Siemens CPU 313C Hardware And Installation Manual page 122

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Your CPU uses a SIMATIC Micro Memory Card (MMC) as a memory module. You
can set up the MMC as a load memory or a portable data medium.
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An inserted MMC is imperative for CPU operation.
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If the CPU is set to RUN and you remove the MMC, the CPU will STOP and
request a memory reset.
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Data on a SIMATIC Micro Memory Card can be corrupted if you remove the card
during write access. In this case you might have to insert the MMC memory in your
PG to delete it, or you format the card in the CPU.
Never remove an MMC in RUN mode. Always remove when power is off or when
the CPU is in STOP state and when the is not writing to the card. Disconnect the
communication lines if you are not sure whether or not the PG is performing write
functions (e.g. load/delete function block).
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Make sure that the MMC to be inserted contains the proper user program for the
CPU (system). An incorrect user program can lead to serious consequences in
your process.
9-10
S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03