This paper proposes a novel .N' -modular redundancy (NMR) structure with concurrent output error location (COEL) capability. The concurrent output error locatable NMR structure consists of a conventional NMR structure and a totally self-checking (TSC) extra circuit with N+ I two-rail code outputs. This extra circuit is used for locating the output error produced by any replicated module or the voter, and the internal fault produced by the extra circuit itself: Such a self-checking extra circuit is called the output error locator (OEL). It is constructed by conventional TSC two-rail code checkers (TRC ' s) and self-testing multi-input comparators. Each comparator is made by adding one extra two-rail code input to the cascaded multi-input comparator (4). The error handling capabilities, hardware complexity and propagation delay are analyzed for the proposed OEL. The performance of the proposed scheme and that of the Gaitanis's scheme (6) are compared for a triple modular redundancy (TMR) structure.