Software redundancy:

• Automatic switchover from failed central controller (master) to standby central controller.

Operation with WinCC operator station:

• Request switchover
• Switch redundancy on/off (activate/disable switchover)
• Display status of redundant coupling
• Display status of ET 200M slaves

Mode of operation

In the event of a fault, the "software redundancy" package initiates a switchover from the master central controller to the standby:

• During switchover:
  no effect on process since the controller outputs remain frozen
• Following switchover:
  continuation of process based on last data received. Data may be several cycles old, therefore only useable for slower processes.

The switchover time between failure of the master and continuation of the process on the standby controller depends on several factors:

• Communication performance of the CPUs used
• Communication medium for master/standby coupling, type of connection and transmission rate
• Quantity of data to be transmitted
• Cause of fault
• Transmission rate of redundant PROFIBUS DP
• Number of slaves on the redundant PROFIBUS DP

For example, the following switchover times result for a system comprising two 414-2 DP CPUs and four ET 200 stations with 4 KB data to be transmitted:

• If the CPUs are coupled via PROFIBUS DP: 1.2 s.
• If the CPUs are coupled via MPI: 1.5 s.