Passive hardware redundancy masks or hides the occurrence of errors rather than detecting them. Recovery is achieved by having extra hardware available when needed.
The most common implementation is Triple Modular Redundancy (TMR). Relies on majority voting scheme to mask error in one of the three hardware units.
Active hardware redundancy attempts to do all the four functions i.e. detect errors, confine damage, recover from errors, and isolate and report fault.
•Hardware duplication with comparison
•Hot standby sparing
•Cold standby sparing
•Pair-and-a-spare
Hardware duplication with comparison is the basic building block for active redundancy.
Hot standby sparing and Cold standby sparing
Most common approaches to hardware redundancy
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TMR Architecture PLC.pdf (587.9 KB)