Abstract: This paper proposes a low-cost distributed control system solution based on a personal computer, utilizing RS485 bus communication technology and the resources of the electronic equipment itself. This solution has been applied to the batching production line control of several cement plants, achieving good results and significant economic and social benefits. Keywords: Personal computer, Distributed control system, RS485, Data acquisition module, Frequency converter 1. Introduction Numerous small and medium-sized enterprises (SMEs) occupy an important position in China's national economy. Most of these enterprises have outdated equipment and backward technology, urgently requiring technological transformation. Utilizing modern electronic information technology to transform traditional industries is a fundamental industrial policy in China and an urgent need for these enterprises to survive, develop, and improve their competitiveness. However, the high cost of modern distributed control systems hinders their application in these enterprises. With the development of modern electronic information technology, the prices of various electronic devices, especially computers, are constantly decreasing while their overall performance is constantly improving. This provides conditions for the development of low-cost distributed control systems. Based on this, we have successfully developed a distributed control system suitable for SMEs, achieving low cost while fully meeting performance requirements. 2. System Composition SME distributed control systems are generally small in scale with few on-site control operations. The main controlled variables include: digital input/output, analog data acquisition, and AC motor start/stop and speed control. Based on these characteristics and considering integration with modern enterprise information systems, we designed a distributed control system with a two-tier architecture. 2.1 Process Control Level: A general-purpose microcomputer with a built-in industrial Ethernet card is used. It communicates with the enterprise information management system and the basic control computer via industrial Ethernet to complete system monitoring functions. 2.2 Basic Control Level: The basic control computer also uses a general-purpose microcomputer with a built-in industrial Ethernet card, communicating with the host computer via industrial Ethernet. A 5V opto-isolated RS232/RS485 converter is installed on its standard RS-232 serial communication interface, forming an RS485 bus system. RS485 uses a balanced transmit/receive method, featuring long transmission distance, strong anti-interference capability, and multi-station functionality. The 4017 is a remote data acquisition module for analog data acquisition; the 4050 is a remote control module for digital input/output; the frequency converter is a Fuji FRN7.5G11S model, used for AC motor control. In the basic control level, the basic control computer acts as the master station, and the modules and frequency converters act as slave stations, forming a master/slave network based on the RS485 communication protocol. 3. System Software Design 3.1 Software Selection Generally, large distributed control systems use industrial control configuration software such as FIX and RSVIEW to create the host computer screen, and ladder logic language to program the PLC. This approach is costly, so in small distributed control systems, the trend is towards using general-purpose high-level language design software, such as VB, VC++, and Delphi, to create monitoring screens. In this approach, we use VB to program the host computer monitoring program and the basic control computer control program. Since most engineering technicians are proficient in VB programming, this avoids the need for initial training in using specialized configuration software and PLC programming software, shortening the system development cycle and saving the cost of purchasing large-scale configuration software. The powerful functions of the VB high-level language make database management easy to achieve, integrating monitoring and management, which is something that general industrial control configuration software cannot do. 3.2 Implementation of Communication between the Basic Control Computer and 485 Interface Devices The basic control computer, remote data acquisition and control modules, frequency converters, and other 485 interface devices form a master-slave communication network via an RS485 bus. The basic control computer acts as the master station, sending control commands to the remote data acquisition and control modules and frequency converters. The modules and frequency converters, acting as slave stations, execute the commands upon receiving them and send response signals to the master station. The modules, frequency converters, and other 485 interface devices are uniformly addressed, each with a different station number. The master station sends control signals to each slave station in turn and receives different return signals from each slave station, thereby controlling each slave station and acquiring data. Slave stations cannot communicate with each other. The communication ports of the computer, modules, frequency converters, and other 485 interface devices are uniformly configured as follows: baud rate 9600, even parity, 8 data bits, and 1 stop bit. 3.2.1 Communication Protocol between 4017 and Basic Control Computer The message (instruction) structure sent to 4017 is as follows: # is a single-byte ASCII # character (value 23H), indicating a request to read an analog input value; ADR is a two-byte ASCII code representing the module address; CR is a single-byte ASCII CR character (value 0DH), indicating the end of the message. The message returned by 4017 is as follows: DATA0-DATA7 are all seven-byte ASCII codes, representing the analog values ​​of channels 0-7, including one sign bit, one decimal place, and five digits. The 4017 analog channel is defined as ±5V. CR is a single-byte ASCII CR character (value 0DH), indicating the end of the message. 