Abstract: To address the shortcomings of existing monitoring systems, such as complex structure, high cost, and inconvenient maintenance, this paper develops a general-purpose monitoring system based on the PROFIBUS fieldbus. This monitoring system includes monitoring instruments and a monitoring host. It features 32 analog input channels capable of connecting various types of sensors and utilizes the PROFIBUS fieldbus, allowing it to work with instruments from other manufacturers conforming to the PROFIBUS fieldbus TN915 protocol, aligning with current international trends in control system development. Keywords: Monitoring system; Input channel; Fieldbus; Sensor 1 Introduction Currently, widely used monitoring systems are distributed control systems (DCS) or employ intelligent sensors. However, DCS suffers from complex structures, relatively high costs with increasing point counts, and inconvenient maintenance. Using intelligent sensors at multiple monitored points with relatively concentrated locations also increases unnecessary system costs. Therefore, this paper develops a general-purpose monitoring system based on the PROFIBUS fieldbus, comprising monitoring instruments (lower-level devices) and a monitoring host (upper-level device). For monitoring in small and medium-scale applications, where a variety of sensors are used and multiple points of digital input and output are required, the monitoring instrument is designed with 32 analog input channels (capable of connecting various types of sensors), 8 digital input (DI) channels, 8 digital output (DO) channels, and 4 closed-loop analog output (AO) channels. Therefore, the monitoring system developed in this paper is a small but comprehensive general-purpose monitoring system. 2 Overall Design Principles and System Composition [align=center] Figure 1 System Composition Block Diagram[/align] The monitoring system developed in this paper should have the characteristics of high reliability, ease of maintenance, simple structure, flexible operation, and easy expansion. Therefore, the overall design principle of this system is as follows: the host computer adopts an industrial control computer, and flexible human-machine interface software is used to realize operation and control; the monitoring instrument adopts a single-chip microcomputer system with a 16-bit single-chip microcomputer 80C96KB as the CPU to realize the acquisition and control of field data, achieving the requirements of simple structure, flexible function, and stable performance; the communication between the host and slave computers adopts RS-485 serial bus, which has strong anti-interference ability, long transmission distance, and can connect multiple slave computers in a simple parallel manner; the communication protocol between the host and slave computers adopts the PROFIBUS international standard, making the system easy to expand and highly interchangeable. Based on the system functional requirements, the system in this paper consists of two parts: the monitoring system and the monitoring host, which are composed of hardware and software. The monitoring instrument is a single-chip microcomputer system with measurement and control functions, serving as the slave computer placed in the field. The monitoring host consists of an industrial control computer and a serial communication interface component. The system composition is shown in Figure 1. 3. Lower-level Computer Composition The lower-level computer uses an 80C196KB microcontroller system for data acquisition, control output, transmission of measurement data to the upper-level computer, and reception of operation commands from the upper-level computer. 3.1 AI Unit The AI ​​unit includes amplification, filtering, and processing circuits for 32 analog input signals, as shown in Figure 2. [align=center] Figure 2 AI Unit Structure Diagram[/align] Main performance of the AI ​​unit: 8-channel Pt100 RTD measurement range: 0～3000°C; 8-channel K-type thermocouple measurement range: 100～8000°C; 8-channel E-type thermocouple measurement range: 100～8000°C; 4-channel 4～20mA current signals; 4-channel 1～5V voltage signals; To ensure the system's measurement accuracy is better than 0.2%, the A/D converter in the AI ​​unit uses a 12-bit AD574, which can complete one A/D conversion within 25μs, and the 32 signals are cyclically sampled once every 2 seconds. 3.2 DI Unit [align=center] Figure 3 DI Unit Block Diagram[/align] The DI unit includes 8 channels of 24V DC opto-isolated switch input signals. Its structure is shown in Figure 3. Function of the DI unit: Simultaneous input of 8 channels of relay on/off switching signals. 3.3 DO Unit The DO unit includes 8 channels of passive contacts with opto-isolated outputs. Its structure is shown in Figure 4. [align=center] Figure 4 DO Unit Structure Diagram[/align] Main performance of the DO unit: Maximum switching capacity: 1A/220V; Relay: EG2-5N1; ON response time: MAX 10ms; OFF response time: MAX 10ms; Internal current consumption: MAX 50mA, 5VDC (8 points simultaneously on). 