Siemens S7-300 - Operation of Mode Selector Switch for Memory Reset

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S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
9-14 A5E00203919-01
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The table below shows the steps required for resetting CPU memory.
Table 9-4 Procedure for resetting the CPU memory
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1. Turn the key to STOP position
2. Turn the key to MRES position
Hold the key in this position until the STOP LED lights up for the second time
and remains on (this takes 3 seconds).
Then release the key.
3. You must turn the key back to the MRES position, push it in within three
seconds and hold it there until the STOP LED flashes (at 2 Hz).
You can now release the switch. When the CPU has completed memory reset,
the STOP LED stops flashing and remains lit.
The CPU has reset the memory.
The procedure described in the table above is only required if the user wishes to
reset the CPU memory without being requested by the CPU to reset the memory
(STOP LED flashing slowly). If the memory reset is requested by the CPU, simply
turn the mode selector switch to MRES briefly to start the reset.
The figure below shows you the sequence of operation.
t
On
Off
3s
max. 3 s
min. 3 s
STOP
LED
1
2
3
CPU
Figure 9-7 Operation of mode selector switch for memory reset
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What should I do if the STOP LED does not flash during the memory reset or if
other LEDs are lit (Exception: BATF LED)?
1. You must repeat steps 2 and 3.
2. If the CPU still does not reset memory, evaluate the diagnostic buffer of the
CPU.