Siemens SIMATIC ET 200SP System Manual page 118

Configuring
6.2 Configuring the CPU
Dependency on the CPU mode
You can execute this function in "RUN" mode as well as in "STOP" mode. The table below
shows the reactions of the CPU in the different modes:
Table 6- 3 Reaction of the CPU depending on the mode
Action System reaction
Overwrite actual The values of all data block tags are
values in "RUN" overwritten during operation. No dis-
mode tinction is made between retentive and
non-retentive values.
Overwrite actual Only the actual values of the retentive
values in tags are overwritten by the snapshot.
"STOP" mode Non-retentive tags are initialized with
their start values during the transition
from STOP to RUN. The values from
the snapshot are not taken into consid-
eration.
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Consequences for the online program
Changing actual values can lead to inconsistencies between
the program and the actual process.
If the volume of data to be transmitted is large, the values are
possibly transmitted in several cycles. If the program accesses
tags before all values are completely transmitted, there is a
risk that inconsistent value combinations may be created and
processed.
The copying of the values of elementary data types can also
possibly take place over several cycles. These values are
potentially invalid until they have been completely transmitted.
Dangerous states may occur if the program accesses these
values before they have been completely transmitted.
Because only the data from the snapshot are transmitted,
there is a risk that inconsistent value combinations may be
created and processed.
System Manual, 12/2015, A5E03576849-AF
Distributed I/O system