7
PROGRAMMING CAUTIONS
7.1 Instructions Restricted in Use for Redundant System
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(b) Relevant Instructions
• PLS
•
P(Examples: MOVP,INCP, etc.)
• SP. ,GP. ,ZP. ,JP. (Intelligent Function Module Dedicated Instructions)
(7) Restrictions on Use of COM and ZCOM Instructions
The COM and ZCOM instructions execute refresh between the Redundant CPU and
network module during program execution.
In a redundant system, there are restrictions on the selection of the refresh items by
the COM and ZCOM instructions.
Table7.4 shows the refresh items by the COM and ZCOM instructions and whether
they can be selected or not in the redundant system.
: Selectable,
: Not selectable
* 1: Tracking is not executed when the COM or ZCOM instruction is executed.
If system switching occurs due to any of reasons for system switching indicated in No. 1 of
Table7.5 from when the COM instruction or ZCOM instruction is executed until tracking is
completed, system switching is done without tracking being performed.
Hence, any change made to the output to the network module by the COM/ZOM instruction in the
control system CPU module will not be reflected on the standby system CPU module.
Since the new control system CPU module provides the old output to the network module after
system switching, the output from the network module may change.
* 2: Not selectable since an intelligent function module cannot be mounted on the main base unit in
the redundant system.
* 3: For the COM instruction, select the refresh item with the special relay "Link refresh selection
during COM instruction execution (SM775)" and special register "Link refresh selection during
COM instruction execution (SD778)".
Table7.4 Refresh Items by COM and ZCOM Instructions and Whether
They Can Be Selected or Not in Redundant System
Instruction
symbol
Refresh item
*3
Selection in redundant
system
COM
I/O refresh
Network module refresh
*
1
Auto refresh of intelligent function module
*
2
Auto refresh of CPU shared memory
General data processing
ZCOM
Network module refresh
*
1
Auto refresh of intelligent function module
*
2
Table7.5 Reasons for System Switching and Output Changes at System Switching
No.
Reasons for System switching
Output change when COM/
ZCOM instruction is used
1
• Control system power off
• Control system CPU module reset
• Control system hardware fault
• Control system CPU module stop error
Since system switching is
executed without tracking, the
output may change at the time
of system switching.
2
• System switching request from network module
• System switching instruction execution
• System switching request from GX Developer or OPS
Since system switching is done
after tracking, the output does
not change at system switching.
