Abstract: This paper introduces a design method for a control system based on the SIEMENS PROFIBUS-DP network. The topology, hardware configuration, and master-slave communication mechanism of the network are analyzed in detail, demonstrating that this is an advanced control technology. Keywords: PROFIBUS-DP; network; control system; communication mechanism 1 Introduction Fieldbus is the embodiment of the development of information technology and network technology in the control field, and is one of the hot topics in automation technology development. It has gained widespread application due to its characteristics of digitalization, openness, decentralization, and adaptability to the field environment. Currently, the main fieldbus technologies that have gradually matured and influenced the process of industrial automation include PROFIBUS, HART, LONWORKS, and FF. Among them, PROFIBUS is one of the most popular fieldbus technologies, and its products are widely used in automation fields such as industry, power, energy, and transportation. It is a fieldbus standard of German national standard DIN 19245 and European standard EN 50170, and is an international, open, and manufacturer-independent fieldbus. According to the application characteristics, it can be divided into three compatible versions: (1) PROFIBUS-DP is used for communication between device-level control systems and distributed I/O with its fast transmission speed and strong anti-interference capability; (2) PROFIBUS-FMS mainly solves the general communication tasks at the workshop level and can provide flexible and large-scale communication services; (3) PROFIBUS-PA is designed for process automation. It is directly connected to the field sensors or actuators and can be connected to the DP bus through the DP/PA interface. It is used in situations with high safety requirements and bus power supply[1]. 2 System topology and configuration[3] According to the connection method from field devices to controller, the topology of fieldbus usually adopts the following three methods: linear, tree and ring. Profibus adopts a linear structure, with a trunk line connecting the controller to the controlled object. The bus cable branches from the trunk cable to the field device. The controller scans the inputs on all I/O stations and can send information to the output channel when necessary to realize multi-master and peer-to-peer communication. This article focuses on the design of the PROFIBUS-DP fieldbus control system. 2.1 Host Computer This system uses an Advantech industrial PC as the host computer. The PC is connected to the Profibus-DP via a CP5611 interface card, thus connecting the IPC and the fieldbus to form a complete control network system capable of configuration, operation, and other functions. The host computer monitoring software can use either Intellution's Windows 9x & NT-based configuration software or Siemens' WinCC software. It can realize functions such as dynamic display, alarms, trends, control strategies, and control network communication, and provides a user-friendly interface, allowing users to generate corresponding application software according to actual production needs. 2.2 SIMATIC S7 Master Station As the DP master station, the CPU is located in the control center. This system uses the CPU315-2DP modular PLC, which integrates a Profibus-DP fieldbus interface device and has powerful processing capabilities (processing 1024 statements in 0.3ms). After the PLC program is compiled in the host computer STEP 7, it is downloaded to the CPU315 and stored. The CPU can automatically run the program, read the status words of all I/O modules on the bus according to the program content, and control the corresponding devices. 2.3 Slave Stations (Slave A, B) In Slave Station A, the SIMOVERT Master drives are AC frequency converters, and CB1 is the corresponding communication processor. The SIMOREG K 6RA24 is a DC converter, and CB24 is the corresponding communication processor. When the S7-300 or S7-400 is used as the master station of the Profibus-DP network, it can drive 32 such slave stations respectively. With repeaters, the maximum number can reach 127. This AC/DC drive product can perform open-loop and closed-loop control of the motor, and store the data received from the DP network into the bidirectional RAM through the CB communication board. Each word in the bidirectional RAM is addressed. The bidirectional RAM at the inverter end can be sorted by the addressed parameters to write control words, set values ​​or read actual values, diagnostic information and other parameters to the inverter [4]. 2.4 Distributed I/O ET200M ET200M is a modular distributed I/O station that is connected to the Profibus-DP fieldbus through the IM-153 interface. For the SIEMENS STEP7 development platform, the address arrangement of the distributed I/O nodes on ET200M is consistent with the traditional centralized address arrangement. Therefore, programming is the same as programming a centralized control program. Moreover, the module address of distributed I/O can be changed according to user needs to adapt to the needs of actual field debugging. 