A
Series 90-70 Redundancy Through Application Logic
The following restrictions apply using the current Series
HDo not use transitional contacts or coils since this information cannot be transferred across the link.
HAvoid using timers, counters or PID in program blocks which are not called every sweep otherwise timer durations may not match up when control is transferred.
HSince PID keeps the actual real time clock in its data structure, you cannot simply execute it all the time while updating its entire data structure. You must omit the clock portion of the data structure when you do the transfer. Another safer method would be to keep the backup unit in manual mode while forcing the manual register to the output of the master. When the
HThe time of day clock cannot be shared between PLCs, therefore it cannot be used to make program decisions.
HDo not use MCRs since the internal value of the MCR cannot be transferred between systems.
HDo not expect information that comes in to the system from an external communications device to be the same in both systems. The communications can change data in one system before it is updated in another.
HAvoid using fault contacts and fault table information to base critical program decisions, some of the information is not available to be passed between systems.
HAvoid using the timed contacts since the internal PLC's real time clocks will not match.
HLocal program block memory (%L) cannot be transferred between the PLCs, therefore data that must be shared should be restricted to the transferred memories.
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