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PLC-based systems are commonly designed according to a two-phase process: the system structure is conceived and detailed by the electro-mechanical project team, then the control project team is asked to design and develop the control software. Because of product complexity, different culture of the two teams, lack of supports to requirement specification and communication, and other problems, the design process presents heavy costs and times. The paper aims at presenting an approach for bridging the gap between the two design environments. It adapts and integrates two techniques resulting from European Unwn funded projects: a family-based product modelling technique from the ESPRIT project 8224 -RUMS (used in the structural design phase) and an object-oriented PLC specification language from the ESPRIT I ESSI project 10542 -EASIER (used in the software designphase).

It is the aim of this paper to present a model conceived for supporting the initial, critical phases of PLC software life cycle, namely requirement specification, requirement analysis and software design, to obtain reusable code. The model, named EASIER, is based on an object-oriented paradigm where the message-method mechanism is replaced by the law and action primitives, since they are more suitable to cope with the real-time, cyclic nature of PLC software. Software re-usability is pursued by identifying independent control modules through a proper bedded encapsulation mechanism, representing system dynamics by means of synchronized state diagrams, expressing context-free behaviour of system components by introducing virtual objects. The EASIER approach is presently validated with respect to relevant reverse and forward engineering applications; from one of these cases we derive the examples used in the paper.