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Synchronous Scan
The figure below shows the sweep components for the active and the backup CPUs. It shows the two communication points in the sweep. The first communication point is immediately after the inputs are scanned. At this point in the sweep the newly read inputs are sent from the active CPU to the backup CPU and synchronization information is passed. In the second communication point, the rest of the data (outputs, internal references, registers) is sent from the active PLC to the backup.
ACTIVE CPU
Housekeeping
Input Scan
Send Inputs
and
Synchronize
Logic Solution
Send Outputs
and
Other Data
Output Scan
Windows
and
1
D A T A
2
D A T A
a47008
BACKUP CPU
Housekeeping
Input Scan
Receive Inputs
and
Synchronize
Logic Solution
Receive Outputs
and
Other Data
Output Scan
Windows
and
1First Data Transfer Occurs: %I, %AI and Synchronization
2Second Data Transfer Occurs: %Q, %AQ, %R, %M
Figure 8. Active and Backup Sweeps
First Data Transfer %I, %AI and Synchronization
There are two points in the sweep where the active CPU will transfer data to the backup unit. The first data transfer will occur immediately after the Input Scan has occurred. The active unit will send all of the configured input data, both discrete (%I) and analog (%AI), to the backup unit. For discrete data, the status, override, and transition information is all transferred; if point faults are configured, point fault data is also sent. This data will overwrite the current input data in the backup unit.
In addition to the input data transfer, a synchronizing message containing the ºStart of Sweep Timeº will be sent from the active unit to the backup unit as soon as the input data has been transferred. The CPU's will stay in synchronization because the active unit will wait on the backup CPU to respond to the synchronizing message before starting its sweep.
Chapter 4 Operation | 59 |