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28 FUNCTIONS
28.6 Program Execution in Both Systems
Operation at system switching
The operation at system switching is different for a program executed in both systems. The following table shows the
operation at system switching.
For the operation of programs which are not set as both systems execution programs at system switching, refer to the
following.
Page 446 Operation at system switching
Item New control system CPU module New standby system CPU module
Program execution Initial execution
type program
When an initial execution type program has not been
completed on the old control system at system
switching, it is executed again from its head.
When an initial execution type program has not been
completed on the old control system at system
switching, it is not executed on the new standby
system because system switching is performed after
the program completion on the old control system.
Scan execution type
program
This type of program is executed from the step 0.
Direct access input (DX) In the program execution after system switching, data is fetched when an instruction using the direct access input
(DX) is executed.
Direct access output (DY) Data is not output.
However, data is output when "Standby System Output Setting" of the CPU parameter is enabled and an
instruction using the direct access output (DY) is executed in the program execution after system switching. (
Page 485 Redundant System Operation Setting)
FROM/TO instructions In the program execution after system switching, this instruction is executed when the instruction execution
condition is satisfied.
Instruction that requires several scans • In the program execution after system switching, this instruction is executed when the instruction execution
condition is satisfied.
• When the systems are switched while the instruction is being executed, the instruction execution continues
and the completion device turns on at a completion of the instruction execution.
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