Mitsubishi Electric MELSEC Q Series Reference Manual page 144

(1) System switching factor
If the data link error status (the D LINK LED is off) is continued for the period of system switching monitoring time
or longer, a system switching request will be issued to the control system CPU.
(2) Setting System switching monitoring time
Set system switching monitoring time in "Network Range Assignment" of the control station. (
Section 6.3)
(3) Disabling system switching that is caused by a data link error
Setting Data-link-error-induced system switching disable flag (SB0017) of both systems to ON in advance
prevents a system switching request from being issued to the control system CPU even if a data link error is
*1
detected. (
*1 Tracking-transfer the Data-link-error-induced system switching disable flag (SB0017).
Remark
Use this flag to prevent an undesired system switching during network maintenance (e.g. cable replacement), which may
caused by a data link error due to incorrect operation.
(4) System switching that is caused by a network module error
Upon detection of an error that disables data link, such as hardware failure of a CC-Link IE Controller Network
module, a system switching request is issued to the control system CPU.
Note that system switching due to a network module error is executed regardless of System switching monitoring
time and the status of Data-link-error-induced system switching disable flag (SB0017).
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Page 568, Appendix 1)
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