Resumo - O esquema de arbitragem tipicamente usado nos barramentos de campo “tempo-real” baseia-se num scheduler do tipo off-line que gera uma tabela estática e cíclica contendo a atribuição de tempo de barramento às transacções associadas a variáveis do processo. Esta abordagem, por exemplo usada no barramento de campo FIP (Factory Instrumentation Protocol) é altamente inflexível no sentido de que qualquer alteração ao sistema, tal como adicionar um sensor, obriga à interrupção do funcionamento do barramento. Neste artigo propomos a utilização de um scheduler do tipo planeamento para resolver esta inflexibilidade. Este scheduler representa uma situação de compromisso entre entre as vantagens e desvantagens do escalonamento (scheduling) dinâmico e estático típicos. É, também, apresentada uma condição suficiente de escalonabilidade (schedulability) que implica um custo de desempenho mínimo (overhead) e que, por essa razão, é adequada para a análise on-line. A possibilidade de utilização do scheduler de planeamento no contexto do barramento FIP também é descrita obtendo- -se, desse modo, um barramento compatível tipo-FIP. Abstract - A typical approach to real-time fieldbus arbitration is the use of an off-line scheduler that generates a cyclic static table containing the allocation of bus-time-slots to the transaction of process control variables. This approach, used in the FIP fieldbus (Factory Instrumentation Protocol), is highly inflexible in the sense that any system changes, such as adding a sensor, requires the interruption of the fieldbus operation. In this article we propose the use of a planning scheduler to overcome such inflexibility. This scheduler compromises between the advantages and disadvantages of typical dynamic and static scheduling. A sufficient schedulability condition is also presented that incurs minimal run-time overhead and, therefore, is suited to on-line analysis. The possibility of using the planning scheduler within the FIP context is also described resulting in a compatible FIP- like fieldbus. I. INTRODUCTION The increasing demand for higher production volumes and lower production costs has been pushing industry towards an ever increasing automation level. This fact has led to a higher complexity of the typical industrial plant including more and more equipment such as sensors, actuators and controllers. Due to processing power, reliability, flexibility and modularity requirements, industrial systems have become distributed, making use of intelligent equipment spread over the factory plant and interconnected by means of an industrial communications network. The typically large number of equipment interconnected led such networks towards a bus configuration in order to reduce cabling cost and increase modularity and ease of maintenance. These networks, known as fieldbuses, convey data which are used within control loops and thus, are under precise timing constraints. Therefore, a network protocol is required which uses a real-time deterministic access arbitration scheme allowing to know in advance whether the conveyed data will meet its timing constraints [1,2]. If these constraints are not satisfied severe consequences to human lives, equipment and/or environment may happen. This problem has been solved by the FIP fieldbus (Factory Instrumentation Protocol) using the producer- -distributor-consumer(s) model [1,2,3,4,5]. A centralised bus arbitrator uses a cyclic static schedule to initiate periodic transactions 1 for the production and broadcasting of process variables 2 (e.g. sensor readouts or controller outputs). Such schedule is produced off-line and must be loaded into the local memory of the bus arbitrator prior to the start of system operation. The FIP protocol suffers from the most common disadvantage of static scheduling: operational inflexibility. In order to improve the use of FIP-like buses in dynamic industrial environments, different scheduling schemes must be used that allow on-line system changes [2]. In this paper we propose the use of a planning scheduler to overcome the referred inflexibility. We will show that this scheduler is a good compromise between static and dynamic approaches. 1 Actually, the FIP protocol also allows a traffic of non-time- constrained sporadic transactions. However, such traffic is isolated from the time-constrained periodic traffic and is not relevant for the real-time behaviour of the fieldbus. Therefore, such sporadic traffic is not mentioned throughout this article. 2 Whenever the term “variable” is used we refer to variables which value has to travel over the fieldbus, only. Improving Flexibility in a Real-Time Fieldbus Network Rosa Pasadas, Luís Almeida, J. Alberto Fonseca