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26 REDUNDANT FUNCTION
26.2 System Switching
Operation at system switching
The following table shows the operations of the CPU modules of when the control system and the standby system are
switched.
These operations are for when both systems are operating and the operating status of the CPU modules are the same.
*1 The operation varies in a program executed in both systems. ( Page 421 Operation at system switching)
*2 The operation varies when the signal flow memory is not transferred. ( Page 436 Instructions whose operations vary depending on
tracking of the signal flow memory)
*3 The operation varies when the standby system output setting is enabled. ( Page 425 Redundant System Operation Setting)
Item CPU module of the new control system CPU module of the new standby system
Program
execution
(except the SFC
program)
Initial execution
type program
This type of program is not executed.
However, when an initial execution type program has
not been completed on the old control system at system
switching, it is executed again from the start.
The program execution stops.
*1
Scan execution
type program
This type of program is executed from the step 0.
Fixed scan
execution type
program
The fixed scan execution interval is measured from 0. The program execution stops.
Standby type
program
This type of program is not executed.
Event execution
type program
• Interrupt program: The program is executed when an
interrupt factor is generated.
• ON of bit data (TRUE): The program is executed
when a specified factor is generated.
• Passing time: A specified time is measured from 0.
Execution of SFC
program
Block active
status
The status before system switching is held.
Step active status The status before system switching is held.
Execution of
action
The program execution is started in the active action
using transferred device data.
The program execution stops.
Tracking transfer Tracking data is transferred to the new standby system. Tracking data is received. However, when a stop error
occurs on the new standby system, the new standby
system does not receive the tracking data.
Online change Write operation at system switching continues.
Device/label memory The state before system switching is held.
Signal flow memory
( Page 396 Tracking transfer
setting for the signal flow memory)
• In backup mode, the signal flow of the old control
system is reflected.
*2
• In separate mode, the signal flow of the old standby
system is reflected.
The state before system switching is held.
Special relay (SM), special register
(SD)
The state before system switching is held.
However, when the CPU module is in the RUN state, SD520 to SD531 are cleared.
Output (Y) The state before system switching is held and output
refresh is performed.
After the output (Y) is turned off, output refresh stops.
*3
Direct access input (DX) Data is input when an instruction using the direct access
input (DX) is executed in the program after system
switching.
No operation is performed because the program does not
operate.
*1
Direct access output (DY) Data is output when an instruction using the direct
access output (DY) is executed in the program after
system switching.
FROM/TO instructions These instructions are executed when the instruction
execution condition is satisfied in the program after
system switching.
Instruction that requires several scans These instructions are executed when the instruction
execution condition is satisfied in the program after
system switching.
• No operation is performed because the program does
not operate.
*1
• When the systems are switched while the instruction is
being executed, the instruction execution continues.
However, the completion device does not turn on in the
new standby system, and it turns on in the control
system after the next system switching.
*1
Constant scan Constant scan is enabled. • The constant scan function is disabled in backup mode.
• The constant scan function is enabled in separate mode.
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