Some Aspects of Networking Needed for an Integrated Distributed Supervisory Control System

Abstract

There has been considerable growth in the use of Local Area Network (LAN) technology for Distributed Process Control systems over the last few years. Mostly this has concentrated on three main areas: proprietary highways linking plant interafce and control equipment, simple links to data processing equipment, and some use of products such as DECnet to link supervisory computers. This paper highlights some current problems, and focuses on some unique new aspects of distributed control which are necessary for providing a properly integrated approach to distributed supervisory control systems.A Subscription Service satisfies the need to efficiently move data between supervisory computers, taking into account the possibilities of multiple sources (redundancy) and multiple users of the data, making use of the broadcast capabilities of Ethernet to minimise overhead. A Hot-Standby Pair configuration allows two supervisory computers on the network to operate in a redundant manner, providing high reliability. A Global Facility Secure service allows legislating or locking of access to a shared resource (such as part of a database) on another supervisory computer. This permits multiple operations (which are divisible in time) to be safely done. A Virtual Display System allows a comprehensive set of Operator Displays on one supervisory computer to be used by an Operator Display Station on another computer. The concept of servicing the display over the network, rather than transferring the data, results in a reduced load, both for the network and for the computers.