Siemens S7-300 Installation Manual page 226

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The CPU Æ operating system distinguishes between different priority classes, e.g.
cyclic program execution, program execution controlled by process interrupt. Each
priority class is assigned Æ organization blocks (OB) in which the S7 user can
program a response. The OBs have different standard priorities which determine
the order in which they are executed or interrupted in the event that they are
activated simultaneously.
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SIMATIC S7 PLC operating modes are: STOP, Æ START-UP, RUN.
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Organization blocks (OBs) form the interface between CPU operating system and
the user program. The processing sequence of the user program is defined in the
organization blocks.
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1. Variable of a 67(3  code block
2. Variable for declaring module response (one or several per module). All modules
have a suitable basic factory setting which can be customized in 67(3 .
There are Æ static parameters and Æ dynamic parameters
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Unlike static parameters, dynamic parameters of modules can be changed during
operation by calling an SFC in the user program, for example limit values of an
analog signal input module.
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Unlike dynamic parameters, static parameters of modules cannot be changed by
the user program. You can only modify these parameters by editing your
configuration in 67(3 , e.g. modification of input delay parameters of a digital
signal input module.
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Æ Programming device
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An automation system in the context of SIMATIC S7 Æ is a programmable logic
controller.
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S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00105492-03