Abstract: Computers and networks have entered the post-PC era. With the resurgence of research on applying Ethernet to industrial automation, the application of Ethernet technology at the field instrument and industrial equipment layer is a development trend of industrial control networks. This paper studies and develops a hardware structure of an industrial field intelligent control module based on industrial Ethernet. Appropriate microprocessors and peripheral storage, communication devices are selected to construct a hardware platform with a network interface that can access the industrial Ethernet field network. Keywords: Intelligent control; Industrial Ethernet; Network interface 1 Introduction Industrial network control systems, as the name suggests, are networked industrial control systems. Currently, the most widely used industrial network control systems at home and abroad are distributed control systems and fieldbus control systems, but the most promising are industrial Ethernet control systems. They embody the development trend of control systems towards networking, integration, distribution, and node intelligence, thus becoming a research hotspot in the control field. This paper combines instrumentation and control networks to design a hardware device for a field intelligent control module with industrial Ethernet. 2 Hardware Structure Design of the Field Intelligent Control Module The main task of hardware design is to design interface circuits to meet the requirements of distributed control. Based on the system functions and centered around the core controller Rabbit2000, the hardware system can be divided into four modules as shown in Figure 1: microcontroller and memory module, digital input/output module, analog input/output module, and communication module. [align=center]Figure 1 Hardware System Implementation Block Diagram[/align] The entire circuit board is powered by 5V DC and integrates a Rabbit2000 microprocessor, 512K Flash RAM, 128K SRAM, and an RJ-45 Ethernet interface. The Rabbit2000 has five 8-bit parallel interfaces. Ports B and part of Port D are used to connect to the Realtek Ethernet card control chip RTL8019AS; Port C can be used for RS485 or RS232 serial communication. The I/O acquisition interface hardware circuit is connected to the bus. The entire controller interface section includes 9 analog input channels, 2 analog output channels, 8 digital input channels, and 8 digital output channels. The digital-to-analog and analog-to-digital converter chips are all 12-bit, and their accuracy basically meets the requirements of general controlled objects. 3. Detailed Hardware Design of the On-Site Intelligent Control Module The system microcontroller selected is the Rabbit2000 microprocessor manufactured by Rabbit Semiconductor. The Rabbit2000 processor is a high-performance 8-bit microprocessor based on the Z80 architecture, specifically designed for next-generation embedded systems. The chip is packaged in a 100-pin PQFP package, operates at 2.7V-5V, and has a maximum clock frequency of 30MHz. For embedded systems, its performance surpasses many 16-bit and 32-bit processors, and its efficiency is superior to similar 8-bit series. The Rabbit2000 has 40 parallel I/O lines (shared with the serial port). Some of these I/O ports are timer-synchronized, allowing for precise generation of edges and pulses under combined hardware and software control. It includes four serial ports, all of which can operate asynchronously in various operating modes. Two of these ports can also operate synchronously, interfacing with serial I/O devices. An on-chip watchdog timer and a built-in battery-powered time/date unit are integrated. 3.1 Analog and Digital Input/Output Modules The analog input interface is used to convert analog signals from the field into digital signals that the microprocessor can recognize and process. Since the measured objects in industrial applications are generally slowly changing signals such as temperature, pressure, and liquid level, a low sampling frequency is required. In this design, the analog signal acquisition is accomplished using TI's new analog-to-digital converter TLC2543. The digital-to-analog converter (DAC) interface is used to output analog signals. Through the DAC interface, the microprocessor can convert the binary representation of the signal to be output into an analog voltage or current signal proportional to the digital signal. In this design, Analog Devices' 12-bit voltage output D/A converter AD5320 is selected. [align=center] Figure 2 Analog Input Circuit Diagram[/align] Because the input analog signal may be close to zero or full-scale, or the internal resistance of the input signal source may be very high, adding a buffer circuit at the front end of the A/D conversion circuit to adjust the input analog signal can improve the full-range conversion accuracy and the measurement accuracy of weak signals. The specific circuit diagram