Consistent Data - Siemens SIMATIC S7-400H System Manual

Consistent data

Overview
Data that belongs together in terms of its content and describe a process state at a specific
point in time is known as consistent data. In order to maintain data consistency, do not
modify or update the data during their transfer.
Example 1:
In order to provide a consistent image of the process signals to the CPU for the duration of
cyclic program processing, the process signals are written to the process image of inputs
prior to program execution, or the processing results are written to the process image of
outputs after program execution. Subsequently, during program processing when the inputs
(I) and outputs (O) operand areas are addressed, the user program addresses the internal
memory area of the CPU on which the process image is located instead of directly accessing
the signal modules.
Example 2:
Inconsistency may develop when a communication block, such as SFB 14 "GET" or SFB 15
"PUT", is interrupted by a process alarm OB of higher priority. If the user program modifies
any data of this process alarm OB which in part have already been processed by the
communication block, certain parts of the transferred data will have retained their original
status which was valid prior to process alarm processing, while others represent data from
after process alarm processing.
This results in inconsistent data, i.e. data which are no longer associated.
SFC 81 "UBLKMOV"
Use SFC 81 "UBLKMOV" to copy the content of a memory range, the source area,
consistently to another memory range, the target range. The copy operation cannot be
interrupted by other operating system activities.
SFC 81 "UBLKMOV" enables you to copy the following memory areas:
● Bit memory
● DB contents
● Process input image
● Process output image
The maximum amount of data you can copy is 512 bytes. Make allowances for the CPU-
specific restrictions listed in the instruction list.
Since copying cannot be interrupted, the alarm response times of your CPU may increase
when using SFC 81 "UBLKMOV".
S7-400H
System Manual, 03/2012, A5E00267695-11
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