Mitsubishi MELSEC-A Series Reference Manual page 314

10. TROUBLESHOOTING
POINT
(1) In the MELSECNET data link, if the power supply of the local or remote I/O
station on both sides of a normally operating local or remote I/O station is
turned off simultaneously (within 100 ms), the data link for the entire system
might be disabled.
If the "automatic return function" is set for these stations, data communication
resumes immediately. However, if the automatic return function is not set for
these stations, they will remain disconnected. To return them to the data link,
perform reset operation.
Example:
If the power supply to L1 and R3 is turned off simultaneously (within 100 ms)
while R2 is operating normally, the data link for the entire system might be
disabled.
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