Siemens CPU 313C Hardware And Installation Manual page 228

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Programmable controllers (PLCs) are electronic controllers whose function is
saved as a program in the control unit. The configuration and wiring of the unit are
therefore independent of the function of the control system. The PLC has a
computer structure; it consists of the Æ CPU (Central Processing Unit) with
memories, I/O modules and internal bus system. The I/Os and the programming
language are oriented to control engineering needs.
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RAM (Random Access Memory) is a semiconductor read/write memory.
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Æ Ground
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Potential with reference to which the voltages of participating circuits are observed
and/or measured.
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A memory area is retentive if its contents are retained even after a power failure
and a change from STOP to RUN. The non-retentive area of memory markers,
timers and counters is reset following a power failure and a transition from the
STOP mode to the RUN mode.
The following can be made retentive:
• flag bits
• S7 timers
• S7 counters
• Data areas
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On CPU startup (e.g. after is switched from STOP to RUN mode via selector switch
or with POWER ON), OB100 (restart) is initially executed, prior to cyclic program
execution (OB1). On restart, the input process image is read in and the 67(3 
user program is executed, starting at the first instruction in OB 1.
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Errors occurred in the PLC (that is, not in the process itself) during user program
execution.
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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03