Real-time fieldbus communications using Profibus networks

TLDR

Profibus uses a simplified timed‑token protocol that is a proven solution for real‑time communication, yet its differences from the standard TT protocol mean that conventional real‑time analysis does not directly apply. The study investigates how to enable real‑time industrial communications over Profibus fieldbus networks by ensuring real‑time messages are transmitted within a maximum bound time. The authors analyze Profibus by proposing two traffic‑profile approaches—an unconstrained low‑priority profile and a constrained low‑priority profile—to guarantee real‑time behavior. The analysis reveals that Profibus cannot be treated with standard TT analysis, and that the unconstrained low‑priority profile yields tighter deadlines while the constrained profile increases non‑real‑time throughput.

Abstract

This paper provides a comprehensive study on how to use Profibus fieldbus networks to support real-time industrial communications, that is, on how to ensure the transmission of real-time messages within a maximum bound time. Profibus is base on a simplified timed token (TT) protocol, which is a well-proved solution for real-time communication systems. However, Profibus differs with respect to the TT protocol, thus preventing the application of the usual TT protocol real-time analysis. In fact, real-time solutions for networks based on the TT protocol rely on the possibility of allocating specific bandwidth for the real-time traffic. This means that a minimum amount of time is always available, at each token visit, to transmit real-time messages, transversely, with the Profibus protocol, in the worst case, only one real-time message is processed per token visit. The authors propose two approaches to guarantee the real-time behavior of the Profibus protocol: (1) an unconstrained low-priority traffic profile; and (2) a constrained low-priority traffic profile. The proposed analysis shows that the first profile is a suitable approach for more responsive systems (tighter deadlines), while the second allows for increased nonreal-time traffic throughput.