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3.4
Processing the User Program
START-UP
CYCLE
CPU 948 Programming Guide
C79000-G8576-C848-04
The complete software on the CPU (consisting of the system program
and the STEP 5 user program) has the following tasks:
• • CPU START-UP
• • Controlling an automation process by continuously repeating
operations (CYCLE).
• • Controlling an automation process by reacting to events occurring
sporadically or at certain times (interrupts) and reacting to errors.
For all three tasks, you can select special parts of your program to run
on the CPU by programming user interfaces (organization blocks
OB 1 to OB 35 - refer to Section 2.2.3).
Before the CPU can start cyclic program execution, an initialization
must be performed to establish a defined initial status for cyclic
program execution and, for example, to specify a time base for the
execution of certain functions. The way in which this initialization is
performed depends on the event that led to a START-UP and on
settings that you can make on your CPU. For more detailed
information, refer to Chapter 4.
You can influence the START-UP procedure of your CPU by
programming organization blocks OB 20, OB 21 and OB 22 or by
assigning parameters in DX 0 (refer to Chapter 7).
Following the START-UP, the system program goes over to cyclic
processing. It is responsible for background functions required for the
automation tasks (refer to Fig. 3-1 at the beginning of this section).
After the system functions have been executed at the beginning of a
CYCLE, the system program calls organization block OB 1 or
function block FB 0 as the cyclic user program. You program the
STEP 5 operations for cyclic processing in this block.
Processing the User Program
3 - 11
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