Fieldbus technology emerged internationally in the mid-1980s. Applied in production environments, it enables bidirectional, serial, multi-node digital communication between microcomputer-based measuring devices. Adapting to the trend of industrial control systems towards decentralization, networking, and intelligence, it quickly became a global hotspot in industrial automation technology, attracting widespread attention worldwide. Since the late 1980s, several fieldbus technologies, such as FF, Lonworks, CANbus, and Profibus, have matured and influenced the progress of industrial automation. Profibus, short for Process Fieldbus, is a fieldbus technology used for workshop-level monitoring and data communication and control of field devices in factory automation. It enables distributed digital control and field communication from the field device level to the workshop level, providing a feasible solution for achieving comprehensive factory automation and intelligent field devices. Belt conveyor is a crucial link in coal mine production, and the implementation of monitoring systems in this area is a trend in modern coal mine production. This article takes the underground belt conveyor monitoring system of Qidong Coal Mine under the Wanbei Mining Bureau as an example to briefly introduce the application of Profibus fieldbus technology in monitoring systems. I. Profibus Fieldbus Technology 1. Profibus Overview Profibus is an international, open, manufacturer-independent fieldbus standard widely used in industrial automation. Based on application characteristics, Profibus is divided into three compatible versions: Profibus-DP, Profibus-FMS, and Profibus-PA. Profibus-DP is a high-speed (data transmission rate 9.6kbit/s～12Mbit/s) and economical device-level network, mainly used for communication between field controllers and distributed I/O, meeting the fast response time requirements of AC/DC speed control systems. Profibus-PA adopts the IEC II 58-2 standard, with a transmission rate of 31.25kbit/s and provides intrinsic safety features, suitable for applications with high safety requirements and bus-powered applications. Profibus-FMS mainly solves workshop-level communication problems, completing cyclic or non-cyclic data exchange tasks with medium transmission speeds. 2. Bus Topology Depending on the connection method from field devices to the controller, the fieldbus topology can take various forms, typically using the following three: linear, tree, and ring. Profibus uses a linear structure, characterized by its simplicity. A main trunk line connects the controller to the mechanical device (controlled object), and bus cables branch from the trunk line to the field devices. The controller scans the inputs on all I/O stations and can send information to the output channel when necessary. Under this bus structure, multi-master and peer-to-peer communication can be realized, and two controllers can share information and I/O stations in the same system. In addition, an I/O device can be removed from the bus without shutting down the bus system, which greatly facilitates the maintenance of the bus system. 3. Profibus-DP device types According to the actual design needs, this system adopts Profibus-DP. Each Profibus-DP system includes the following three different types of devices: (1) DP master type 1 It is the central component of the Profibus-DP application. Within a specified, repetitive information cycle, the central controller or PC exchanges information with the distributed slaves (DP slaves). The non-cyclically transmitted data does not change frequently compared to the cyclically transmitted measurements, so this data is transmitted along with the fast-cyclically transmitted useful data, but it is transmitted with a lower priority. Interrupt acknowledgment in the master station ensures reliable transmission of interrupts from DP slave stations. (2) DP Master Station Type 2 This type of device (such as a programmer, configuration device, or operating device) is used for the startup, configuration, or operation of the DP system during normal operation (such as diagnostics). This type of master station can read input, output, diagnostic, and configuration data from the device. (3) DP Slave A DP slave is an I/O device that reads input information and provides output information to the I/O. The number of input and output information depends on the device type, with a maximum of 244 bytes. 