Basic design principle of led display screen in subway train

December 20, 2022
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Basic design principle of led display screen in subway train

The basic design principle of subway led display screen; As a public oriented information display terminal in subway, indoor led display has a very wide range of civil and commercial value.

At present, subway vehicles operating in china are generally equipped with indoor led display, but there are few additional functions and single screen display content. In order to cooperate with the use of the new metro passenger information system, we have designed a new multi bus metro LED dynamic display screen.

The display screen not only has multiple bus interfaces in external communication, but also adopts single bus and I2C bus devices in the internal control circuit design.

There are two kinds of LED screens on the subway: one is placed on the outside of the carriage to display the train running section, running direction and the current station name, which is compatible with Chinese and English; Other service information can also be displayed according to operation needs; The text display can be static, scrolling, translation, waterfall, animation and other effects, and the number of characters displayed is 16 × 12 16 dot matrix characters. The other is the terminal indoor LED display, which is placed in the train. The terminal indoor LED display can preset the terminal according to the train operation requirements, and display the current terminal in real time, as well as the current temperature in the train, with 16 characters × Eight 16 dot matrix characters.

System composition

The LED display system screen is composed of a single-chip microcomputer control unit and a display unit. A single display unit can display 16 × 16 Chinese characters. If a certain size of LED graphic display system is produced, it can be realized by using several intelligent display units and the method of "building blocks". Serial communication is used between display units in the system. In addition to controlling the display unit and transmitting the instructions and signals of the upper computer, the control unit is also embedded with a single bus digital temperature sensor 18B20. Thanks to the module design of the control circuit, if there are requirements for humidity measurement, 18b20 can be upgraded to the module circuit composed of DS2438 from Dallas and HIH23610 from HoneywELL. In order to meet the communication needs of the whole vehicle, CAN bus is used for communication between the upper computer and each control unit in the vehicle.

hardware design

The display unit is composed of LED display panel and display circuit. The LED display unit board is composed of 4 dot matrix modules × 64 dot matrix universal intelligent display unit, a single display unit can display 4 16 × 16 dot matrix Chinese characters or symbols. Serial communication is used between display units in the system, so that the work of the whole system is coordinated and unified. The display circuit consists of two 16 pin flat cable ports, two 74H245 tristate bus drivers, one 74HC04D six inverter, two 74H138 eight decoders and eight 74HC595 shift latches. The core of the control circuit is the high-speed microcontroller 77E58 of WINBOND, and the crystal frequency is 24MHz AT29C020A is a 256K ROM for storing 16 × 16 dot matrix Chinese character library and 16 × 8 dot matrix ASCII code table. AT24C020 is an EP2ROM based on I2C serial bus, which stores preset statements, such as subway station names, greetings, etc. The temperature in the vehicle is measured by the single bus digital temperature sensor 18b20. SJA1000 and TJA1040 are CAN bus controller and transceiver respectively.

Control circuit unit design

The whole system takes the dynamic microcontroller 77E58 of Winbond as the core. The 77E58 adopts a redesigned microprocessor core, and its instructions are compatible with the 51 series. However, because the clock cycle is only 4 cycles, its running speed is generally 2~3 times higher than the traditional 8051 at the same clock frequency. Therefore, the frequency requirements for the microcontroller in the dynamic display of large capacity Chinese characters are well solved, and the watchdog is also provided. The 77E58 controls the flash memory AT29C020 through the latch 74LS373, with a size of 256K. Since the memory capacity is greater than 64K, the design adopts the paging addressing method, that is, P1.1 and P1.2 are used to select pages for the flash memory, which is divided into four pages. The addressing size of each page is 64K. In addition to selecting AT29C020 chips, P1.5 ensures that P1.1 and P1.2 will not cause misoperation of AT29C020 when they are reused on the 16 pin flat cable interface. The CAN controller is the key part of communication. In order to improve the anti-interference ability, a 6N137 high-speed optocoupler is added between the CAN controller SJA1000 and the CAN transceiver TJA1040. The microcontroller selects the CAN controller SJA1000 chip through P3.0. 18B20 is a single bus device. It only needs one I/O port for the interface between the device and the microcontroller. It can directly convert the temperature into a digital signal and output it serially in a 9-bit digital code mode. P1.4 is selected in the control circuit to complete the chip selection and data transmission functions of 18B20. The clock cable SCL and bidirectional data cable SDA of AT24C020 are respectively connected to P1.6 and P1.7.16 pin flat wire interfaces of the microcontroller, which are the interface parts of the control circuit and the display circuit.

