Siemens S7-300 Installation Manual page 218

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Chassis ground is the totality of all the interconnected passive parts of a piece of
equipment on which dangerous fault-voltage cannot occur.
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Memory bit which can be used to generate clock pulses in the user program
(1 byte per memory bit).
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Note in the case of S7-300 CPUs that the clock memory byte is not overwritten in
the user program.
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A SIMATIC S7 code block contains part of the 67(3  user program. (In contrast:
a Æ Data Block (DB) only contains data.)
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Communication processors are modules for point-to-point and bus communication.
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The programming device online function "Compress" is used to align all valid
blocks contiguously in the RAM of the CPU at the start of the user memory. This
eliminates all gaps which arose when blocks were deleted or modified.
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Assignment of modules to racks/slots and (e.g. for signal modules) addresses.
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Data whose contents are related and which should not be separated are known as
consistent data.
For example, the values of analog modules must always be handled consistently,
that is the value of an analog module must not be corrupted by reading it out at two
different times.
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Counters are part of CPU --> system memory. The content of "Counter cells" can
be modified by 67(3  instructions (e.g. up/down count).
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S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
A5E00105492-03