4 |
The Start of Sweep Time message transfer repeatedly coordinates the elapsed time clocks (upon which timers are based) in the redundant CPUs. The system time is continuous as long as one of the two systems is running. When a switchover occurs, the same time continues to be kept in the new active unit.
Output Data Transfer to the Backup Unit
After the initial data transfer, both CPUs operate independently until the end of the program logic solution. Before the output scan starts, a second automatic data transfer occurs. In this time, the active unit transfers the selected control and output data to the backup unit. This includes the %Q, %AQ, %R, and %M memories. For discrete data, the status, override, and transition information is transferred. If point faults are configured, point fault data is also sent.
ACTIVE CPU
Housekeeping
Input Scan
Send Inputs
and
Synchronize
Logic Solution
Send Outputs
and
Other Data
Output Scan
Windows
and
1
D AT A
2
D AT A
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
2Second Data Transfer Occurs: %Q, %AQ, %R, %M
The data is transferred in blocks. Each block is checked for data integrity. The backup CPU holds the transferred data a temporary area until all the data has been received and verified. Then the backup CPU copies the data into the actual PLC memories. If the full transfer fails to complete properly, the backup unit disregards the data in the temporary area and instead uses the values it obtained during its own logic solution.
After the second data transfer, the active and the backup CPUs independently perform their output scans and run their programmer and system communication windows. They continue to operate independently until they synchronize again after the next input scan.
Chapter 4 Normal Operation |