4. Profibus-DP Communication Protocol The Profibus fieldbus adopts the physical layer and data link layer of the OSI model, as shown in Figure 1. Its transmission rate is 9.6kbps to 12Mbps, and the maximum transmission distance is 100m at 12Mbps and 400m at 1.5Mbps. It can be extended to 10km with repeaters. Its transmission medium can be either twisted pair or optical fiber, and a maximum of 127 stations can be connected. The Profibus-DP physical layer is identical to Layer 1 of the ISO/OSI reference model, employing the EIA-RS485 protocol. Depending on the data transmission rate, it can use either twisted-pair cable or fiber optic cable as the transmission medium. The Profibus-DP data link layer protocol's Media Access Control (MAL) portion uses a controlled access token bus and master-slave architecture. The token bus is consistent with the IEEE 8024 local area network protocol. Tokens are passed between masters on the bus, and the master holding the token gains control of the bus. This master communicates with slaves or other masters according to a relational table. The master-slave data link protocol differs from the LAN standard; it conforms to the Unbalanced Normal Response (NRM) mode in HDLC. This mode is characterized by: one master controlling multiple slaves on the bus; the master establishing a logical link with each slave; the master issuing commands; and slaves responding; slaves can continuously send multiple frames until no more information is sent, the required number of frames is reached, or the master stops them. The frame transmission process in the data link consists of three phases: data link establishment, frame transmission, and link release. The frame format transmitted between the master and slave stations in normal response mode is shown in Figure 2. F is the frame flag field (8 bits). A is the slave address field. The control field C represents the frame type, number, command, and control information, dividing HDLC frames into three types: information frames, monitoring frames, and unnumbered frames. Information frames are used for transmitting application data and piggybacking on responses; monitoring frames are used to monitor normal operation on the link and respond to various link statuses (such as acknowledgment frames, requests for retransmission, or pauses); unnumbered frames (without an information field) are used to transmit various unnumbered commands and responses, such as establishing link operating modes, releasing the link, and reporting special situations. The information field consists of application data in PKW + PZD. PKW is used to read and write parameter values, such as writing control words or reading status words, and is generally 4 bytes long. PZD is used to store the specific control values ​​of the controller and set parameters for stations or status words, and is generally 2 to 10 bytes long. For example, the second byte of PZD can be set as the start/stop control bit for devices 0# to 7#. FCS is the Frame Check field, which performs Cyclic Redundancy Check (CRC) on the entire frame content. HDLC frames can be up to 24 bytes long. Profibus-DP does not use the ISO/OSI application layer, but instead sets up its own user layer. This layer defines the functions, specifications, and extension requirements of DP. In summary, Profibus-DP's real-time performance is far superior to other LANs, making it particularly suitable for industrial sites. II. Hardware Structure of the Underground Conveyor Belt Monitoring System Profibus-DP is applied to the underground conveyor belt monitoring system of Qidong Coal Mine, Wanbei Mining Bureau. The hardware system is shown in Figure 3. The entire system consists of a host computer, a Profibus-DP master station, Profibus-DP slave stations, and their field devices. The Profibus-DP bus connects all devices. Both the Profibus-DP master station and Profibus-DP slave stations use the SIMATIC S7-300 module series; the master station uses the CPU315-2DP series module, and the slave stations use corresponding I/O modules. (1) Distributed I/O System This system uses the ET200 communication module to connect with Profibus-DP. The ET200 fully utilizes the SIMATIC S7-300 module series, connecting all S7-300 I/O modules to the fieldbus through the interface template IM153. The actuators and sensors under the I/O modules are connected to the field devices. The I/O modules provide output data to the field devices in master/slave mode and feed input data to the CPU or host computer. The I/O modules belong to the DP slave station. (2) CPU As the DP type 1 master station, the CPU is located in the control center. This system uses the CPU315-2DP modular medium-sized PLC, which has powerful processing capabilities and integrates the Profibus-DP fieldbus interface device. It also has a speed of processing 1024 statements in 0.3ms. After the PLC program is compiled in the STEP7 programming tool of the host computer, it is downloaded to the CPU315 and stored in the CPU315. The CPU315 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 hardware devices. (3) The host computer is a DP type 2 master station. This system uses an Advantech industrial computer as the host computer, and connects the industrial computer to the fieldbus through the fieldbus interface card CP5611. In this way, the industrial PC and the fieldbus network segment are connected to form a complete control network system that can complete configuration, operation and other functions. In order to ensure the stability of the system, the system uses dual-machine redundancy, and another industrial computer is connected to the fieldbus through the same fieldbus interface card CP5611. If one of the industrial computers fails, the other can continue to run. III. Software structure of the underground conveyor belt monitoring system The software structure includes the Windows NT operating system, lower-level programming software, and upper-level monitoring software. 