Siemens CPU 948 - Principle of Program Execution

3.1 Principle of Program Execution
You can process your STEP 5 user program in various ways.
Cyclic program execution is most common with programmable
controllers (PLCs). The system program runs through a program loop
(the cycle, refer to Section 3.4) and calls organization block OB 1
cyclically in each loop (refer to Fig. 3-1).
Call OB1
BE
Call PB 20
BE
PB 20
OB 1
Update inter-
processor comm.
flag outputs
image outputs
(PIQ)
Update process
Trigger cycle time
Update inter-
processor comm.
flag inputs
image inputs
(PII)
Update process
from start-up
System program User program
Fig. 3-1 Principle of cyclic program execution
Principle of Program Execution
CPU 948 Programming Guide
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