Abstract: This paper describes how, in a fieldbus control system, the MODBus fieldbus communication protocol based on the RS485 physical layer is used, and the master/slave principle is adopted to realize real-time control and data acquisition of equipment. Trial operation shows that the system works well. Keywords : Fieldbus; MODBUS; DS100 controller; observation and control of air conditioning 1 Introduction As we all know, every complex control system is composed of various sensors, transmitters and other detection devices to promptly reflect the operating status of field equipment and various parameters of the controlled object to the controller or control computer, thereby realizing precise control of the entire system. With the development of computer, communication, integrated circuit and sensor technology, a new control technology has emerged in the field of control, namely fieldbus (FCS). The emergence of fieldbus has provided a new means for data acquisition and system control. 2 Fieldbus Technology 2.1 Concept of Fieldbus Technology According to the definition of the International Electrotechnical Commission IEC 1158, fieldbus is "a serial, digital, multi-point communication data bus between field devices/instruments installed in the production process area and automatic control devices/systems in the control room." In other words, fieldbus is a network system and control system that uses individual, distributed, digital, and intelligent measurement and control devices as network nodes, connected by a bus to exchange information and jointly complete automatic control functions. 2.2 Advantages of Fieldbus The simplified structure of fieldbus systems demonstrates superiority in the design, installation, commissioning, normal production operation, and maintenance of control systems. 1. Reduced hardware quantity and investment. Because the intelligent devices distributed at the front end of the equipment in the fieldbus system can directly perform various sensing, control, alarm, and calculation functions, the number of transmitters can be reduced, thus saving a significant amount of hardware investment. The reduction in control equipment also reduces the floor space required for the control room. 2. Reduced installation costs. The wiring of the fieldbus system is very simple, saving both investment and design and installation workload. According to calculations from typical experimental projects, installation costs can be reduced by more than 60%. 3. Reduced maintenance costs. Because the field control equipment has self-diagnostic and simple fault handling capabilities, and sends relevant diagnostic and maintenance information to the control room via digital communication, users can query the operation, diagnostic, and maintenance information of all equipment to analyze the cause of faults early and quickly resolve them. 3 System Hardware Design The simplified connection diagram of the entire system is shown below: In this experiment, I adopted a strategy of connecting the DS100 controller to the computer's serial port via a 485/232 adapter. The computer's serial port serves as the master control device for the RS-232 interface, and everything is controlled by the computer. It is important to note that before starting the experiment, the controller's built-in software `setdevice` should be run to set the controller's communication parameters, such as the baud rate. During the acquisition process, the computer sends a read command in a specific frame format to the controller, and the controller returns the data transmitted from the sensor as a response, thus realizing the data acquisition function. The controller, connected to the ambient temperature and humidity and water temperature regulating devices, is responsible for system control. During control, the computer sends a write command containing data. Upon receiving the correct write command, the controller responds to the computer and transmits the data to the connected regulating device. The regulating device, upon receiving the command, adjusts the water temperature or ambient temperature and humidity according to the given temperature, thus realizing the system's control function. 4. System Software Design The system's module block diagram is as follows: The main module, as an independent page, selects whether the air conditioning monitoring and control is performed on the indoor or outdoor side. The main module contains four independent pages: system settings, data display, system control, and experiment report generation. Before starting the system, configuration is required, including serial port settings and channel selection. Data acquisition, as an independent module, is responsible for receiving and sending data. Received data is sent to a buffer. The data processing module is responsible for processing the data. It reads data from the buffer, sends data to the data display module for display, and performs calculations to obtain the desired results. The data display module refreshes and displays data periodically. The system control module mainly implements the write function. Control data to be written is sent to the data acquisition module, written to the serial port, and then transmitted by the controller to the corresponding air conditioning control equipment. The experiment report generation module generates an EXCEL spreadsheet-style experiment report to reflect the important data of the entire experiment. 4.1 Data Acquisition The data acquisition module implements read and write operations on the DS100 controller. The main data to be acquired includes: water flow rate, dry-bulb temperature, wet-bulb temperature, inlet water temperature, outlet water temperature, and power. A set of data acquired during the experiment is as follows: 4.2 Data Processing The DS100 uses the MODBUS protocol in ASCII transmission mode, which has its own corresponding frame format. The data processing module performs encoding before writing to the DS100 controller and decoding after reading. It also calculates the cooling capacity, energy efficiency ratio, and other necessary quantities from the acquired data. Taking the outdoor side as an example: Outdoor side calculation quantities (2 in total): The corresponding cooling and heating capacity and energy efficiency ratio calculated based on the above collected quantities are as follows: 4.3 System control controls the water inlet temperature and the ambient temperature and humidity to test the performance of the air conditioner under different conditions. During the control process, the computer sends a write command with data. After receiving the correct write command, the controller responds to the computer and transmits the data to the connected regulating device. After receiving the command, the regulating device will adjust the water temperature or ambient temperature and humidity according to the given temperature to realize the system control function. 4.4 Performance demonstration is responsible for displaying the data (including directly collected and calculated data). The calculated quantities of air conditioner performance (energy efficiency ratio, cooling and heating capacity, etc.) can be displayed by dynamically demonstrating their changes through images. The two figures above are the relationship curves of energy efficiency ratio and cooling capacity changing over time, where the scale unit for cooling and heating capacity is W, the scale unit for energy efficiency ratio is W/W, and the scale unit for time is half an hour. The test calculates the energy efficiency ratio and cooling and heating capacity every 15 minutes. The smaller the change in the curve, the more stable the performance of the air conditioner. As can be seen from the above figure, the performance of the tested air conditioner is relatively stable. 5. Experimental Results The designed air conditioning measurement and control system underwent a six-month trial run in the Power Building II laboratory. The results showed that the entire measurement and control system operated stably, performed reliably, and was easy to operate. 6. Conclusion The air conditioning measurement and control system designed in this paper using fieldbus technology has the characteristics of simple hardware structure, strong practicality, and low operating and maintenance costs. It is a solution worthy of promotion in independent air conditioning measurement and control. References: 1. Zou Yiren, Ma Zengliang, Pu Wei, Design and Development of Fieldbus Control System, National Defense Industry Press, 2003.1 2. Xu Ming, Yang Ping et al., Application of Fieldbus Technology in Power Plants, Microcomputer Information, 2006, No.1, pp.25-27 3. Lu Wenjun. 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