Access To The Working Memory Of The Cpu - Siemens SIMATIC S7-400H Manual

Installation of a CPU 41x-H
4.10.2

Access to the Working Memory of the CPU

The communication functions of the operating system access the working memory
of the CPU in fixed block lengths. The block length depends on the CPU; for
S7-400 CPUs it is 32 bytes.
This ensures that the interrupt reaction time is not increased when communication
functions are used. Since this access is perform asynchronous to the user
program, you cannot transmit an unlimited number bytes of consistent data.
The following explains the rules that you should keep to guarantee data
consistency.
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Automation System S7-400H Fault-tolerant Systems
A5E00068197-07