With the rapid development of electronic technology and distributed network systems, many trains have adopted communication network technology and realized the intelligentization, informatization and networking of the train control system. As an important component of the train communication network, MVB, due to its real-time Characteristics such as good performance and high communication rate are attracting more and more attention. This article analyzes the design of MVB multi-protocol communication gateway.
With the rapid development of my country’s high-speed railways, train communication networks have been widely used in new trains. Among them, my country’s Harmony EMU uses communication network control technology. As an important component of the train communication network, the communication gateway MVB , is also being paid attention to by more and more people.
1TCN and MVB Overview
TCN, also known as train communication network, is a communication network developed on the distributed control system of trains for train control and information data diagnosis. TCN is a standard specially formulated for train communication networks. It has the characteristics of reliability, real-time and manageability. This technology has been supported by many railway companies and suppliers. TCN consists of two parts: the twisted train bus WTB and the multi-function train bus MVB. Among them, MVB is a serial data communication bus between interconnected devices with interoperability and exchange requirements. It is an important component of TCN. Now it is on-board Many of these devices are commonly used industrial interfaces, but the actual application of these devices requires the MVB industrial interface to be connected to the MVB bus. Now many gateways have a relatively simple protocol conversion function and cannot support wireless LAN. The transmission method has a great impact on the intelligent development of trains and the principle of openness. The bus data transmission method is wired, which limits the acquisition of real-time dynamics in important places and brings inconvenience to the physical connection of equipment and gateways. Gateway industriale
2 Key technologies of MVB multi-protocol communication gateway
·. MVB multi-protocol communication gateway mainly includes six communication protocols: MVB bus protocol CAN bus, Ethernet, RS-232.RS-485 and Wi-Fi wireless network. These six communication protocols must be organically integrated in one gateway. It is a very difficult thing, but its research is still necessary. Among them, the TCN of the MVB bus protocol has two levels in the topological structure, namely the train WTB, the equipment and controller bus and the vehicle-level MVB. The train bus can automatically identify the direction and position of the train’s parking space, and can transmit reliable data; among them, the data types of the MVB bus mainly include the following three types: ① data with source addressing and periodic broadcast process; ② message data with destination addressing and transmission on demand, mainly including diagnostic information and management service information: ③ Monitoring data transmitted during network reconstruction and initialization. The device types of the MVB bus are mainly divided into six types of devices from 0 to 5, but they can also be divided into master devices and slave devices. A master device refers to a device that can send information to slave devices on the bus and can allocate slave frame sending rights. equipment. It can be divided into strong master device and weak master device. Real-time protocol RTP is a key software processing mechanism adopted by the MVB bus. It mainly provides two data communication services: variables and messages. Its hierarchical structure includes physical layer, network layer, transport layer, link layer and session layer. Message data is transmitted point-to-point, and the communication process can be summarized as: sender call, connection establishment, message transmission, and recipient response. As shown in Table 1:
3.1 Overall design of MVB multi-protocol gateway hardware
The MVB multi-protocol gateway design can be divided into six modules in terms of hardware, which mainly include basic communication of the core system, performance testing, manual interaction, extended functions and simulation debugging.Among them, the work focuses on the design of core circuits and the design and debugging of basic communication function interface circuits. The overall design of the gateway
Line peripherals are driven by the FSMC controller integrated within the central processor. Its function is to convert the data, address and control signals of the AHB system bus, thus meeting the requirements of different peripheral timing sequences. FSMC converts the external memory into It is divided into four 256M byte memory blocks. The memory block with the address space Ox60000000-Ox6FFFFFFFF can be divided into four identical memory areas. Each memory area has a dedicated chip select signal. In order to make the The speed is consistent and avoids constant switching back and forth of FSMC configuration. External SRAM, SSMV62AD and SSMV62AD fast devices can be stored in the first three storage areas of the storage block.
As shown in Table 2, the external expansion uses a low-power memory chip, which can store user data and data systems. It also supports byte access and standby mode. Its access speed is very fast and does not require refresh operations. It It is still composed of 512K 16bit units, and uses 19 address buses and 16 data buses for addressing, reading and writing operations. The external expansion NORFLASH chip is mainly used in storage system applications, and also uses a 16-bit access mode. The basic communication circuit mainly includes the MVB board driver circuit, RS-485 interface circuit, CAN field bus circuit, wi-Fi board interface circuit and RS-232 communication circuit. Among them, the MVB board has 8 bit data access mode, and supports four modes: debugging, parallel, serial, and production. It also allows physical media communication forms of EMD and ESD. S–MOEDO is used to select the physical layer interface in MVB. S- MOED[2:1] is used to select the working mode of MVB. In order to avoid unexpected signal conflicts, connect the hardware of S-MOED[2:0] to the ground and use EMD for communication. In order to facilitate debugging and testing, the MVB multi-protocol gateway has also designed an external 5V DC power supply and the power supply mode of the PC, which are switched through the S901 switch. Among them, the system power supply is separated using two low-voltage voltage stabilizing chips. Design, this can effectively meet the different needs of different functional circuits for working current, and also reduce the mutual interference between them. MVB multi-protocol communication gateway can also be designed using PCB. Its process can be summarized as: schematic design, device layout, electrical wiring, teardrop addition, copper layer placement and rule detection. In order to effectively save the processing cost of the circuit board, the PCB uses double-sided panels. Its design process uses the bottom-up design method of the schematic diagram, dividing each device and module into 9 groups according to their functions, and then The design of the circuit diagram, based on the top-level principles, electrically connects these nine discrete drawings. Due to the proximity principle of devices, plug-ins and interference sources, the ergonomics and height space of the PCB board can be used for layout. The power supply wiring needs to use a tree shape. The topology structure is used to keep the current in the same direction to avoid the occurrence of backflow. The ground wire of the component needs to be directly connected to the copper layer and the power supply, while the high current device requires a separate connection. In order to reduce signal noise and ensure good contact between the top and bottom ground wires, copper can be laid into a double-layer ground plane. However, in this process, in order to reduce the dead copper area, a large number of digital ground vias can be placed for correction and improvement. The functional circuit uses a shielded wiring surround method.