Stage 2
Designing the program
Stage Activity
1 Based on the improved block diagram, decide on the
types of processing required of your program (cyclic
processing, time-controlled processing etc.) and select
the OBs required for this.
2 Divide the types of processing into technological
and/or functional units.
3 Check whether the units can be assigned to a program
or function block and select the blocks you require
(PB x, FB y etc.)
4 Find out which timers, counters and data or results
memory you require.
5 Specify the tasks for each of the proposed logic blocks
and the data for flags and data blocks which may be
required. Create flow diagrams for the logic blocks.
Notes on the scope of
cyclic processing
When deciding on the types of processing, keep the following
conditions in mind:
•• The cycle must run through quickly enough. The process statuses
must not change more quickly than the CPU can react. Otherwise
the process can get out of control.
•• The maximum reaction time should be taken as twice the cycle
time.
The cycle time is determined by the cyclic processing of the
system program and the type and scope of the user program. It is
often not constant, since the cyclic user program may be
interrupted when time and interrupt-driven program sections are
called.
How to Tackle Programming
CPU 948 Programming Guide
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