Box Behavior, e Study

Prior to designing a system, customers and contractors should agree on required black box (externally apparent) system behavior. To define this behavior, practical, precise, design independent methods are needed. This paper describes results of a case study’ in which formal event-based approaches are used, demonstrating that a combination of history based traces and guarded event-action statements is practical for dejining black box behavior. Externally apparent modes (states) simplify the specification to promote human understanding. The spec$?er is allowed to use both traces and event-action statements in a single specification, as requirements that define sequential events are best specijied using traces, and requirements that are conditional are best specified using eventaction statements. Graph generation from the model, as opposed to graph definition makes this type of specification easier to define and maintain. roblems with current methods Methods commonly used in industry to model system requirements and design are not adequate for specifying externally apparent (black box) behavior. A specification defining such behavior should identify what a system does (the response) when it receives a specific input (the stimulus). The stimulus may be singular, or may consist of a sequence or set of stimuli. The same can be said for the system response. High level models and methods for specifying interfaces (e.g. context diagrams) do not relate system stimuli to system response. Detailed models that show the transformation from input to output include design decisions. Black box specifications should not include design decisions, as they make the specification harder for customers to understand, and Iimit designers. Engineers frequently use Context Diagrams and Interface Control Documents (ICDs) to specify the interface between a system and its environment. Normally (e.g. in structured analysis) Context Dia,orars specify information flowing to/from the system, events ’ Research funded by Office of Naval Research (ONR) under Engineering of Complex Systems Technology Program, Contract N60921-94-C-0073. O-8186-7614-0/96 $5.00