1 Introduction
The LED display is a new type of flat panel display device that appeared in the 1990s. It is popular because of its high brightness, clear picture and bright colors. The full-color LED display utilizes the principle of three primary colors to produce a color effect by superimposing three kinds of LEDs of red, green and blue; the true color LED display has 8-bit gray value for each single primary color, that is, the single pixel is composed of 24-bit RGB binary. The number indicates. The image source of the LED control system is generally a real-time digital signal. To achieve smooth video display, the image with high refresh rate should be transmitted in real time according to the pixel signal corresponding to the LED dot matrix. When the display resolution is large, the transmission is performed. The demand for system bandwidth is huge. Ethernet is a common network standard in life. Applying existing Ethernet technology to LED screen transmission system not only simplifies system design, reduces design cycle, but also facilitates the formation of transmission standards for LED screen control systems. The current Gigabit Ethernet LED control system is limited to the use of a single transmission medium, and cannot combine the advantages of long-distance transmission of optical fibers and low-cost twisted pair networking. In order to solve such problems, this paper analyzes the similarities and differences between Gigabit Ethernet transmissions using two transmission media by studying the related protocols of Gigabit Ethernet [1], and introduces the transmission of 1000BASE-T and 1000BASE-LX physical layer signals in detail. In this way, a media converter based on two standards was designed and applied to an LED transmission system. For the design of the FPGA main controller, the on-chip buffering method of the transmission frame structure and image data is mainly analyzed. The system saves the cost of the LED long-distance real-time transmission system under the premise of ensuring the transmission rate. 
2 Design principles
2.1 Gigabit Ethernet
Gigabit Ethernet technology currently has two standards: IEEE802.3z[2] and IEEE802.3ab. IEEE 802.3z sets standards for fiber and short-range copper connection options, including: 1000BASE-LX, 1000BASE-SX, and 1000BASE-CX. IEEE 802.3ab sets the standard for long-distance connection schemes on five types of twisted pair: 1000BASE-T. This paper selects 1000BASE-LX and 1000BASE-T as the representatives of optical transmission and twisted pair transmission respectively, discusses the application of 802.3z and 802.3ab in LED transmission system and the conversion process between them.
2.2 LED transmission system
The structure of the Gigabit Ethernet LED transmission system [3-5] for twisted pair transmission is shown in Figure 1. The digital video interface obtains the display video source from the PC host; the data cache portion is used to achieve the rate matching between the video source and the network transmission; The Gigabit Ethernet MAC layer at the transmitting end encapsulates the data and constructs a top-level transmission control protocol; the Gigabit network physical layer completes the channel codec of the data, generates the transmission clock, and controls the transmission of the symbol on the physical channel; The layer realizes the extraction of the transmission clock and the channel decoding; the MAC layer performs frame unpacking and checks the data; the data is divided into small area pixel blocks according to the requirements of the scanning system [5-6] according to the area allocation part, and is sent Into the scanning system; the scanning system is responsible for the area display of the LED driver chip control [7-8].
The structure of the Gigabit Ethernet LED transmission system using optical fiber transmission is similar to that of Figure 1, and the specific implementation methods are different. The two transmission media have great advantages in network construction cost and long-distance transmission respectively. The flexible selection of transmission mode can improve the cost performance. In order to combine the advantages of the two transmission modes, it is necessary to design an LED transmission system media converter.
3 Design Ideas 3.1 System Composition
The structure of the Gigabit network media converter is shown in Figure 2. Image source data enters the system through two RJ45 interfaces, each with a rate of 1 Gbit/s. The network transformer converts the signal on the twisted pair into a format that the symbol can recognize. The physical layer is divided into three sub-layers [9], which are a physical coding sublayer (PCS), a physical medium connection sublayer (PMA), and a physical medium dependent sublayer (PMD). The functions implemented by the 1000BASE-T physical layer control chip include the A/D conversion function of the PMD sublayer; the transmission clock recovery function of the PMA sublayer; and the Viterbi decoding and descrambling functions of the PCS sublayer. Two 125M clock 8bit data signals are recovered before the data frame enters the master FPGA. The master FPGA completes the function of the MAC layer, and the two channels of data are separately unpacked and buffered, and the data is divided into 16 bits to form a new frame and added to the control information, and then transferred to the universal Gigabit transceiver; The data is first encoded by the 8B/10B of the fiber PCS sublayer, then sent to the PMA sublayer for parallel and serial conversion, and after the corresponding differential level conversion, enters the PMA sublayer, ie, the SFP optical module, to achieve electro-optical conversion, and finally It is transmitted as an optical signal to a single mode fiber. The data flow direction of the receiving end is completely reversed, and the data enters from the fiber end. In the main control FPGA, the 16-bit frame is re-encapsulated and buffered, converted into two 8-bit frame signals, and then transmitted in the original twisted pair data format. Return to the original receiving system.
