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3.6 Operation Processing during Momentary Power Failure
When the input voltage supplied to the power supply module drops below the specified range, the CPU
module detects a momentary power failure and performs the following operation.
(1) When a momentary power failure occurs for a period shorter than the allowable power
failure time
The CPU module registers error data and suspends the operation processing.
The CPU module, however, continues measurement in the timer device and holds the output status.
(a) When resume start is specified for the SFC program
Data in the system is saved.
(b) When power is recovered after a momentary power failure
The CPU module restarts its operation processing.
(c) Watchdog timer (WDT) measurement during a momentary power failure
Even if operation processing is suspended due to a momentary power failure, the CPU module
continues the measurement of the watchdog timer (WDT).
For example, when the WDT setting of PLC parameter is 200ms and the scan time is 190ms, if a
momentary power failure occurs for 15ms, "WDT ERROR" occurs.
(2) When a momentary power failure occurs for a period longer than the allowable power
failure tim
The CPU module starts its operations initially.
Operation processing will be the same as that when any of the following is performed.
• Programmable controller is powered on.
• The CPU module is reset by the RUN/STOP/RESET switch.
• The CPU module is reset by GX Developer (the remote reset operation).
● In a redundant power supply system, the CPU module does not suspend its operations if a momentary
power failure occurs in either of the power supply modules. However, if a momentary power failure occurs
under the condition where the power is supplied to only one of the power supply modules, operations are
suspended.
● Information of a momentary power failure occurred in a redundant power supply system will be stored in
SM1782 to SM1783 and SD1782 to SD1783.
On the other hand, information of a momentary power failure occurred in a single power supply system
will be stored in SM53 and SD53. (
CHAPTER 12)
Figure 3.1 Operation processing during a momentary power failure
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Momentary power
failure occurrence
Power recovery
The CPU module suspends
its program operations.