Research On The Real-time Message Scheduling Method Of Fieldbus Network Based On Mac Mechanism

Fieldbus is a real-time communications network. The tasks in fieldbus real-time applications should meet the requirements of real-time, reliability, predictability and etc, and real-time property is the key component of industrial control network, therefore, to become the industrial automation mainstream control network, real-time performance is its prior problem to be solved. In the fieldbus communication activities, real-time scheduling is a core part of the process. However, in fieldbus protocols, only the mechanism that nodes scheduling communication by Medium Access Control (MAC) mechanisms was given, but for the problems of real-time scheduling process and the scheduling algorithm have not been expatiated systematically. According to regulations, specifications were met, the way of scheduling algorithms for real-time scheduling could be independently developed by various manufacturers, and be served as their own intellectual property rights. This subject is the deep study and research of real-time scheduling problems of fieldbus, under the background that real-time scheduling process and algorithm not be definitely stipulated in fieldbus protocols, and be lack of deep study and reports in the relative information and reference both at home and abroad. Therefore, the research of this subject has great significance for the research of fieldbus protocol and its equipment.For the Industrial real-time applications, a node give rise to real-time requirements of many kinds of message in different situations, the message is divided into two categories: real-time message and non-real-time message. As a branch of Operational Research and Probability Theory, Queuing Theory was used as a mathematical tool for the fieldbus communication system based on the centralized MAC mechanism modeling for multiple vacation M/G/1 queuing model. Three kinds of service which include exhaustive type, gate type and limited type was adopted in the study of the waiting time of two differente message in different service mode, and the mixed service strategy scheduling method (hereinafter referred to CMR) has been presented to service real-time message by using exhaustive type service strategy, and to service non-real-time message by using limited type service strategy. This method makes the centralized MAC mechanism based scheduling process have the real-time property and without losing its flexibility, and avoid the occurrence of the phenomenon of the traditional scheduling method for all types of message pre-allocated bandwidth, which led to channel blocking or bandwidth wasted.By analyzing the distributed MAC mechanism, the distributed MAC mechanism based real-time message scheduling method (hereinafter referred to DMR) in which two stages of random competition phase and orderly polling period are contained in an arbitration cycle, has been presented. The random competition phase and orderly polling phase have been combined organically, and in each arbitration cycle a polling period to be set for the real-time message, so as to ensure its real-time property, while non real-time message has the opportunity to be sent at random competition period only. Thus both problems have been solved, of that the high-priority nodes alternately occupy the bus which led to low-priority nodes have no right to use the bus for long time, and that the priority of many messages with the same deadline can not be set separately.On the basis of deep analysis of PROFIBUS bus MAC mechanism and MAC frame, the target token rotation time is identified as an important parameter to affect the real-time property. The existing literatures only give the simple formula of calculating the minimum target token rotation time, but no description for how to select the parameters in the formula. In practical applications, especially in multi-master network, the problem becomes more complex, because the master equipments of PROFIBUS bus usually come from different manufacturers, and the configuration parameters which are configurated by the configuration tool, for the entire network might not be optimal or the most practical. In view of this phenomenon, the mathematic model of multi-master station system to calculate the value of target token rotation time has been presented in this paper using the CMR method, the quantitative relationship between the system load and the target token rotation time value has been established, a new way to be explored to find a solution to this problem. The mathematic model of the relationship between target token rotation time and real-time message waiting time was given. A typical application system of PROFIBUS-DP was designed to study the effect on system's property enhancement under the condition of different loads, in which CMR algorithm was applied to calculate the target token rotation time.By analyzing the LonTalk protocol of LonWorks communication protocol, due to a distributed MAC mechanism was adopted, for LonWorks bus of multi-node network, various conflicts between node and node, messages and messages will likely occur. Therefore, there is a problem of real time property in the arbitration mechanism of LonTalk MAC layer, it is difficult to meet the hard real-time requirements. The problems in the arbitration mechanism of LonTalk MAC layer have been resolved by using DMR algorithm in this paper, for the characteristics of each conflict, a solution has been presented, so that the transmission delay time of the priority packet in any node is limited, to meet real-time requirements. Through experiments for different loads, the waiting time of priority and non-priority packet and network efficiency were analysed, the role of the real-time message scheduling method, which was presented in this paper, for improving the real-time property of fieldbus communications system has been verified. Experimental results show that, DMR are correct in theory, and are feasible in practical application.