Synchronous producer-consumer transactions for real-time distributed process control

Some activities of a modern Industrial Process Control & Measurement System feature critical time constraints which impose strict synchronization between the producers (e.g sensors or controllers) and consumers (controllers and actuators) of information. These devices communicate through particular common bus networks, called fieldbuses, using communication protocols developed to guarantee precise delivery times for the variables transmitted. In process control environments, different control units may need to share the data collected by a sensor, or to broadcast processed data to the various actuation units. Thus, transactions involving one producer and several consumers of a real time variable have to be supported. The transaction model proposed is based on the definition of a group associated to each real time variable. A group is a set of entities comprising the producer of the variable and its consumers. The paper proposes a time constrained group communication approach for real time distributed process control, and presents two transaction models suitable for the scenarios typical of Industrial Process Control & Measurement Systems. In particular, the authors focus on the mapping of the group communication model onto the data link layer protocol of the IEC/ISA FieldBus, as it provides a series of facilities which help the application process to check that some of the required time constraints are met.