3.2.2 Communication Protocol between 4050 and Basic Control Computer The message structure sent to the 4050 is as follows: # is a single-byte ASCII # character (value 23H), indicating a request to change the module's output state; ADR is a two-byte ASCII code representing the module address; NUL is a two-byte ASCII NUL character (value 00H), indicating a change in all 8 bits of the state; DATA is a single-byte area representing the 8-bit output state; CR is a single-byte ASCII CR character (value 0DH), indicating the end of the message. The message returned by the 4050 is as follows: > is a single-byte ASCII > character (value 3EH), indicating that the module has accepted the command; CR is a single-byte ASCII CR character (value 0DH), indicating the end of the message. 3.2.3 Communication Protocol between Inverter and Other 485 Interface Devices and Basic Control Computer The FRN7.5G11S inverter is one of Fuji's 5000 series inverters. This series of inverters is very powerful and can control the motor in multiple ways. It can be controlled locally via buttons on the front panel, via digital input terminals, or remotely via an RS485 port. Before establishing communication between the computer and the inverter, the relevant parameters of the inverter must first be set. The parameters set according to actual needs are as follows: H30: RS485 connection function. When this parameter is 3, frequency settings and run commands input from the RS485 port are valid. H31: Used to set the inverter's station address. H34: Transmission speed. When this parameter is 2, it is 9600 bits/s. H35: Data length. When this parameter is 0, it is 8 bits. H36: Parity check. When this parameter is 1, it is even parity. H37: Stop bit. When this parameter is 1, it is 1 stop bit. Because this series of frequency converters is very powerful, it sends many types of commands and has various message formats. Only one format is listed here. The frequency converter start and stop message formats are as follows: SOH is a single-byte ASCII SOH character (value 01H), used to indicate the start of the message. ADR is a double-byte area containing the address of the slave frequency converter. ENQ is a single-byte area indicating a command issued by the master station. f is a single-byte ASCII f character, controlling the frequency converter's operation. DATA is a four-byte area, where "0000" indicates stop, "0001" indicates forward rotation, and "0002" indicates reverse rotation. EXT is a single-byte ASCII EXT character (value 03H), used to indicate the end of the message. BCC is a double-byte area used for message verification; its value is the last two digits of the sum of all bytes from LGE to HSW in the message. If the frequency converter receives a message with an error result, it will discard the message and refuse to respond. Other 485 interface devices, including totalizers and flow meters, all use similar protocols. 3.2.4 Design of Basic Control Computer Communication Software The MSComm control is an ActiveX control provided by Microsoft. It implements reading data from and writing data to the microcomputer's serial port. By changing its properties and writing corresponding program code, we can easily develop communication software for the basic control computer. The main properties of the MSComm control are as follows: Commport: Sets and returns the communication port number. Setting it to 1 selects serial port 1. Settings: Sets and returns the baud rate, parity bit, data bits, and stop bits. Set uniformly to "9600, 8, e, 1". Portopen: Sets and returns the communication port status, used to open or close the port. Input: Retrieves data from the communication control's buffer; read-only at runtime. Output: Writes data to the transmission buffer. This data can be in text or binary format; read-only at runtime. InBufferCount: Returns the number of characters in the receive buffer; setting this property to 0 clears the receive buffer. Inputmode: Sets or returns the data type retrieved by the Input property. Set 0 to text format, set 1 to binary format. Taking the control of the frequency converter as an example, the motor start program when the frequency converter address is 6 is as follows: Dim X (7) as String MSComm. Settings="9600,e,8,1" MSComm. InBufferCount=0 MSComm .Inputmode= comInputmodeBinary X (0)= CHR$(01H):X (1)="06":X (2)=CHR$(05H):X (3)="f":X (4)="0001":X (5)= CHR$(03H) X (6)="92" X=X (0)+X (1)+X (2)+X (3)+X (4)+X (5)+X (6) MSComm. Output=X 3.3 Monitoring Software Design The monitoring software is programmed using VB programming language. Compared to other high-level languages, VB's graphical controls make it relatively easy to create aesthetically pleasing monitoring screens. WinSock is used to implement Socke sockets for communication within a local area network, and VB's database functions are used to store historical data and print reports, thus achieving the monitoring function. 4. Practical Application Case The Hebei Qian'an Shaluan Cement Plant has adopted this technology for one raw material batching production line and two clinker batching production lines, controlling the proportion of seven to eight materials on each line. The DCS outputs switch signals to control the vibratory motor to put materials into the weighing hopper, inputs analog signals to detect the weight, and outputs switch signals to control the opening and closing of the door motor to release materials from the weighing hopper, thereby controlling the flow rate and proportion of each material to meet process requirements, and centrally monitoring the production status of each production line. If a general distributed control system were used, the equipment cost would be approximately 200,000 yuan, while this system only costs 60,000 to 70,000 yuan. Actual operation shows that this solution meets user requirements and can operate stably for a long time. 5. Conclusion In small-scale control system applications, if some popular process control networks are used, the investment in communication, analog signal processing, etc., is very large. This solution fully utilizes the resources of computers and frequency converters themselves, as well as general software technology, to achieve cost reduction. While maintaining the same performance, the investment is only 35% to 50% of popular solutions, and more than twenty systems have already been successfully applied in enterprise production processes.