Abstract: This paper discusses the network access mechanism of PROFIBUS-DP fieldbus in detail, establishes models of token access between PROFIBUS-DP master stations and master-slave access between master stations, and qualitatively and quantitatively discusses the real-time performance, determinism and network utilization efficiency of PROFIBUS-DP fieldbus by analyzing the coding efficiency, communication process and communication time parameters, and draws conclusions with theoretical and practical value. Keywords: Fieldbus, PROFIBUS-DP, MAC, Real-time Abstract: This paper provides a detailed discussion of the MAC mechanism of PROFIBUS-DP. The model of message transmission among master stations and between master and slave stations is established to analyze network throughput and time parameters. Through this analysis, the real-time properties, determinism, and network utility of PROFIBUS-DP are presented, and meaningful conclusions are drawn that aid in the integration and configuration of industrial communication systems with PROFIBUS-DP. Key Words: Fieldbus, PROFIBUS-DP, MAC, Real-time 0 Introduction Fieldbus has replaced the traditional point-to-point, 4-20mA analog signal transmission method, adapting to the requirements of modern control systems such as modular components, distributed control functions, centralized fault diagnosis, ease of maintenance, and low construction costs. Therefore, it has seen significant development and has been successfully applied in various fields. However, connecting multiple devices to a common data channel inevitably leads to bandwidth contention issues. Different fieldbus protocols have different network access mechanisms... The real-time performance and reliability of the control systems constructed by them are also different. Studying the MAC mechanism of the fieldbus protocol and analyzing the network performance is meaningful for the application of fieldbus. The PROFIBUS-DP fieldbus discussed in this paper is one of the international open fieldbuses. In recent years, it has been gradually promoted and applied in large-scale automated factories in China. By analyzing the MAC mechanism of the PROFIBUS-DP fieldbus, its performance can be analyzed, which has a realistic basis and practical value for the application research of fieldbus technology in China. 1 MAC layer protocol of PROFIBUS-DP As a public bus applied at the device layer, the PROFIBUS-DP fieldbus must solve the problem of network access rights competition among nodes and ensure the real-time performance of communication. Referring to the Open Systems Interconnection Protocol (OSI), the bus access control protocol (MAC) is defined in the second layer of the PROFIBUS protocol, the fieldbus data link layer (FDL) protocol. The efficiency, utilization, reliability and real-time performance of the communication network largely depend on the MAC protocol method of the protocol [1]. 1.1 Overview of PROFIBUS-DP fieldbus MAC protocol PROFIBUS-DP divides the nodes on the bus into two categories: master station and slave station, as shown in Figure 1. Its Media Access Control (MAC) protocol comprises two parts: master stations acquiring bus access rights through token passing and master-slave communication between connected slave stations. PROFIBUS master-to-master communication uses a simplified Time Token (TT) protocol; the IEEE 802.4 Time Token protocol is a proven and successful real-time communication protocol. PROFIBUS allows four message service modes: Acknowledged Data Transfer (SDA), Unacknowledged Data Transfer (SDN), Send-Request-Response (SRD), and Periodic Send-Request-Response. The PROFIBUS-DP protocol only allows SRD and SDN message service modes. Regardless of whether it's SDN or SRD, the master station always receives an immediate response, either from a remote station or the local station. This feature is crucial for ensuring the real-time performance of PROFIBUS-DP. For details, please click: PROFIBUS-DP Fieldbus Performance Analysis