Fault coverage design and analysis tools for fault tolerant systems

Complex systems require severe availability objectives related to the importance of the service being provided; for this reason, they are usually fault tolerant. Fault-tolerant designers try to create mechanisms to detect 100% of faults, because in complex systems, uncovered faults lead to latent highly undesired situations. Starting from the basics of FMECA, a design methodology and a tool have been developed (DIANA=Diagnostic ANAlysis). The basic idea of DIANA is to perform coverage analysis during hardware and firmware design together with reliability engineering analysis. To this purpose, DIANA has been integrated into the CAD tools in the same way that logic simulation, timing analysis and analog transmission simulation are performed. Two main results have been obtained by the DIANA project: the first is to provide the designers with a tool that helps them to think in such a way to prevent uncovered fault situations; the second is to calculate the effects of faults on diagnostics in order to provide transition rates to system availability models when real, rather than ideal, cases are taken into account. And we can find the point of diminishing returns.