A technique for specifying interface modules for real-time systems

Documentation plays a key role as a component of design process, and a preview of a task before it comes to be executed. It is particularly useful for aviation and military applications in which reliability and maintainability are very important aspects for judging the success of a project. Interface modules (IM) encapsulate input or output device hardware and the related software, so that the application software can be written without specific knowledge of the particular devices used. An IM specification must therefore use a combination of notations and formalisms. In this paper, we present a technique for IM specification that is an extension of a system requirements documentation technique. It is based on the software cost reduction (SCR) method. An IM is specified as a "subsystem" that interacts with both the external environment and other software modules in the larger system. The interface quantities are modeled as functions of time and the behaviour is described in terms of conditions, events and mode classes. This technique facilitates concise and formal description of the module behaviour, including tolerances and delays.