Consistent Data - Siemens SIMATIC S7-400 Reference Manual

Structure of a CPU 41x
1.13

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. To maintain consistency, the
data should not be changed or updated during processing or transmission.
Example
To ensure that the CPU has a consistent image of the process signals for the
duration of cyclic program scanning, the process signals are read from the process
image inputs prior to program scanning and written to the process image outputs
after the program scanning. Subsequently, during program scanning when the
address area "inputs" (I) and "outputs" (O) 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.
SFC 81 "UBLKMOV"
With SFC 81 "UBLKMOV" (uninterruptible block move), you can copy the contents
of a memory area (= source area) consistently to a different memory area (=
destination area). The copy operation cannot 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 outputs
The maximum amount of data you can copy is 512 bytes. Take into consideration
the restrictions for the specific CPU, which are documented in the operations list,
for example.
Since copying cannot 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.
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Automation System S7-400 CPU Specifications
A5E00165965-01