1. Introduction There are many methods for displaying data on-site, including on-site PCs, displays, and digital display instruments. Compared with PCs and displays, digital tube display instruments have the advantage of low price and are also more suitable for harsh production environments. We undertook the automation transformation project of the vertical shaft furnace pelletizing production process at Daye Iron Mine of Wuhan Iron and Steel Mining Group, and adopted a multi-layer DCS control network based on CAN bus. To solve the data display problems in some sections, we designed an intelligent on-site display instrument based on bus listening technology. 2. System Structure In the pelletizing section control system of Daye Iron Mine's pelletizing plant, Advantech's ADAM-5000 series intelligent I/O modules are used to realize data acquisition and output functions. A total of three ADAM-5000 racks are distributed in three processes on-site. The three ADAM-5000 series modules and the upper-level industrial control computer are connected to each other via RS485 bus, forming a bottom-level RS485 bus network. Data communication within this bus network uses the ADAM-5000 communication protocol. The system structure diagram is shown in Figure 1. The data for the intelligent display instrument in the pelletizing section originates from data monitoring on the RS485 bus. The host PC transmits data according to the ADAM-5000 module frame format, sending commands to be sent to the lower-level ADAM-5000 modules on the 485 bus. Each data frame contains the correct address of the lower-level machine that should receive the command; only the lower-level machine with the target address matching the frame can receive the command frame. Bus monitoring technology overcomes this limitation. It dynamically monitors the data transmitted on the bus in real time, receiving all frames transmitted on the bus. After listening to all communication data from the serial port of the monitoring microcomputer, it analyzes and decodes the communication data to obtain the required data. 3. Hardware Composition of the Field Display Instrument The hardware schematic of the digital tube display instrument is shown in Figure 2. It consists of an AT89C52 microcontroller, a digital display circuit, and an RS485 interface circuit. The RS485 bus transceiver used is the TI SN75176. Due to the large amount of data to be displayed and the high brightness requirements of the digital display in the field, this solution adopts static display. To save microcontroller resources, the CD4094 serial-to-parallel converter with gating function is used as the display driver. This way, the 20+ digit display only occupies 3 microcontroller I/O lines, as shown in Figure 3. In the actual operation of the field display instrument, the AT89C52 microcontroller first receives data frames on the RS485 bus through the SN75176 RS485 bus transceiver chip, then analyzes and selects the data frames, and sends the required data to the digital tube display. 4. Software Implementation Since this system only needs to display a portion of the analog input data acquired by the ADAM-5000, the following only describes the frame format of the AI ​​module in the ADAM-5000 series. It is divided into command frames and response frames. The ADAM-5000 command frame format is shown in Table 1. Table 1 shows the ADAM-5017 command frame format. The first byte of the command frame is the start character, which can be either $ or #. Commands starting with $ can retrieve version information, correct AI modules, etc.; commands starting with # are used to read 5017 channel values, which is the command format we will be using; the address range is 00～FF; the slot number i ranges from 0 to 7; otherwise, when the start character is $, there are 9 formats, which will not be described here. When the start character is #, there are 2 formats: one is empty, the command content is to read all channel values ​​of the i-th slot module; the other is channel value j, the command content is to read the j-th channel value of the i-th slot module. The ADAM-5000 response frame format is shown in Table 2. For command frames starting with #, there are two response frame formats. Table 2 shows the ADAM-5017 response frame format. A start character of > indicates that the command received by the module is valid. Following this are all channel values ​​or a specific channel value, depending on the command frame format; a start character of ? indicates that the command received by the 5017 is invalid. The address thereafter is the same as the address of the received command frame. The key to the listening software lies in receiving data frames on the bus, which is mainly divided into two parts: analysis of command frames and parsing of response frames. When the PC communicates with the ADAM-5000, the display instrument continuously listens for data frames transmitted on the bus. After determining that it is a command frame, it compares the address of the received complete frame with the address of the frame containing the parameter to be displayed. If they match, it continues to listen for the response frame of the ADAM-5000, extracts the data information, and sends it to the digital tube for display; if they do not match, it does not take any action and waits for the next command frame. The communication software is written in C51 language, and the flowchart of the interrupt subroutine is shown in Figure 4. 5 Conclusion The field intelligent display instrument based on RS485 bus listening technology designed in this paper has been applied in the pelletizing section of the Daye Iron Mine vertical shaft furnace pelletizing plant, effectively solving the data display problem in the section. It is also inexpensive, versatile, and can be applied in other systems. This bus listening technology is also suitable for other fieldbuses such as CAN bus.