Siemens S7-300 Installation Manual page 227

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Æ Programmable controller
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The S7 CPU operating system provides up to 26 priority classes (or "Program
execution levels"). Specific OBs are assigned to these classes. The priority classes
determine which OBs interrupt other OBs. If a priority class includes several OBs,
they do not interrupt each other, but are executed sequentially.
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The process image is part of CPU Æ system memory. At the start of cyclic program
execution, the signal states at the input modules are written to the process image
of the inputs. At the end of cyclic program execution, the signal status of the
process image of the outputs is transferred to the output modules.
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A process interrupt is triggered by interrupt-triggering modules as a result of a
specific event in the process. The process interrupt is reported to the CPU. The
assigned Æ organization block is then processed, according to interrupt priority.
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The product version identifies differences between products which have the same
order number. The product version is incremented when forward-compatible
functions are enhanced, after production-related modifications (use of new
parts/components) and for bug fixes.
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The PLC distributes controls for digital, analog and intelligent modules as well as a
wide range of field devices to EN 50170, part 3, for example, drives or valve
blocks, to processes at external locations - even across distances exceeding
23 km.
The modules and field devices are connected to the programmable controller via
the PROFIBUS-DP fieldbus and addressed in the same way as centralized I/Os.
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Programming devices are essentially personal computers which are compact,
portable and suitable for industrial applications. They are equipped with special
hardware and software for SIMATIC PLCs.
S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00105492-03
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