Modeling On-line tests in Safety-Critical Systems

This paper proposes a methodology to evaluate the dependability of safety-critical systems that use on-line tests. The methodology is structured as a two-steps process, accounting for error generation and propagation dynamics. First, the unconstrained flow of error propagation within the system is modeled by using templates of its internal hardware components and composing them based on the error propagation dynamics. This compound model of the system is then augmented to include internal error detection mechanisms and diagnostic tests, which constrain the error propagation flow. Each mechanism is modeled as a plug-in template that can be separately added and/or removed to/from the global model of the system to evaluate different designs. The proposed methodology is applied to analyze the effectiveness of the set of mechanisms employed in an interlocking system, and some considerations about the methodology computational cost and the acceptability of the introduced approximations are discussed.