Siemens SIMATIC ET 200AL, System Manual

Page: 1158 / 2255

Communications services
3.5 Data consistency
Communication
Function Manual, 05/2021, A5E03735815-AJ
37
3.5 Data consistency
Definition
Data consistency is important for data transfer and you need to take this into account when
configuring the communication task. Otherwise, malfunctions may occur.
A data area which cannot be modified by concurrent processes is called a consistent data
area. This means that a data area which belongs together and which is larger than the
maximum size of the consistent data area can consist in part of new and of old data at the
same time.
An inconsistency can occur when an instruction for communication is interrupted, for
example by a hardware interrupt OB with higher priority. This interrupts the transfer of the
data area. If the user program in this OB now changes the data that has not yet been
processed by the communication instruction, the transferred data originates from different
times:
The following figure shows a data area that is smaller than the maximum size of the
consistent data area. In this case, when transferring the data area, it is ensured that there is
no interruption by the user program during data access so that the data is not changed.
① The source data area is smaller than the maximum size of the consistent data area ( ② ). The
instruction transfers the data together to the destination data area.
② Maximum size of the consistent data area
Figure 3-3 Consistent transfer of data