Real-time communication in FieldBus multiaccess networks

Abstract

There has been an increasing need of timely and predictable communication services for embedded real-time systems in automated factories and industrial process controls. Work has been done on real-time communication with deadline guarantees in point-to-point, token bus/token ring/FDDI, and DQDB (Distributed Queue Dual Bus) networks. However, due to the random access nature of the CSMA/CD type multiaccess networks, they are not suitable for applications with stringent timing constraints. We consider real-time communication services with absolute deadline guarantees in multiaccess local area networks equipped with a centralized scheduler, such as the SP-50 FieldBus, an industrial standard protocol for process control and manufacturing applications. Similar to most token-passing networks, in a centralized scheduling multiaccess network, the access to the bus is controlled by a token. Only the station currently holding the token has the exclusive right to use the multiaccess bus. Unlike the token bus, token ring, or FDDI network, the multiaccess network uses a centralized token scheduling scheme and the token need not be allocated to the stations in a cyclic fashion. We show that the pinwheel and the distance-constrained scheduling techniques can be adapted to schedule the token in centralized-scheduling multiaccess networks to guarantee message deadlines.