is shown in Figure 2. To reduce quantization errors in the A/D conversion, the reference voltage for the A/D converter is chosen to be 4.096V, provided by the Zener diode LM4040-4.096. Therefore, the input signal range must be converted to 0-4.096V before being sent to the A/D converter input. Resistors R39 and R41 in the diagram are used to adjust the signal isolation gain, allowing the input voltage to be converted within the 0-4.096V range. The RC circuit composed of R41 and C64 acts as a low-pass filter. The analog input signal passes through the input buffer circuit to the A/D converter input port. The A/D converter selected is the TI TLC2543, a high-speed, low-power, switched-capacitor successive approximation 12-bit analog-to-digital converter (ADC). It requires only 1mA of supply current. In addition to providing a high-speed conversion capability with a maximum sampling rate of 66ksps, it can also use the general-purpose and flexible SPI interface for data transmission with the microprocessor. It also features an on-chip 14-channel multiplexer, which can switch between 11 AD input channels and 3 internal test voltages. Therefore, it can be widely used in data acquisition systems. The analog output channel uses a single-channel 12-bit voltage output D/A converter from Analog Devices, which operates with a single power supply and a voltage range of 2.7V to 5.5V. The on-chip high-precision output amplifier provides full-power amplitude output, with its reference derived from the power input, providing a large dynamic output range. It uses a 3-wire serial interface compatible with standard SPI, QSPI, MICROWIRE, and DSP interface standards to exchange data with the microprocessor, making the interface simple. Its interface circuit is shown in Figure 3. [align=center] Figure 3 D/A Output Circuit Diagram[/align] During operation, the write sequence is started when SYNC is set to low. During this stage, the SYNC line must remain low until the 16th falling edge of SCLK. The DAC is updated at this 16th falling edge. If SYNC is pulled high before this, it means that the write sequence is interrupted, and the shift register is reset. Data from the DIN line is fed into a 16-bit shift register on the falling edge of SCLK. The data in the shift register is 16 bits wide. The first two bits are irrelevant, the next two bits are control bits that determine the operating mode of the controller, and the last 12 bits are data bits that represent the voltage value that the DA converter will output. On the 16th falling edge of the clock, the last bit of data is input with the clock and executes the programmed function according to the given content. Digital quantities are only represented by two states, such as the on or off states of a relay in a control system. An embedded microprocessor is a digital signal processing system, and the control quantity it provides is inherently a digital quantity. However, in order to prevent strong electromagnetic interference or power frequency voltage from entering the measurement and control system through the input and output channels, the entire controller is generally isolated from the peripherals, i.e., isolation technology is required. Therefore, the technology of digital input and output channels is mainly for anti-interference rather than accuracy. 3.2 Communication Module [align=center] Figure 4 Schematic diagram of communication module[/align] As shown in Figure 4, this module mainly consists of an Ethernet controller RTL8019AS, a network transformer, and a serial port level conversion chip. The communication module consists of three parts: an RS232 serial communication interface, an RS485 serial communication interface, and an Ethernet communication interface. The Rabbit2000 is responsible for the network controller's initialization, data reception, and transmission. The Ethernet interface chip RTL8019AS is responsible for converting data packets into physical frame formats for transmission over the physical channel, and restoring received physical signals back into data in a specified format stored in the chip's RAM for the host program to read. The serial interface chips MAX483 and MAX232 are responsible for converting TTL level signals to RS-232 and RS-485 level signals. Considering the need for integration with traditional bus-type control systems, an RS-485 serial communication interface was added to the controller. RS-485 is characterized by its simple structure, low cost, rich hardware and software support, convenient installation, compatibility with traditional DCS, simple interface with field instruments, and easy system implementation. Especially in China, RS485 bus systems will remain the primary form of some small and medium-sized control systems for some time to come. RS-485 bus has a long transmission distance (up to 1200 meters at 90KB/s), transmits signals in a differential balanced manner, and has strong immunity to common-mode