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37 FUNCTIONS
37.1 System Switching
Error in the redundant function module
• When an error has been detected on a redundant function module, a continuation error occurs on the CPU module, and the
control system and standby system continue operating without being switched. Check the error code, and perform online
module change if the redundant function module has failed. ( MELSEC iQ-R Online Module Change Manual)
• When the redundant function module was removed from the base unit or a failure has occurred in the base unit and a
communication error has been detected in the communication between the redundant function module and a CPU module,
a stop error occurs on the CPU module and the systems are switched. In this case, check the error code and eliminate the
error cause.
When the CC-Link IE Field Network module has not been mounted and a communication error has been
detected in the communications between the redundant function module and the CPU module, the systems
may not be switched.
To switch the systems even though the redundant function module has been removed from the base unit or a
failure has occurred in the base unit, mount the CC-Link IE Field Network module.
When both systems operate as standby systems
• If a communication error is generated due to a tracking cable error during system switching, both systems may operate as
standby systems. When an error occurs with the tracking cable, each L ERR LED of the redundant function modules turns
on. In this case, replace the tracking cable with a new one as soon as possible. If both systems operate as standby
systems, connect a new tracking cable properly and turn off and on or reset the CPU module of one system so that the
other system will operate as the control system.
• If a WDT error or a hardware failure of the CPU module has occurred in the control system while a stop error exists in the
standby system, the control system is switched to the standby system and the both systems operate as standby systems.
In this case, eliminate error causes, and then power off and on or reset the CPU modules in the both systems.
The following table lists the operations of when both systems operate as standby systems.
System switching disabled by online change
Manual system switching is disabled during an online change.
To disable the manual system switching during an online change, the CPU module is set to the manual system switching
disabled state before an online change starts. After the online change is completed, it is set to the manual system switching
enabled state.
If a communication with the engineering tool is interrupted due to communication cable disconnection or other causes during
an online change, the online change cannot be completed, and the CPU module remains in the manual system switching
disabled state. When the CPU module is in the manual system switching disabled state, the systems cannot be switched by
manual system switching or a system switching request from a network module.
If an online change has failed, refer to the following and take actions.
Page 662 Action for when an online change has failed
Item Operation of both systems
LED of the
redundant function
module
BACKUP LED Flashing
CTRL LED Off
SBY LED On
Special relay SM1635 (Standby system judgment flag) On
System switching
request
System switching request from a network module System switching disabled
System switching request by the SP.CONTSW
instruction
System switching request from the engineering tool
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