Siemens SIMATIC ET 200AL, System Manual

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Communications services
3.5 Data consistency
Communication
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Function Manual, 05/2021, A5E03735815-AJ
The following figure shows a data area that is larger than the maximum size of the consistent
data area. In this case, the data can be changed during an interruption of the data transfer.
An interruption also occurs if, for example, the data area needs to be transferred in several
parts. If the data is changed during the interruption, the transferred data originates from
different times.
① The source data area is larger than the maximum size of the consistent data area ( ③ ). At time
T1, the instruction only transfers as much data from the source data area into the destination
data area as fits in the consistent data area.
② At time T2, the instruction transfers the rest of the source data area to the destination data
area. After the transfer, data from different points in time exist in the destination data area. If
the data in the source data area has changed in the meantime, an inconsistency may result.
③ Maximum size of the consistent data area
Figure 3-4 Transfer of data larger than the maximum consistency area
Example of an inconsistency
The figure below shows an example of changing data during the transfer. The destination
data area contains data from different points in time.
① Maximum size of the consistent data area
Figure 3-5 Example: Changing data during the transfer