S5-95F Introduction to STEP 5
7.4.1 START-UP Program Processing
In the START-UP mode, the operating system of the programmable controller automatically calls up
a start-up OB if the OB has been programmed.
The following start-up OBs are available:
• OB21 is called up for a manual cold restart.
• OB22 is called up for an automatic cold start after power recovery if the programmable controller
was previously in the RUN mode.
If you have programmed start-up OBs, then they are processed before the cyclic program pro-
cessing starts. The start-up OB program is appropriate, for example, for a one-time presetting of
certain system data.
If you access the PII in OB21/22 or if you directly access the onboard I/Os, then you always receive
the value ”0”. If you write to the PIQ or to the onboard I/Os via direct access in the OB21/22, then
your change is stored and becomes effective only after OB1 has been processed.
Features of the start-up blocks (OB21, OB22):
• The red and green LEDs light up.
• Timers TO to T63 are processed, timers T64 to T127 are not processed
• Scan monitoring is not activated.
• Interrupts are not being processed.
EWA 4NEB 812 6210-02
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