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What is the purpose of external the battery for M221?

1) The help content give us the information the battery of the M221 allow:
-to save the content of the whole %MW area ( %MWi  with i>50;  note: %MW0-%MW49 are also save without battery)
- to save the RTC clock 
2) But in reality the battery is always needed/mandatory because without battery the PLC can’t start in RUN after a PowerOFF, PowerON of the PLC
Within %S75 the PLC program can check when the battery has the be replaced before it become out of order  (in average every 4/5 years):
            When this bit is Set to 1, it means external battery power is low, so has to be replaced soon
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