Mitsubishi Electric MELSEC iQ-R Series User Manual page 425

Program execution time
Set a program execution time of the standby system to be shorter than that of the control system. When a program execution
time of the standby system is longer than the total program execution time of the control system, the control system starts the
next scan before the standby system has received tracking data, causing a continuation error. If the systems are switched in
this state, the latest data may not be reflected on the new control system.
When a program execution time of the standby system cannot be shortened, set SD1662 (Tracking transfer data receive
completion wait time) to extend a waiting time for tracking data receive completion of the control system. With this setting, the
CPU module of the control system starts the next scan after the standby system has received tracking data. Even if the
systems are switched, the control can be continued based on the latest data.
In a redundant system with redundant extension base unit, set a program execution time of the standby system to be within
200ms. When the program execution time exceeds 200ms, the standby system detects a continuation error. If operation is
performed while the program execution time exceeds 200ms, a hardware failure of the CPU module of the control system
causes system switching, and because a stop error occurs in the CPU module of the new control system, control may not be
continued.
Tracking transfer
• Do not set global devices used in a program executed in both systems as a tracking transfer target. Due to tracking
transfer, the standby system data is overwritten with that of the control system, and the standby system program may
operate in an unintended way.
• When using a label in a program executed in both systems, use a local label.
• When using an FB in a program executed in both systems, use a local FB. If a global FB is used, a program execution
result of the control system is transferred and written over a program execution result of the standby system, causing an
unintended operation.
26 REDUNDANT FUNCTION
26.6 Program Execution in Both Systems
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