Distributed On-line Digitized Control Architectures for Process Control

Abstract

A tremendous amount of investigations and realisations has been carried on for the last ten years in the domains of communication media and protocols, distributed database management, distributed operating systems and reliability resulting in significant progresses towards standardization. However, most of these results apply to on-line systems and relatively very little of this work is related to hard-real-time environments whereby specified bounds on processing delays are stringent and ranging from few millisecunds to few hundreds of milliseconds.Based on the ISO OSI-Reference Model analysis, rationale are presented for a reliable connectionless-mode, message due-date-dependant qualities of services and flexible conversation protocols above the transport layer. So as to support time-dependant transmission protocols, it is shown that the IEEE 802 Local Area Network standards must be enhanced in order to provide message transmission scheduling based on message due-dates. A task priority is defined as a function of both its static priority and its due-date. Data access serialization induces precedence constraints on task scheduling. Task resource allocation conflicts are solved though a unique task-priority-dependant resolution rule.