A proposal modification of CAN protocol to support a dynamic priority policy being able to be implemented on CAN fieldbus controller components

Industrial local area networks, called controller area networks (CAN), are used in the framework of real-time distributed industrial applications. Such applications cover the drinking or used water adduction; the transportation and the distribution of electricity; the systems of purification stations; the oil or gas fields exploitation; the automatons and robot systems in manufacturing; the distributed processing systems, etc. The bit to bit arbitration of the CAN protocol defines a static priority to messages exchanged. This static priority is more important that the identifier attributed to the exchanged object is lower, CAN networks guarantee sufficiently short time latency and it has been shown that these systems exceed in performance token-based networks. Nevertheless, when the offered traffic approaches the system nominal bandwidth, it results a significant growth of transmission periods to a number of stations, from there, excluding the CAN to be a real time network. The authors propose in this paper a dynamic priority policy, based on the reserve bits expected by the protocol and on the needs expressed in necessary bandwidth, improving message transmission periods and reducing blocking probability to the higher identifier objects. CAN controller components constructors would have to allow the configuration of reserve bits so as to leave to the user the choice of their utilization, even if, the CAN protocol does not anticipate a precise utilization at the present time.