Slot swapping mechanisms for process control networks

Modern industrial plants feature a wide use of computer networks for the interconnection of various environments which present very different time constraints on the exchange of information. The existence of heterogeneous traffic flows entails the use of different network topologies to support the different kinds of traffic. Communication between field devices in a single control cell, i.e. between sensors, actuators and control devices (PLC) is supported by Fieldbus networks. In an Industrial Process Control and Measurement System (IPCMS), control cells do not operate in an isolated fashion, but their activities are strictly correlated and need a continuous exchange of information. This exchange is supported by bridges which interconnect various FieldBuses. Because of the presence of various bridges, a variable that has to cross several bridges may accumulate an excessive delay which may cause the associated deadline to be exceeded. In this paper we propose the Slot Swapping Protocol (SSP), a new protocol for the interconnection of FieldBus networks. We will describe the architecture of the system and the mechanisms used by the SSP protocol, based on the idea of slot-swapping. Although the solution proposed provides for totally distributed management of access to the ring, it guarantees that each FieldBus will transmit messages to other Fieldbuses within the time limits set for each message, overcoming the limits of interconnection by means of chains of bridges.