Technical data

5.5 Frequent Errors in the User Program
The system program can detect faulty operation of the CPU, errors in
the system program processing or the effect of user errors in the
program.
This section contains a list of errors most likely to occur when you
first run your user program.
You can avoid these errors easily by remembering the following
points when you write your STEP 5 program:
•• When specifying byte addresses for I/Os, make sure that the
corresponding modules are plugged into the central controller or
the expansion unit.
•• Make sure that you have provided correct parameters for all
operands.
•• Make sure that outputs, flags, timers and counters are not
processed at different points in the program with operations that
counteract each other.
•• Make sure that all data blocks called in the program exist and are
long enough.
•• Check that all blocks called are actually in the memory.
•• Be careful when changing existing function blocks. Check that the
FBs/FXs are assigned the correct operands and that the actual
operands are specified.
•• Make sure that timers are scanned only once per cycle (e.g. A T1).
•• Make sure that scratchpad flags (intermediate flags) are saved by
interrupt and time-driven programs and are loaded again on
completion of the inserted program when they are required by
other blocks (e.g. standard FBs).
Frequent Errors in the User Program
CPU 928B Programming Guide
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