Abstract: Based on a review of DCS and fieldbus technologies, this paper proposes the concept of a multi-intelligent instrument control system. A control PC is connected to the intelligent instruments via a serial communication interface card and an RS-485 bus to construct a multi-intelligent instrument DCS system. The system structure is analyzed, and a communication protocol is proposed. The controlled object can be either an actual industrial process or a controlled object model system. The monitoring software provides a clear control interface, facilitating control experiments using various control algorithms. Keywords: Intelligent instrument, Distributed Control System (DCS), Fieldbus, Communication Protocol, Serial Communication. Modern production demands drive process control towards multi-input multi-output requirements, and the increasing decentralization of control and centralization of management in modern industry have led to the emergence of Distributed Control Systems (DCS). Given the high cost of foreign fieldbus equipment and DCS systems, the transformation of distributed systems in state-owned enterprises can only follow a path with their own unique characteristics. This paper addresses the above problems and application requirements, using intelligent instruments as field control devices. It studies the relevant technologies in the composition of DCS and its applications, and proposes a design scheme for a multi-intelligent instrument control system. An experimental multi-intelligent instrument DCS system is constructed, and considering the actual needs of the experiment, a controlled object model and computer monitoring system software are designed. The experimental system consists of a PC, intelligent instruments, a serial communication interface card, an RS-485 bus, and a controlled object model system. 1. Architecture of the Multi-Intelligent Instrument Control System 1.1 General DCS Architecture Since the mid-1970s, the development of process computer control systems has entered a period characterized by distributed control systems (DCS) employing 4C technologies (computer, communication, control, and CRT display technology). From a system structure perspective, DCS can be divided into three basic parts: distributed process control devices, centralized operation and management systems, and communication systems. The relationship between these three parts is shown in Figure 1. Functional layering is a characteristic of DCS architecture, fully reflecting the characteristics of distributed control and centralized management in distributed control systems. According to the functional layering method, the multi-intelligent instrument control system can be divided into field control stations, process control stations, control levels, and production management stations. 1.2 Architecture of Multi-Intelligent Instrument Control System Figure 1 DCS Structure Diagram (1) Overall Structure As mentioned above, the multi-intelligent instrument control system is geared towards the needs of technological transformation of small and medium-sized enterprises in China. Therefore, after integrating the functions of each layer of a general DCS, we propose the network structure of the multi-intelligent instrument control system, as shown in Figure 2. Field-level components are mainly composed of intelligent instruments. The control PC can be composed of industrial control computers or ordinary PCs. The intelligent instruments and the control PC are connected by a fieldbus. The fieldbus protocol should be designed according to the control requirements and the characteristics of the intelligent instruments. Management-level components are mainly composed of commercial PCs or other computers, which themselves constitute a management information system (MIS). The management PC and the control PC are connected by a general computer network. The specific type can be determined according to the requirements of the MIS system. Figure 2 Network Structure Diagram of Multi-Intelligent Instrument Control System (2) The emergence of built-in microprocessors in intelligent instruments has brought about fundamental changes in the design methods, circuit structures and functional operations of intelligent instruments compared with traditional instruments. In intelligent instruments, except for the adjustment function, which is almost entirely implemented by the microprocessor, the microprocessor is the core and works in an orderly manner under the control of the instrument software. In a multi-intelligent instrument control system, the information exchanged between the intelligent instrument and the control PC can be divided into: measured value; set value; control quantity; status parameters (one is the numerical value or 0/1 logic value reflecting the working or production status of the intelligent instrument, and the other is the setting of the working status and parameters of the intelligent instrument, the most typical being the PID parameter setting). (3) Control PC The control PC is mainly used to realize centralized operation and advanced control. It is the bridge between decentralized control and centralized management of the management PC. It generally completes the functions of display and printing, control management and operation, and system configuration. (4) Management level To achieve the above control objectives, higher-level control and management must also be carried out. Modern enterprises use MIS systems to achieve this. (5) Fieldbus Intelligent instruments and fieldbus technology are inseparable and complementary. At present, domestic intelligent instruments generally use general-purpose single-chip microcomputers such as the MCS-51 series. Therefore, considering the technical implementation, economy and production reality, when adopting many fieldbus standards for the technological transformation of state-owned enterprises, the RS-485 bus is a better choice. The multi-intelligent instrument control system uses the RS-485 bus as the physical transmission medium for communication protocol. 2. Communication Protocol for Multi-Intelligent Instrument Control System The communication function of intelligent instruments is the foundation for the implementation of an intelligent instrument DCS system. Based on existing protocols, we rationally designed the hardware and software for the communication function and proposed a multi-intelligent instrument control system protocol. Its overall communication strategy adopts a reservation protocol, where time is divided into time slices, and each intelligent instrument uses its pre-reserved time slice to communicate with the host. The network layer model of the fieldbus draws on the OSI seven-layer protocol and has been partially extended, focusing on industrial applications to form a communication network for the industrial field. The specific network specifications in this design adopt the application layer, network layer, data link layer, and physical layer of the OSI protocol. At the physical layer, the transmission medium of the fieldbus in this system uses RS-485 twisted-pair cable. At the data link layer, its main functions are: assembly and decomposition of information frames, error control and processing. The baud rate uses standard 1200, 2400, 4800, and 9600, which can be selected according to system design requirements; the transmission frame adopts the standard UART format. On the fieldbus of a multi-intelligent instrument control system, transmission frames can be divided into address frames and data