PC Control / Profi Control 645-12M Instruction Manual pdf - Kuhnke

4.9.1. Data consistency secured by OS_CRIT
Kuhnke controller such as PC Control 645-12M have an
operating system with multi-tasking capability. Tasks such
as processing the user program or PROFIBUS communication
are acyclically arranged. Task changes are timer controlled,
i.e. they do not relate to the processing step of the
preceding task.
This may lead to the PROFIBUS task updating the process
image between two user program commands. Consequently,
the external operands read after the next user
program command are more up-to-date than the data
read previously.
This may cause problems if the different bits of information
are functionally related, because misinterpretations cannot
be excluded in this case. Just imagine a 24-bit analogue
value supplied by a positioning slave that is to be read by
two commands, and then there is a task change right between
the two reading processes. The interpretation of the
analogue value might deliver an entirely wrong result.
KUBES module OS_CRIT remedies this situation by influencing
the operating system to prevent task changes within
limited user program sections.
OS_CRIT is delivered with the KUBES program package.
A constant value is specified as its starting parameter.
Example
Prevent task change (constant = 1)
JPK OS_CRIT , _____
1 -|_____|-
LD BI01a00. ; low word
=D BM00.00
LD BI01a02. ; high word
=D BM00.02
Allow task change (constant = 0)
JPK OS_CRIT , _____
0 -|_____|-
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