1. Objective

Outline the failover mechanism in ABB 800xA PM865 controller pairs using Sync Units.

2. Core Components

• PM865 CPUs
• Sync Units (RCU)
• AC800M bus
• CI854 (if Profibus used)

3. Method

Procedure adapted from official ABB documentation and field observation.

1. Connect both PM865 CPUs to a shared Sync Unit.
2. Flash identical control logic using Control Builder.
3. Monitor heartbeat signal on AC800M bus.
4. Simulate controller loss and observe failover time.

4. Observations

• Failover occurs within 30s (ABB default).
• CPU B takes over with state restoration.
• Heartbeat visible via OPC tag RedundancyStatus.

5. Commentary

This matches vendor expectations. Still pending: testing with different firmware versions.

6. Related Topics

• [[ABB 800xA System Overview]]
• [[Profibus Redundancy Behavior]]