interference, allowing one transmitter on a twisted pair to drive multiple load devices. Therefore, many industrial field control systems use this bus standard for data transmission. RS-232 is currently the most commonly used serial standard interface, suitable for asynchronous communication between IBM-PCs and other external devices. To ensure correct transmission of binary data and accurate device control, it is necessary to maintain consistent signal levels. To meet this requirement, RS-232 specifies the voltage range for data and control signals. It uses negative logic, defining any voltage between +3V and +15V as a logic "1" level, and any voltage between -3V and 15V as a logic "0" level. Since most computer interface chips use TTL or CMOS levels, and RS-232 levels differ from TTL and CMOS levels, level conversion is necessary during communication, primarily using the MAX232 chip. The MAX232 is a dual-channel RS232 transceiver manufactured by Maxim Integrated. It operates on +5V power, supports TTL and CMOS level inputs, meets the EIA RS232C standard, and has low power consumption. 3.3 Ethernet Interface Module Implementation Principle The main challenge in the hardware design of the field intelligent control module lies in the design of the network interface module. When the field control module needs to send data, it should first perform Manchester encoding, then pre-distort the encoded data to make it suitable for transmission over a 10BaseT Ethernet network, and finally send the processed data to the Ethernet network at a speed of 10 Mbps. Simultaneously, to ensure effective data transmission, the system should also have collision detection and retransmission functions. From the above data transmission process, it can be seen that directly implementing the above functions using a common low-speed microcontroller is very difficult. The solution involves using a dedicated network interface chip, the RTL8019AS. This chip conforms to the CSMA/CD protocol defined by IEEE 802.3. Besides providing the electrical performance required for the physical link, it also offers Manchester encoding, collision detection, and retransmission capabilities. It can complete data transmission and reception with minimal external circuitry. The Rabbit2000 processor only needs to provide initial configuration and a data interface to the chip. The RTL8019AS provides a standard ISA interface for the microprocessor. The ISA bus has 98 signals. Directly implementing the ISA interface is complex and unnecessary. Since the design goal is a web server running in a small embedded system, analysis of the network card's operating principle allows for direct control via the RTL8019AS's data and address lines, minimizing the number of interface signal lines. The network card interface circuit can be divided into two parts: one is the connection to the computer's ISA bus, including the introduction of data bus read/write, memory read/write signals, and port read/write signals; the other is the operation of the network card's internal components, including reading and writing to the buffer RAM, controlling the RTL8019AS, reading the stack address PROM, and the bootstrap ROM. When the network card is set to 8-bit mode, operates in jumper mode, and uses automatic detection for the network interface type, the control signals only need to include the read/write control IORB and IOWB, the BD5, BD6, and BD7 signals required for configuring the network card I/O and interrupts, and the reset signal RSTDRY; the network card's base address is selected as 0x300, requiring only 5 address lines to access internal registers; the data lines are 8 bits. Based on the above analysis, Rabbit2000 only needs to provide 6 control lines (the reset line is connected to the system reset), 8 data lines, and 5 address lines to control the RTL8019AS. The author's innovation lies in the fact that industrial Ethernet has broad application prospects and has already attracted attention in the control field worldwide. The hardware design idea and specific implementation scheme of the field intelligent control module are proposed. The scheme of a multi-purpose module with field measurement and control function is realized by using Rabbit2000 microprocessor and RTL8019AS network card chip as the core, combined with data acquisition and processing circuit. References: [1] Yang Peng et al. Development and technical characteristics of industrial Ethernet [J], Microcomputer Information, 2006, 22-4: 32-24. [2] Wu Kuanming. 80C51 XA 16-bit controller system design [M], Devices and Applications Development, Beijing University of Aeronautics and Astronautics Press, 1996. [3] Shi Jiugen, Zhang Peiren, Chen Zhenyong. CAN fieldbus system design technology [M], National Defense Industry Press, 2004. [4] Zhou Guoqing, Yin Yanlei, Zhang Liuquan, Zhang Yungang. Research on power intelligent monitoring terminal based on Ethernet [J]. Microcomputer Information, 2007, 3-2: 37-38