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If the switch occurs due to a controlled condition such as toggling the unit selection switch on the Redundancy Communications Module or forcing a switch in the user logic program with a SVC_REQ, or because of a fault detected by the PLC CPU, then the
If the switch occurs due to a failure of the PLC CPU (including loss of power), then the switch will occur after the backup unit determines that the active unit has failed to rendezvous at the synchronization point. Failure to rendezvous may take up to 2 failwait timeouts (1 for each link) to determine that a failure has occurred. Control will not transfer, in this case, until both links have been tried unsuccessfully.
Configurable Backup Data Size
The maximum size of the backup data (Shared I/O) is 20 KBytes of Input data and 28 KBytes of Output data. The shared I/O data configuration must be the same in both the Primary and Secondary units. This shared I/O data is transferred from the active CPU to the backup CPU during the CPU sweep process. A total of up to 56 KBytes of user memory is consumed by this data transfer. A maximum of 48 KBytes of the 56 KBytes is the total Shared I/O (20 KBytes %I, %AI; 28 KBytes %Q, %M, %AQ, and %R), while the remainder (8 KBytes) is used by the system for internal data transfers, including synchronizing data.
On-Line Programming
A connection and disconnection of the parallel programmer cable should only be made with the programmer properly grounded, and Logicmaster 90 software properly booted up and in
On-Line Repair
A Hot Standby CPU Redundancy system allows you to do
After replacing the failed component and returning power to the rack, the backup unit will resynchronize with the currently active unit. The unit which had failed and was previously the active unit will determine its role in the system as part of the resynchronization process. If it is the Primary unit (with Serial Bus Address 31) it will once again become the active unit, the unit with Serial Bus Address 30 (Secondary unit) will again become the backup unit. For more detailed information on replacing failed components and resynchronization, see Chapter 4, ºSystem Operationº.
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