A method of reverse engineering requirements for process-control system software is presented, along with a domain-specific functional structure. Techniques are demonstrated on the executable pseudocode of a commercial avionics control system. Resulting requirements are expressed as a state-based model of externally visible behavior specified completely in the language of process control.>

The paper describes an approach to writing requirements specifications for process-control systems, a specification language that supports this approach, and an example application of the approach and the language on an industrial aircraft collision avoidance system (TCAS II). The example specification demonstrates: the practicality of writing a formal requirements specification for a complex, process-control system; and the feasibility of building a formal model of a system using a specification language that is readable and reviewable by application experts who are not computer scientists or mathematicians. Some lessons learned in the process of this work, which are applicable both to forward and reverse engineering, are also presented.>