3 Profibus-DP communication function [1,2] The Profibus-DP network uses a physical layer, a data link layer and its own user layer. The physical layer uses ETA RS485 transmission technology and uses shielded twisted pair copper wire or optical fiber transmission cable. To prevent surges, terminating resistors are connected at both ends of the bus. In actual use, it is important to ensure that the terminating resistor at the intermediate node is in the OFF position. The data link layer, also known as the fieldbus data link layer (FDL), generally has two media access methods: token bus (Token Bus protocol conforms to IEEE 802.4) and master-slave mode. A token is a special message used to transfer control between master stations. In token bus mode, a master station that obtains a token gains control of the bus for a pre-defined time period. During this period, the master station is allowed to perform its tasks and communicate with all master stations or all slave stations according to the master-slave relationship table. If the master station has no frames to send, has sent all required frames within the specified time, or its control time has ended, it passes the master station token to the next master station. This ensures that even under heavy load, each master station can obtain bus access within a defined time, avoiding the congestion problem common in Ethernet under heavy load, and retaining the advantages of a simple bus network structure and the ability to freely add or remove stations. The data link protocol in master-slave mode differs from LAN standards. It conforms to the Unbalanced Normal Response (NRM) mode in HDLC. The characteristics of this mode are: one master station on the bus can control multiple slave stations. The master station establishes a logical link with each slave station. The master station issues commands, and the slave stations respond. Slave stations can continuously send multiple frames until no more information is sent, the required number of frames is reached, or the master station stops them. The frame transmission process in the data link consists of three stages: data link establishment, frame transmission, and link release. U represents an unnumbered frame in HDLC; U:A, SNRM, P indicates that slave station A is selected in SNRM mode, and P is the polling bit; U:A, UA, F indicates that the master station establishes the data link using the unnumbered acknowledgment command UA of the U frame as a response, and F is the termination bit. I represents an HDLC information frame. N(S) = 0 indicates the sequence number. N(R) = 0 indicates that the slave station A has not received the frame. N(S) = 1, N(R) = 0, and N(S) = 2, N(R) = 0 indicate that the second and third consecutive information frames sent by the master station are received. The third frame uses the probe bit P to indicate that the slave station also has an information frame to send. N(R) = 3 indicates that slave station A has correctly received I frames with sequence numbers 2 and up. The terminator F indicates that the slave station only sent one frame. Finally, when neither the master nor the slave station has any information frames to send, or when the master station is about to establish a data link with another slave station, the master station uses the U release connection command DISC. Slave station A confirms this via the UA in the U frame, and the link transmission process ends. 4 User Interface Development The control system based on the Profibus-DP network mainly has the following human-machine interface interfaces. (1) Information display screen: mainly displays the current operating status information, such as the speed of the motor, the operating frequency of the inverter, and some fault information; (2) Equipment control screen: although the S7-300 can realize data acquisition and control signal output on the fieldbus and has some control algorithms, complex control functions still need to be manually controlled on the host computer. Clicking the corresponding device button in the interface can control the equipment such as the inverter and converter individually; (3) Real-time alarm processing: judges the data collected by the system in real time, issues alarm signals, and processes them according to technical requirements and automatically controls the corresponding equipment; (4) Report printing and data curve display, etc. 5 Conclusion Profibus fieldbus is a digital communication network. Digital signals are used for communication between bus devices in the middle layer or different layers of the system. Since digital signals replace analog signals, multiple signals (including multiple operating parameter values, multiple device statuses, and fault information) can be transmitted on a single wire while providing power to multiple devices. A/D and D/A conversion components are no longer needed outside the field devices, which greatly reduces the number of wires and connecting accessories and improves the reliability and anti-interference capability of the system. Therefore, this bus control system can be applied to almost all industrial control systems and building automation systems, and it is also a "fully integrated automation" system bus [1]. It uses a brand-new concept of distributing control logic throughout the entire system, which greatly improves the level of industrial automation, maximizes production efficiency, and realizes the integration of factory management and control. With its own characteristics and advantages, it will inevitably become the mainstream direction in the field of automatic control. References: [1] Yang Xianhui. Fieldbus Technology and Its Application [M]. Beijing: Tsinghua University Press, 1999. [2] Pan Xiaoning, et al. Communication Principle of Profibus-DP Industrial Control Network [J]. Electrical Drive, 1999 (5): 25-28. [3] SCHICKHUBER. McCarthy Distributed Fieldbus and Control Network System [J]. Computing & Control Engineering Journal [J]. 1997, 8 (1): 53-55. [4] Zhao De'an, et al. A Type of Tension Adaptive Control System Based on PLC and Profibus-DP Bus [J]. Information and Control, 2002, 31 (2): 122-126.