Display unit connection and control

The display circuit part is connected with the 16 pin flat wire port of the control circuit part through the 16 pin flat wire port (1), which transmits the instructions and data of the microcontroller to the LED display circuit. The 16 pin flat wire (2) is used for cascading multiple display screens. Its connection is basically the same as the 16 pin flat wire port (1), but it should be noted that its R end is connected to the DS end of the eighth 74H595 from left to right in Figure 2, When cascading, it will be connected in series with the 16 pin flat cable (1) port of the next display screen (as shown in Figure 1). CLK is the clock signal terminal, STR is the row latch terminal, R is the data terminal, G (GND) and LOE are the row light enable terminals, and A, B, C, D are the row select terminals. The specific functions of each port are as follows: A, B, C, D are row selection terminals, which are used to control the specific sending of data from the upper computer to the designated row on the display panel, and R is the data terminal, which accepts the data transmitted by the microcontroller. The working sequence of the LED display unit is as follows: after the CLK clock signal terminal receives a data at the R terminal, the control circuit manually gives a pulse rising edge, and the STR is in a row of data (16 × 4) After all 64 data are transmitted, a rising edge of pulse is given to latch the data; The LOE is set to 1 by the microcontroller to light up the line. The schematic diagram of the display circuit is shown in Figure 3.

Modular design

Metro vehicles have different requirements for indoor led display according to the actual situation, so we have fully considered this when designing the circuit, that is, under the condition of ensuring that the main functions and structures remain unchanged, specific modules can be interchanged. This structure makes the LED control circuit have good expansibility and ease of use.

Temperature and humidity module

In the hot and rainy areas in the south, although there is a constant temperature air conditioner in the car, the humidity is also an important indicator that passengers care about. The temperature and humidity module designed by us has the function of measuring temperature and humidity. The temperature module and the temperature and humidity module have the same socket interface, both of which are single bus structures and are controlled by P1.4 port, so it is convenient to exchange them. HIH3610 is a three terminal integrated humidity sensor with voltage output produced by Honeywell Company. DS2438 is a 10 bit A/D converter with a single bus communication interface. The chip contains a high-resolution digital temperature sensor, which can be used for temperature compensation of humidity sensors.

485 bus expansion module

As a mature and cheap bus, 485 bus has an irreplaceable position in industrial field and traffic field. Therefore, we have designed a 485 bus expansion module, which can replace the original CAN module for external communication. The module uses MAXIM's photoelectric isolation MXL1535E as the 485 transceiver. To ensure control compatibility, both MXL1535E and SJA1000 are chip selected through P3.0. In addition, 2500VRMS electrical isolation is provided between RS2485 side and controller or control logic side through transformer. TVS diode circuit is added to the output part of the module to reduce line surge interference. Jumpers can also be used to decide whether to load bus terminal resistance.

Software design

The system software is composed of upper computer management software and unit controller control software. The upper computer management software is developed on the Windows22000 operating platform using C++BUILD6.0, including display mode selection (including static, flashing, scrolling, typing, etc.), scrolling direction selection (including up and down scrolling and left and right scrolling), dynamic display speed adjustment (i.e. text flashing frequency, scrolling speed, typing display speed, etc.), display content input, display preview, etc.

When the system is running, the system can not only display the characters such as station announcement and advertisement according to the preset settings, but also manually input the required display characters. The control software of the unit controller is programmed by KEILC of 8051 and solidified in the EEPROM of the single chip computer 77E58. It mainly completes the communication between the upper and lower computers, data acquisition of temperature and humidity, I/O interface control and other functions. During actual operation, the temperature measurement accuracy reaches ± 0.5 ℃ and the humidity measurement accuracy reaches ± 2% RH


This paper introduces the design idea of subway indoor LED display screen from the aspects of hardware schematic diagram design, logic structure, composition block diagram, etc. Through the design of field bus interface module and temperature humidity module interface, the indoor LED display screen can adapt to the requirements of different environments, and has good scalability and versatility. After many tests, the indoor led display screen has been used in the new passenger information system of domestic metro, and the effect is good. The practice proves that the display screen can well complete the static display of Chinese characters and graphics and various dynamic displays, and has the characteristics of high brightness, no flicker, simple logic control, etc., which fully meets the display requirements of subway vehicles for LED screens.