Consistent Data - Siemens SIMATIC S7-400H Manual

Installation of a CPU 41x-H
4.10

Consistent Data

Data that belongs together in terms of its content and a process state written at a
specific point in time is known as consistent data.. To maintain consistency, the
data should not be changed or updated during processing or transmission.
Example 1:
To ensure that the CPU has a consistent image of the process signals for the
duration of cyclic program scanning, the process signals are read from the process
image inputs prior to program scanning and written to the process image outputs
after the program scanning. Subsequently, during program scanning when the
address area "inputs" (I) and "outputs" (O) are addressed, the user program
addresses the internal memory area of the CPU on which the image of the inputs
and outputs is located instead of directly accessing the signal modules.
Example 2:
Inconsistency can arise if a communication block (e.g. SFB 14 "GET", SFB 15
"PUT") is interrupted by a process interrupt OB with a higher priority. If the user
program in this process interrupt OB now changes the data which have already
been processed in part by the communication block, the transferred data originate
in part before and in part after the time the process interrupt was processed.
This means that these data are inconsistent (not coherent).
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Automation System S7-400H Fault-tolerant Systems
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