Some Features of two Fieldbuses of the IEC 61158 Standard

The IEC 61158 is on the way to become an international fieldbus standard after a long period of contrasts and misunderstandings. The final standard document results from the joining of eight among the most diffused fieldbus protocols.This paper takes into consideration one of the most popular fieldbus of the final standard document, Profibus, and examines the differences with the original IEC 61158 Technical Specification (which is also included in the standard). The analysis concerns the data link layer of the two fieldbuses: Profibus is based on a token passing technique, while the IEC fieldbus realises a mixed access combining token passing with a centralised scheduling procedure managed by a special node named Link Active Scheduler (LAS). Two parameters of the data link layers have been in particular considered: the cycle time and the medium access efficiency. In particular, the evaluation of the cycle times has been carried out using analytical models which are valid under the hypothesis that the token holding times of the stations are sufficiently long to satisfy all the service requests coming from the users, that is a typical situation of a network not overloaded. Considerations have also been made to investigate the behaviours of the cycle times when the network traffic increases.