The Planning Scheduler: Compromising Between Operational Flexibility and Run-Time Overhead

Abstract

The operational flexibility of fieldbuses is becoming an important issue in modern industrial automation systems. In these systems, control and measurement equipment is normally connected to a fieldbus. For certain applications it is interesting to dynamically change the system configuration by either removing or adding equipment, or just by changing certain properties of existing one. The difficulty in doing so is to guarantee that the modifications done on-line do not cause any timing constraints in the communication system to be missed.This article discusses the existing approaches to real-time message scheduling in what concerns the schedulability analysis of bus traffic. The use of a planning scheduler, as proposed previously by the authors, is briefly described and the impact on system flexibility, scheduler run-time overhead and schedulability guarantee is discussed. Some new results concerning the worst-case execution time of the scheduler and the worst-case message delays are presented.The article also includes a discussion on the applicability of the planning scheduler in the context of existing fieldbus standards. It is shown that this scheduler can improve the operational flexibility of fieldbuses based on static scheduling approaches and allow on-line schedulability analysis in fieldbuses based on dynamic best-effort approaches.