1 Introduction With the increasing popularity of Internet technology, the degree of information sharing continues to increase. According to the prediction of network experts, about 70% of the information transmitted on the Internet in the future will come from small embedded devices, and Internet applications are turning to embedded devices as the center. Because many embedded devices used in industrial measurement and control systems, smart instruments, smart home appliances, etc., their external communication interfaces are still easy-to-use and highly reliable serial ports, so how to use serial ports that have been widely used in industry The access of embedded devices to the network has become an important and urgent issue. Under this background of application status, this paper designs and implements the interconnection system of embedded equipment with RS-485 serial port and Ethernet with IP2022 network processor as the core. The system implements the functions of universal asynchronous transceiver (UART), Ethernet driver peripherals and TCP / IP network protocol stack, and provides a hardware and software platform for embedded devices to access Ethernet. 2 System hardware design 2.1 Introduction to IP2022 IP2022 is a powerful network communication processor launched by Ubicom of the United States. Under the drive of a 4.8MHz crystal oscillator, using its internal phase-locked loop (PLL) circuit, the CPU operating frequency can reach 120MHz, and most instructions are single Periodic execution, its throughput can meet the requirements of various new network connectivity applications. The internal structure of IP2022 is shown in Figure 1. IP2022 contains a full-duplex serializer / deserializer (Ser / Des), which can be directly connected to various common network interfaces, and can realize on-chip 10Base-T Ethernet (MAC and PHY), UART and other various fast Serial protocol. IP2022 implements peripheral communication and control functions in the form of software modules (ip Modules), which has greater system design flexibility than traditional hardware. IP2022 also provides a TCP / IP network protocol stack, and has a series of additional software necessary for a complete end-to-end connectivity solution. Because IP2022 contains two Ser / Des components, it can easily convert from one format to another format, so it can also be used as a protocol converter. 2.2 Overall structure of system hardware The hardware of the system is mainly composed of IP2022 network processor, RS-485 interface circuit, Ethernet interface circuit and power supply circuit. The IP2022 network processor is the core of the entire system, which mainly completes data conversion and system control. The Ethernet interface circuit realizes the connection with 10Base-T Ethernet. RS-485 interface circuit realizes connection with embedded equipment with RS-485 interface. The power supply circuit provides the + 5V, + 3.3V, + 2.5V power required by the system. The hardware structure of the system is shown in Figure 2. 2.3 RS-485 interface circuit of the system The RS-485 standard has become the preferred standard for data transmission in distributed systems due to its advantages of good noise immunity, long transmission distance, and multi-station transmission capabilities. However, if it is used improperly in the actual use process, an error will occur in data transmission. In order to suppress bit errors, in this design, we used two SN75LBC184 with transient voltage suppression function to separate the input and output lines so that the input and output signals do not interfere with each other. SN75LBC184 has a built-in high-energy transient noise protection device, which improves the reliability against transient noise on data synchronous transmission cables. The chip transmits information in a differential manner, has strong resistance to common mode interference, allows a transmitter on a pair of twisted pairs to drive multiple load devices, and its maximum allowable communication distance reaches 1200 meters. Figure 3 shows the principle diagram of the system's RS-485 interface circuit. Through trial and error, install a 3.3K bias resistor on the input end and connect a 1K series resistor in series to make the input signal waveform the best; on the output end, add an 18K bias resistor. The working voltage of SN75LBC184 is + 5V, while the working voltage of I / O end of IP2022 is + 3.3V. In this way, when receiving an external signal, the I / O port of the IP2022 may be damaged due to excessive voltage. Therefore, a voltage conversion chip 74LVX4245 was added between SN75LBC184 and IP2022 during design. The signal at one end of the chip uses + 5V, and the signal at the other end uses + 3.3V. The enable terminal controls the signal flow, which effectively maintains the stability and safety of the IP2022. 2.4 The system's Ethernet interface circuit [5] The IP2022 chip contains a full-duplex serializer / deserializer (SER / DES), which can implement on-chip 10Base-T Ethernet (MAC and PHY). When designing the external interface circuit, there is no need to use a dedicated Ethernet interface chip, just connect the input and output signal lines of IP2022 directly to the network transformer. Figure 4 shows the schematic diagram of the system's Ethernet interface circuit. IP2022 uses 6 I / O pins, which are connected to serializer / deserializer 1. The four pins used for transmission are each connected with a resistor, and these four resistors jointly give an impedance of 100Ω. TX + / TX- is a differential transmission line pair, RX + / RX- is a differential reception line pair, using differential balanced transmission, the transmission speed can reach 10Mbps. FB2022 is an internal network transformer with a low-pass filter. It is responsible for amplifying the differential voltage of the IP2022 output and passing it to the RJ45. It reduces the input differential voltage and passes it to the IP2022 to protect the IP2022 chip from damage. Connect a 100Ω coupling resistor between RX + and RX- to improve the stability of the communication line. RJ45 is a standard twisted pair Ethernet interface socket.
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