3.4 AO Unit The AO unit includes 4 channels of 4~20mA analog output signals. Its structure is shown in Figure 5. They can be configured to form a closed-loop control with any one of the 32 AI channels. [align=center]Figure 5 AO Unit Structure Block Diagram[/align] AO Unit Main Performance: Load Range: 0Ω～300Ω; Output Range: 4～20mA; To ensure output accuracy, the AO unit uses a dual-channel 12-bit serial DAC-TLC5618, which amplifies the output voltage signal and then converts it into a 4～20mA current output via a V/I conversion circuit. 3.5 Serial Communication Interface Unit The serial interface of this measuring instrument adopts the opto-isolated RS-485 interface standard, and the communication cable uses shielded twisted-pair cable. The RS-485 bus has advantages such as long transmission distance and strong anti-interference ability, making it suitable for production environments. 3.6 Program Memory and EEPROM An 8K program memory is used in the lower-level machine to store the lower-level machine's monitoring program. The system also uses a 128K EEPROM-DS 1213D to store the current system settings and data acquisition information, which can realize the power-off protection function. The EEPROM battery life is 10 years. 4. Host Computer The host computer of the system uses an Advantech 610 industrial PC to achieve high reliability for the entire system. It includes: host computer, monitor, keyboard, and mouse. The basic configuration of the host computer is as follows: CPU: P4-1.8G; Memory: 256M; Hard disk: 20G; Optical drive: 52X; Video memory: 32M; Monitor: PHILIP107E; Standard keyboard and mouse; Advantech communication interface card: PCL-745B; The industrial PC is expanded to have two RS-422/485 serial ports through the dedicated communication interface card PCL-745B. This system only uses one of these ports. When there are many slave computers, the amount of information communicated between the host and slave computers is large, with a maximum available baud rate of 19200bps. 5. System Working Principle The system working principle can be briefly summarized as follows: The system selects any one of the 32 external signals to be measured as input via a relay array. After the signal is input into the system, one of five processing methods is selected. These five methods are: resistance temperature detector (RTD) circuit; type E thermocouple circuit; type K thermocouple circuit; standard 1-5V voltage input; and standard 4-20mA current input. If either of the two standard input methods is selected, the signal is directly connected to the next stage signal conditioning circuit after passing through the path. If the thermocouple or RTD temperature detection path is selected, the signal output from the temperature detection circuit first passes through a current output interface integrated circuit AM402 before being connected to the signal conditioning circuit. The analog signal output from the signal conditioning circuit is converted into a digital signal by an A/D converter and then sent to the CPU for data processing. The CPU issues a call command to the A/D converter to retrieve the data for processing. The processed data is communicated with the host computer in real time via a serial port and further processed by the host computer. The host computer's response information is stored in memory at a specified address by the CPU and is also output by the CPU. The CPU outputs 8-bit parallel data, which undergoes parallel-to-serial conversion, digital-to-analog conversion, and voltage-to-current conversion to enable the output quantity to drive the controlled system. The system also outputs switching quantities as control signals. 6. Main Performance Indicators of the System The main performance indicators of the system are as follows: (1) 4 channels of 4~20mA analog input, which allows the host computer to set the channel number and can be retained after power failure. (2) 4 channels of 1~5V analog input, which allows the host computer to set the channel number and can be retained after power failure. (3) 16 channels of thermocouple (K type, E type) input, which allows the host computer to set the channel number and can be retained after power failure. (4) 16-channel RTD (Pt100) input, allowing the host computer to set the channel number, and can retain the value even after power failure. (5) 8-channel switch input (24V DC opto-isolated), allowing the host computer to set the channel number, and can retain the value even after power failure. (6) 8-channel switch output (passive contact, opto-isolated, 1 A/220 V), allowing the host computer to set the channel number, and can retain the value even after power failure. (7) 4-channel analog output (4~20mA), allowing the host computer to set one of them as the closed-loop control output of the AI ​​channel, and the other channel numbers can also be set in the same way, and can all retain the value even after power failure. (8) The serial port adopts the opto-isolated RS-485 interface standard, and the communication protocol follows the PROFIBUS bus standard. (9) The CPU adopts 80C196. (10) The sampling interval is less than 2 seconds. (11) The measurement error is less than 0.2%. (12) The local address can be set by the host computer and can be retained after power failure. (13) Real-time display and storage function of monitoring data and system working status. 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