Siemens CPU 313C Hardware And Installation Manual page 230

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System diagnostics refers to the detection, evaluation and signaling of errors which
occur within the PLC, for example, program errors or module errors. System errors
can be displayed with LED indicators or in 67(3 .
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A system function (SFC) is a Æ function integrated in the operating system of the
CPU that can be called, as required, in the STEP 7 user program.
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A System Function Block (SFB) is a Æ function block integrated in the CPU
operating system. If required, it can be called in the STEP 7 user program.
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The system memory (RAM) is integrated on the central processing unit. System
memory contains the address areas (e.g. timers, counters, memory bits) and the
data areas required internally by the --> operating system (e.g. buffers for
communication).
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The system status list contains data describing the current status of an S7-300.
You can always use this list to obtain an overview of:
• The S7-300 configuration
• The current CPU configuration and the configurable signal modules
• Current status and processes in the CPU and configurable signal modules.
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A terminating resistor is used to terminate data links in order to prevent reflections.
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Æ Timer
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Timers are part of CPU Æ system memory. The content of "timer cells" is
automatically updated by the operating system, asynchronously to the user
program. 67(3  instructions are used to define the exact function of the timer
cells (for example on-delay) and initiate their execution (e.g. start).
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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03