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This paper presents an online diagnosable fault-tolerant system:N-unitt-fault tolerable system. The number of unitsNin the system can be either odd or even. The relationship betweenNandt(the number of faulty units which can be tolerated) is presented. The approach of an optimumN-unitt-fault tolerable system is also given. As an example, a 4-unit 2-fault tolerable system is discussed. The reliability and mean time to failure of 4-unit 2-fault tolerable system are shown to be higher than 5MR (5-modular redundancy) and TMR (Triple Moduler Redundancy) system reliabilities. The amount of hardware components in a 4-unitt-fault tolerable system is simpler than 5MR. The complexity of switching circuit forN-unitt-fault tolerable system increases only linearly with respect of the number of modules. Our scheme is also simpler than the hybrid redundancy system. Some theorems for the online diagnosis ofN-unitt-fault systems are given and proved.

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