An implementation of a proposed modification of CAN protocol on CAN fieldbus controller component for supporting a dynamic priority policy

Abstract

CAN industrial local area networks, called controller area network, are used in the framework of real-time distributed industrial applications and distributed processing systems. Such applications cover the drinking or used water adduction; the transportation and the distribution of electricity; the purification stations; the oil or gas fields exploitation; the vehicle and space engines, the automatons and robot systems in manufacturing, etc. CAN networks guarantee relatively short time latency. Due to its short frame, when one PDO (process data object) is implemented as a single CAN data frame or CAN remote frame and to its bit-to-bit arbitration. It has been shown that this random access scheme, based on CSMA/CA protocol exceeds in performance other likely techniques for wired and wireless LAN. However, this type of arbitration defines a static priority to messages exchanged. When the offered traffic approaches the system nominal bandwidth, it results in a significant growth of transmission periods to a number of stations, that appear in the calculation of the worst-case queuing delay and blocking probabilities in both MAC and physical sublayers, from there, excluding the CAN to be a realtime network. Suitable techniques for identifiers allocation have been developed to find a satisfactory way to attribute identifiers to exchanged objects, allowing all possible period limits to applications. However, some mission-critical systems (time-critical PDO) will require deterministic behavior in communication during service. Even at maximum load, the transmission of all safety related messages must be guaranteed. To solve this problem, the ideal solution is to determine the point of time when a message with a given priority will be transmitted with a high precision. Some suboptimal solutions have been proposed, like TTCAN (time triggered CAN), DiPP (distributed priority policy) and DPP (dynamic priority policy). We have proposed a dynamic priority policy, based on the reserve bits left by the protocol and on the needs expressed in necessary bandwidth, to improving the message transmission periods and to reducing the blocking probability in maximal load. We demonstrate in this paper the easy implementation of the proposed modification of CAN protocol in a CAN controller component and its compatibility with older controllers.