S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
7-1
Consistent Data
7
7.1 Basics
Overview
Data that belongs together in terms of its content and describes a process state 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:
● Bit memory
● DB contents
● Process Image of Inputs
● Process Image of Outputs
The maximum amount of data you can copy is 512 bytes. Remember the restrictions for the
specific CPU as described, for example, in the operations list.
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.
For information on SFC81, refer to the corresponding online help and to the
System and
Standard Functions
manual.
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