Consistent Data - Siemens SIMATIC S7-400H System Manual

7.2

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. 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 execution, 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 scanning 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 image of the inputs and outputs 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. When the user program
modifies any data of this process alarm OB which 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 area of the source consistently to
another memory area, namely the destination area. The copy operation can not be
interrupted by other operating system activities.
SFC 81 "UBLKMOV" enables you to copy the following memory areas:
● Memory markers
● DB contents
● Process image of the inputs
● Process image of the outputs
The maximum amount of data you can copy is 512 bytes. Make allowances for the CPU-specific
restrictions listed in the operation list.
Since copying can not be interrupted, the interrupt reaction times of your CPU may increase
when using SFC 81 "UBLKMOV".
The source and destination areas must not overlap. If the specified destination area is larger
than the source area, the function only copies as much data to the destination area as that
contained in the source area. If the specified destination area is smaller than the source area,
the function only copies as much data as can be written to the destination area.
S7-400H
System Manual, 09/2007, A5E00267695-03
S7-400H in PROFIBUS DP mode
7.2 Consistent Data
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