1. Lower-level programming software This system uses the SIMATIC S7-300's matching programming tool STEP7 to complete hardware configuration, parameter setting, PLC program compilation, testing, debugging and document processing. Usually, the user program consists of organization blocks (OB), function blocks (FB, FC) and data blocks (DB). Among them, OB is the interface between the system operation program and the application program under various conditions, used to control the operation of the program. FB and FC are user subroutines. DB is a user-defined storage area for accessing data. In this system, it is the data interface point between the host computer monitoring software and the STEP7 program. By configuring the corresponding DB block in MPI, the data interface between the host computer monitoring software FIX and the STEP7 program can be realized. 2. Host computer monitoring software FIX industrial control configuration software is a large-scale application software based on Windows 9X & NT developed by Intellution, Inc. of the United States. It integrates control technology, human-machine interface technology, graphics technology, database technology and network technology. It includes components such as dynamic display, alarm, trend, control strategy, and control network communication, providing a user-friendly interface so that users can generate corresponding application software according to actual production needs. (1) Interface with PROFIBUS fieldbus ① Data flow FIX uses I/O drivers to read and write data from the device. Each I/O driver supports its specific hardware. For the PROFIBUS network of this system, the MPI driver is used to obtain the data on it. The FIX configuration software first acquires data from the process hardware in the field through the MPI driver software interface and stores it in the DIT driver image table (the driver image table is actually a memory area during system operation). The FIX internal database (PDB) retrieves the required data from the DIT table through the SAC program. Application software (such as the FIX screen running program and report generation program) retrieves information from the process hardware from the FIX internal database through the internal database access software. This allows for the dynamic display of the operating status of each process hardware in the field on the industrial process screen. Data can also be written back to the process hardware in reverse order to execute control operations. The corresponding data acquisition process is shown in Figure 4. ② MPI Configuration: A crucial issue in the application of the MPI driver is the address translation between STEP7 and FIX. The DB blocks set in STEP7 should be converted to MPI DB blocks, which needs to be implemented in the MPI configuration. MPI configuration includes channels, devices, starting addresses, and other parameters to ensure that the MPI DB blocks correspond to the DB blocks set in STEP7, enabling the FIX application to acquire field data. (2) User Interface Development The human-machine interface developed for this control system includes the following: ① Information Display Screen: The information display screen mainly displays the current operating status information of each conveyor belt, such as the current conveyor belt speed, the position of the coal storage bin, and some fault information, such as belt deviation, blockage, slippage, etc. Different colors can be used to indicate whether the current status is normal or abnormal. ② Equipment Control Screen: Although the lower-level computer program can realize data acquisition and control signal output on the fieldbus, and implement some simple control algorithms such as PID control, complex control functions still need to be manually controlled on the upper-level computer. Clicking the corresponding equipment button on the screen allows for individual control of the equipment. ③ Real-time Alarm Processing: The system judges the data collected in real time, issues alarm signals, processes them according to technical requirements, and automatically performs corresponding equipment control, such as unlocking and restoring conveyor belt fault signals. ④ Report Printing: Real-time reports are developed using FIX's DDE function and have a print function at any time. ⑤ Real-time Data Curve Display: The system monitors the trend curves of important parameters, thereby allowing understanding of the equipment's operating status over a period of time. ⑥ The historical trend screen function is similar to the real-time data curve, except that it displays the equipment's operating parameter values ​​over a past period. The underground conveyor belt monitoring system of Qidong Coal Mine under Wanbei Mining Bureau is now in operation, with good equipment performance, significant economic benefits, and high praise from users.