Analysis of real-time data transmission in the DLL of IEC/ISA fieldbus

Fieldbus provides real-time data communication among field devices in process control and factory automation systems. IEC and ISA recently approved the Committee Draft of the data link layer of fieldbus standard for use in industrial control systems. The data link layer of IEC/ISA fieldbus is operated on the basis of token-passing and scheduling mechanisms. In this paper, the authors analyze the performance characteristics of the IEC/ISA fieldbus, especially the capability of real-time data transmission. A discrete-event simulation model of the data link layer is developed. Using the simulation model, this paper identifies the network parameters that influence the performance of priority mechanism and real-time data transmission, and analyzes the delay performance of IEC/ISA fieldbus with respect to the change of the network parameters. The fieldbus simulation model also integrates a continuous-time simulation model of a control system as an application process. Using the simulation model, this paper investigates the effect of network-induced delay on the performance of a control system