Siemens S7-300 Installation Manual page 229

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The reduction rate determines the send/receive frequency for Æ GD packets on
the basis of the CPU cycle.
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Æ System function block
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Æ System function
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Æ Bus Segment
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Signal modules (SM) form the interface between the process and the PLC. There
are digital and analog I/O modules (input/output module, digital or analog).
(input/output module, analog)
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A slave may only exchange data with the Æ Master on request.
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A STARTUP routine is executed at the transition from STOP to RUN mode. Can be
triggered by the Æ mode selector switch or after power on, or by an operator action
on the programming device. An S7–300 performs Æ a restart.
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Programming language for developing user programs for SIMATIC S7 PLCs.
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Substitute values are configurable values which output modules transfer to the
process when the CPU switches to STOP mode.
In the event of an input access error, a substitute value can be written to the
accumulator instead of the input value which could not be read (SFC 44).
S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00105492-03
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