Omron SYSMAC C Series Operation Manual page 134

Specifications and General Information Section 7-1
5. A voltage (current) may be output momentarily from the output terminal
when the power supply to the PC is turned ON or OFF.
Approximately 1 V will be output for several hundred milliseconds after the
power is turned ON or OFF.
If this causes a problem, provide countermeasures so that the power supply
or output timing of external devices differs from the ON/OFF timing of the
power supply at the PC.
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