Supporting Real-Time Communications with Standard Factory-Floor Networks
Ref: HURRAY-TR-9917 Publication Date: 1, May, 1999
Supporting Real-Time Communications with Standard Factory-Floor NetworksRef: HURRAY-TR-9917 Publication Date: 1, May, 1999
Fieldbus networks are widely used as the communication support for distributed computer-controlled systems (DCCS), in applications ranging from process control to discrete manufacturing. Usually, DCCS impose real-time requirements to the communication network; that is, traffic must be sent and received within a bounded interval, otherwise a timing fault is said to occur. This motivates the use of communication networks where the Medium Access Control (MAC) protocol is able to schedule messages according to their real-time requirements. In the past, the scope of fieldbuses was dominated by vendor-specific solutions, which were mostly restricted to specific application areas. More recently, vendor-independent standardised fieldbuses, supporting the open system concept, have started to be commonly used. Particular relevance must be given to the European Standard EN 50170, which encompasses three widely used fieldbuses: P-NET, PROFIBUS and WorldFIP. The main research objective of this thesis is to develop analysis and methodologies to guarantee, prior to run-time, that real-time distributed computer-controlled systems (DCCS) can be successfully implemented with standard fieldbus communication networks, such as those defined by the European Standard EN 50170. In this thesis, we characterise the MAC temporal behaviour for each one of these three EN 50170 profiles. More importantly, we provide analytical formulae for the evaluation of the worst-case response time of messages in these networks. These formulae constitute a set of powerful tools to guarantee the timing requirements of distributed time-critical applications, where distribution is supported by one of the EN 50170 profiles. Finally, we also show how priority-based scheduling mechanisms can be implemented at the application process level of P-NET and PROFIBUS masters, in order to overcome the limitations of their FCFS outgoing queues. Moreover, we demonstrate how the methodologies used to guarantee the timing requirements of tasks scheduling, can be successfully adapted to encompass the characteristics of P-NET and PROFIBUS networks.
PhD Thesis, Faculdade de Engenharia da Universidade do Porto.