Contemporary industrial systems are challenged by fast-growing requirements for agile and effective reactivity to rapidly changing market demands. To achieve this set of requirements, new technologies and paradigms like Internet-of-Things (IoT), Big Data Analytics, Internet-of-Services (IoS), and service-oriented architecture (SOA) are being introduced into the industrial environments. These advances result in the confluence of two dissimilar, but complementary domains: the physical operational technologies (OT) and the cyber information technologies (IT) domains. Convergence of these two domains in a cross-layer fashion implies a new set of two major requirements of components and systems: 1) structural connectivity and 2) functional interoperability. However, the wide variety and heterogeneity of industrial systems-especially in the factory floor-entails integration complexity, which is in contrast with the mentioned requirements. In this paper, we focus on how legacy industrial systems can migrate in a cost-effective manner to the new paradigm of integrated IT-OT levels. We propose an interoperability layer requiring no changes on the legacy device that maps field device data into an ISA95-based information model. The performance of our contributed open-source implementation is evaluated in several deployment configurations.