26 REDUNDANT FUNCTION
26.11 Precautions on Programming
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Precautions for using the COM or ZCOM instruction
When refresh is performed by using the COM or ZCOM instruction, output from the remote I/O station or a module on the
extension base unit may change after system switching. To prevent this, do not perform refresh with the COM or ZCOM
instruction. For the COM instruction, whether or not to perform refresh can be set using SM775 (Selection of refresh
processing during the COM instruction execution) and SD775 (Selection of refresh processing during the COM instruction
execution). Set SM775 and SD775 and perform only the device/label access service processing with the COM instruction.
: Selectable, : Not selectable
*1 At the timing when the COM or ZCOM instruction is executed, tracking transfer is not performed. Thus, when the system switching is
attempted after execution of the instruction and before completion of tracking transfer, the system switching is performed without
tracking transfer. Thus, even though output to the module is changed by the COM or ZCOM instruction with the CPU module in the
control system, the change will not be reflected to the CPU module in the standby system, but after the systems are switched, the CPU
module in the new control system outputs the signals before the system switching, the output may change.
*2 This processing is not selectable because a multiple CPU system cannot be built in a redundant system.
Precautions for using the ADRSET instruction
Even though a file is written to both the control system and standby system, different addresses are assigned to the systems.
To continue the processing in the new control system, use the ADRSET instruction to obtain indirect addresses again.
Instruction Refresh processing In a redundant system
COM instruction I/O refresh
*1
Network module link refresh
*1
Intelligent function module refresh
*1
Refresh using the CPU buffer memory of the multiple CPU system (END)
*2
Device/label access service processing (communications with the engineering tool, GOT,
or other external devices)
ZCOM instruction Network module link refresh
*1
Intelligent function module refresh
*1
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