Meaning
System performance comparisons between two identical processing units identify errors without needing a third vote. In a dual modular redundancy configuration, both components perform the same calculation and the system compares the outputs. If the results differ, the system identifies a fault and initiates a safety protocol.
This approach is common in industrial controllers where continuous operation is less critical than preventing incorrect outputs.
Error Identification
Mismatched data between the two streams signals that one unit has failed or encountered a transient glitch. Because dual modular redundancy only uses two units, it can detect that an error has occurred but cannot determine which unit is correct. This limitation requires the system to halt or enter a fail safe state until the discrepancy is resolved.
System Reliability
Probability of a dangerous failure is reduced because both units must fail in the exact same way to produce an undetected error. Implementing dual modular redundancy increases the hardware cost and the physical footprint of the control cabinet. Designers use this architecture when the cost of a system shutdown is lower than the cost of an incorrect command.
Availability Tradeoff
Maintenance requirements increase because two separate units must be monitored and repaired. While dual modular redundancy improves the safety of the process, it can lead to more frequent shutdowns due to false alarms from a single unit glitch. Redundancy levels are balanced against the target uptime defined in the service level agreement.
Total system health depends on the synchronization of the internal clocks.