frames. The difference lies in the fact that the 9th data bit in the address frame is "1", while the 9th data bit in the data frame is "0". Error checking uses CRC checksum. At the network layer, the fieldbus is a multi-node collinear network, and its network layer management is completed by the control PC. The address frame is sent by the control PC to wake up a specific intelligent instrument on the bus to establish communication. The intelligent instrument only begins the communication process after receiving an address frame containing specific data bits (usually its instrument address number). The communication process of the intelligent instrument begins by sending a data frame containing specific data bits (instrument address) back to the control PC; the control PC confirms the establishment of communication with the intelligent instrument upon receiving the data frame. The communication between the control PC and the intelligent instrument is the process of exchanging information multiframes. At the application layer, for a multi-intelligent instrument control system, the application layer protocol defines the information multiframes transmitted between the control PC and the intelligent instruments. The application layer protocol is shown in Table 1: Table 1 Application Layer Protocol The interface conversion circuit between the PC interface and the fieldbus of the intelligent instrument DCS experimental system uses the PCL-743/745 serial communication interface card manufactured by Advantech. Each interface card has two RS-485 serial communication ports, each port has a Universal Asynchronous Receiver/Transmitter (UART) with a 16-byte First-In-First-Out (FIFO) buffer. It buffers the data into 16-byte packets before placing the data onto the bus, which greatly reduces the CPU load and avoids data loss when the system is busy or cannot handle interrupts in time. This is especially important for high-speed serial I/O ports under the Windows operating system. Before installing the interface card, the I/O base address and interrupt request are set through some jumpers on the card. The communication function of the PCL-743/745 serial communication interface card is implemented by reading and writing to its internal registers. 3. Programming Implementation of PC Communication Functions and Monitoring Software Design The communication function of the PC is implemented using Visual C++ 6.0, with a single communication thread, ThreadProcForComm (LPVOID param). This communication thread only handles data transmission and reception, including variables such as flag, data_tra, data_rec, and address. flag indicates whether communication was successful, data_tra is the transmit data buffer array, data_rec is the receive data buffer array, and address is the smart meter address array. The monitoring software should implement the following functions: monitoring and operation, printing, and storage. In the entire software, all control functions are implemented through control sub-functions. For modifications to setpoints and control values, as well as changes to the control method, the software provides a Visual C++ class with three member functions: `SettingvalueModify(int h, WORD new)`. The parameter `h` represents the instrument number, and `new` represents the newly set value. For the control algorithm interface, the software also provides a Visual C++ class with two member functions: `Data_in(measureIn[], setpoint[])` and `Data_out(controlOut[])`. The former takes the measured and setpoint values, while the latter outputs the control values. `measureIn[]`, `setpoint[]`, and `controlOut[]` represent arrays of measured, setpoint, and control values, respectively, and are all global static variables. Users can freely write specific control algorithms within this class. The design of the software's storage and printing functions will not be discussed here. 4. Example This design is an improvement on the SEU-211 intelligent temperature controller, resulting in the USTS-100 intelligent instrument. The SEU-211 intelligent temperature controller is also an intelligent control instrument for DCS applications. Its control object is an electric heating furnace, and it uses thermocouples as temperature sensors. The input of the instrument is a thermocouple signal, and the output is a thyristor trigger signal. In terms of communication function, the communication protocol is based on RS-485 and adopts a structure of mutual data frame transmission after address call/response. In the design of the USTS-100 intelligent instrument, the following hardware improvements were made: (1) It can also be connected to standard voltage and current signals for measuring input/output signals, and integrates the input and output circuits of multiple signals to form an input and output circuit that can take into account different circuit structures and circuit parameters, thereby enhancing its versatility. It also designed a controlled object model system, so that the characteristics of different industrial process control objects can be realized through software, thereby enhancing the application capability of the DCS system. (2) In order to realize complex control algorithms, the instrument microcomputer has reserved a power-loss protection memory that can be expanded to 32KB, which can realize arbitrary configuration of program space and data space. (3) The core device for level conversion in the communication part was changed to the MAX485 with better performance. Therefore, improvements to the hardware circuit mainly enhance the instrument's adaptability, while improvements to the communication function are primarily focused on the software. To balance the strict sequential nature of the instrument's internal measurement and control functions with the randomness of the communication functions, the instrument software design borrows from a multi-task operating system module, setting up a task list and task trigger/start mechanism. This ensures the strict sequential requirements between modules. Simultaneously, the communication function is decomposed into smaller modules, and a data buffer is set up for sending and receiving. Based on the interrelationships between each communication function module and each measurement and control function module, the task trigger and start mechanisms are determined. The design details in this part are quite detailed and will not be elaborated upon here. 5. Conclusion This paper addresses the industrial production automation technology transformation of state-owned enterprises. Based on the characteristics of intelligent instrument networking, it proposes a multi-intelligent instrument control system using a control PC connected to intelligent instruments via an RS-485 bus. Based on a reference to representative fieldbus protocols, a communication protocol for the multi-intelligent instrument control system is proposed, application software is developed, and a USTS-100 temperature controller with this communication function is designed, thus forming a practical multi-intelligent instrument control system and completing a targeted experiment. Experiments have demonstrated that the experimental system operates well and can complete various complex control experiments. This research will help improve the research and application level of intelligent instruments in China, expand their application areas, and meet the technological transformation needs of small and medium-sized enterprises in China, thus possessing both theoretical significance and practical value.