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The transfer of the redundancy data during each sweep will be in blocks with each block checked for data integrity. The transferred data will be held in a temporary area by the backup CPU until all data has been received and verified from the active unit. Then the backup CPU will copy that data from the temporary area to the actual PLC memories. The transfer is capable of being performed on either RCM link. If one RCM link fails then the transfer will switch to the other RCM without causing a loss of synchronization in the system. If at any time during a transfer the active unit fails or the full transfer fails to complete properly (both RCMs fail) then the backup unit will disregard the data that has been transferred to the temporary area and proceed with the values it already obtained during its input scan.
The last part of the input data transfer is the synchronizing message containing the ºStart of Sweep Timeº. The ºStart of Sweep Timeº is the universal start of sweep time for the redundancy system. Normally each PLC CPU operates its elapsed time clock on which timers are based independently and the clocks are always started over from zero on a power cycle. Two independent clocks would cause time discontinuity at switch over time. In addition the clocks will tend to drift from each other over time. The redundancy system corrects for this by keeping a single elapsed time value for the entire redundancy system. The time will be continuous as long as one of the two systems continue to run and the active unit will continuously pass the time to backup unit to correct for any natural drift in the clocks. When a switch over occurs, the same time will continue to be kept in the new active unit.
Data Transfer from Backup Unit to Active Unit
Eight bytes (4 registers) of data can be transferred from the backup unit to the active unit during the input data transfer before the logic solution. To initiate this transfer, the backup unit executes SVCREQ #27 (Write to Reverse Transfer Area) to copy eight bytes of data from the reference specified by PARM to a temporary buffer (SVCREQ #27 on the active unit will have no effect). This transferred data will be stored in a temporary buffer on the active unit.
In the active unit, SVCREQ #28 (Read from Reverse Transfer Area) is then executed to copy the eight bytes of data from the temporary buffer to the reference specified by PARM (SVCREQ #28 will have no effect on the backup unit).
Note
There is always a one sweep delay between sending data to the active unit using SVCREQ #27 and reading the data using SVCREQ #28 on the active unit.
This data copied from the buffer is not valid
Hduring the first scan after either unit has transitioned to RUN;
Hwhile the backup unit is in STOP mode;
Hif the remote unit does not issue a service request to the sending unit.
An example of this data transfer is described below.
This data should not be used if REM_RDY is off or if REM_RDY is transitioning to on.
If the following two rungs are placed in the program logic of both units, the backup unit will send %P0001 through %P0004 to the active unit. The active unit will read the data into %P0005 through %P0008. %P0001 through %P0004 on the active unit and %P0005
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