11 BASIC CONCEPT OF REDUNDANT SYSTEM
11.5 Operation Modes of a Redundant System
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11.5 Operation Modes of a Redundant System
A redundant system operates in one of the following two modes.
*1 When programs or CPU parameters are modified while labels (except those with a device assigned) are being used, perform the
following operations when the redundant system is in backup mode.
When the program is modified: Execute the online change function for both systems.
When the CPU parameters are modified: Write data to the programmable controller when the CPU module is in the STOP state.
If programs or CPU parameters are modified only in the standby system, tracking transfer of labels is stopped. ( Page 404 When
data is different between the control system and the standby system)
A redundant system starts up in backup mode. To switch the operation mode, use the engineering tool. ( Page 371
Operation Mode Change)
The following table summarizes the differences between the backup mode and the separate mode.
Operation mode Description
Backup mode A mode to operate a redundant system.
When an error or a failure occurs in the control system, the standby system is switched to the new control system and continues
operation. When a system failure occurs in the control system, the standby system can take over the control because data is
transferred from the control system to the standby system at every scan.
Separate mode A mode to perform system maintenance of a redundant system without stopping control.
In separate mode, the CPU module of the control system and the CPU module of the standby system can execute different
programs. In the standby system, programs and CPU parameters can be modified
*1
and the operation of programs can be checked
with the data transferred from the control system without stopping the control operation of the control system.
Item Backup mode Separate mode
Program operation The control system executes programs.
The standby system executes programs according to "Both
Systems Program Executions Setting" in "Program Setting" of
"CPU Parameter". The standby system does not execute
programs by default.
For how to change the mode to the backup mode, refer to the
following.
Page 372 Changing the operation mode to the backup mode
Both of the control system and the standby system execute
programs.
For how to change the mode to the separate mode, refer to the
following.
Page 371 Changing the operation mode to the separate
mode
System switching Both automatic system switching and manual system switching
are supported.
Only manual system switching is supported.
Tracking transfer Data is transferred according to "Tracking Setting" of "CPU
Parameter".
Data is transferred according to "Tracking Setting" of "CPU
Parameter".
However, the following data is not transferred.
• Local devices, local labels
• Special relay, special register
• PID control instruction information
• Signal flow memory
Memory copy from
control system to
standby system
The following copy operations can be performed.
• Auto memory copy
• Memory copy using the engineering tool
• Memory copy using the special relay and special register
The following copy operations can be performed.
• Memory copy using the engineering tool
• Memory copy using the special relay and special register
System consistency
check
The system consistency is checked. The system consistency is not checked.
Program execution
type
■When the operation mode is changed to the backup mode
The control system and the standby system take over the
program execution type before the operation mode change.
■When the operation mode is changed to the separate mode
The operation differs between the control system and the
standby system.
• The control system takes over the program execution type
before the operation mode change.
• The standby system executes the program with the execution
type specified in the CPU parameter.
Interrupt program
execution enable/
disable status
■When the operation mode is changed to the backup mode
The control system and the standby system take over the
interrupt program execution enable/disable status before the
operation mode change.
■When the operation mode is changed to the separate mode
The operation differs between the control system and the
standby system.
• The control system takes over the interrupt program execution
enable/disable status before the operation mode change.
• The standby system can no longer execute interrupt programs.
I/O refresh The output (Y) refresh of the standby system depends on
"Standby System Output Setting" of "CPU Parameter".
The output (Y) of the standby system is not refreshed by default.
Even if "Standby System Output Setting" is set to "Disable" in
"CPU Parameter", the output (Y) of the standby system is
enabled.
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