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5.3 The System Switching Function
5.3.6 System Switching Precautions
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5.3.6 System Switching Precautions
(1) System Switching Failure
The control system may disappear and the redundant system may not function in any
of the following cases:
(a) A tracking cable error (including tracking cable disconnection) has
occurred during system switching (due to hardware failure, power off, or
CPU module reset).
(b) A communication error has occurred between the CPU modules during
system switching due to a user instruction or a stop error.
(c) During system switching due to a user instruction or a stop error, a tracking
cable error (including tracking cable disconnection) has occurred
concurrently with any of the following standby system conditions:
• The standby system is powered off or its CPU module is reset.
• A stop error occurs in the standby CPU module.
• Control system and standby system CPU module operating statuses are
different.
• A standby system network module failure
Operations when the system switching fails due to one of (a) to (c) shown in the
Table5.40 below.
If the system switching has failed, power OFF the system A and B, connect a perfect
cable to the CPU module, and then power ON the both systems simultaneously. With
this operation, the system A will operates as the control system.
(2) Monitoring the Watchdog Timer During System Switching
The redundant system suspends the watchdog timer from being monitored during
system switching.
Therefore, the "WDT ERROR (error code: 5001)" will not occur in the system
switching scan even if the scan time exceeds the monitoring time set in the scan time
watchdog timer.
Table5.40 System A and B CPU Module Operations When the System Switching Fails
Function
System A and System B CPU Module Operation
CPU Module Operating Status
Standby System Operation
LED Indication
BACKUP
Red light ON (in Backup Mode)
Amber light ON (in Separate Mode)
CONTROL
Light OFF
Special Relay
SM1516
(Operation System Status)
ON (Standby system)
System Switching
Request
System Switching Request
from Network Module
Cannot execute system switching (ignores system switching request)
System Switching Instruction
Cannot execute system switching (instruction cannot be executed due to
standby system operation)
System Switching Request
from GX Developer, etc.
Cannot execute system switching
Returns error code 4240H on startup (Can't execute on standby system).
Communication from GX
Developer, etc.
Communication specified by
control system CPU module
Communication error
Returns error code 4248H on startup (inconsistency).