control Program
Figure 2-4. Example of Organization Block Use
PB 1 SB 1
FB 2 FB 200
Introduction to STEP 5
2-9