Cyclic Program Processing - Siemens S5-100U User Manual

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Introduction to STEP 5

Cyclic Program Processing

The operating system calls OB1 cyclically. If you want to

have structured programming, you should program only jump

operations (block calls) in OB1. The blocks you call up, PBs,

FBs, and SBs, should contain completed functional units in

order to provide a clearer overview.

A time monitor is triggered at the beginning of each program

processing cycle (scan cycle trigger). If the scan cycle time

trigger is not reset within the scan monitoring time, the CPU

automatically enters the STOP mode and disables the output

You can set the monitoring time (see Table 6-6). You could

have a control program that is so complex that it cannot be

processed within 300 ms. With CPU 103 and higher, you can

use OB31 (see section 9.3) to lengthen (retrigger) the scan

monitoring time in the control program.

Monitoring time is exceeded, for example, if you program

endless loops or if there is a malfunction in the programmable

Figure 7-13. Cyclic Program Processing

Cycle trigger

EWA 4NEB 812 6120-02

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