Siemens Simatic M7-400 Reference Manual page 90

Programmable controllers module specifications
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Power Supply Modules
Battery Voltage on the Backplane Bus
The battery voltage is either supplied by the backup battery or externally into the
CPU or receive IM. In its normal state, the level of the battery voltage is between
2.7 V and 3.6 V.
The battery voltage is monitored for the lower limit. Violation of the lower limit is
indicated by the BAF LED and reported to the CPU.
BAF lights up if the battery voltage on the backplane bus is too low. Possible
causes of this include:
• Battery (batteries) empty or battery polarity has been reversed.
• External supply via CPU or receive IM is defective or supply from secondary
power supply module is defective or missing.
• Short circuit or overload on the battery voltage.
Note
Due to internal capacities, if you remove the battery or switch off the external
supply, some time may elapse before BAF, BATT1F, or BATT2F lights up.
3-10
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07

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