“Wireless communication has been widely used in mobile communication, vehicle dispatch command and air traffic control system. Most of the VHF/UHF radio stations widely used in our country are analog voice radio stations, and mainly use short-wave and ultra-short-wave voice communication. Therefore, the ability of direct data transmission is very low, and it cannot meet the digital needs of current data transmission; although digital radio can be designed To complete the data transmission, but it cannot make full use of the existing VHF/UHF analog radio, resulting in waste, and specially redesigning the digital radio has the disadvantages of long design cycle and high cost.
Author: Ding Shengxi; Zhang Qishan
Wireless communication has been widely used in mobile communication, vehicle dispatch command and air traffic control system. Most of the VHF/UHF radio stations widely used in our country are analog voice radio stations, and mainly use short-wave and ultra-short-wave voice communication. Therefore, the ability of direct data transmission is very low, and it cannot meet the digital needs of current data transmission; although digital radio can be designed To complete the data transmission, but it cannot make full use of the existing VHF/UHF analog radio, resulting in waste, and specially redesigning the digital radio has the disadvantages of long design cycle and high cost. This paper proposes a scheme to design high-speed wireless MODEM using MX919 chip. The chip is connected to the voice radio through the audio interface, and connected to the CPU through the parallel port, which can realize the wireless transmission of data signals with the traditional voice radio, effectively using the existing equipment, and has been used in oilfield geographic information systems, bank cash transport vehicles , The GPS vehicle monitoring and dispatching system of the bus has been well tested.
1 MX919 Features
MX919 is a wireless modulation and demodulation chip of MX.COM Company, which can complete 4-level FSK modulation and demodulation, and has programmable working registers inside. It can easily communicate with the host and radio, and has its own data packaging protocol. In the data transmission mode, the digital data from the host is received and packaged; in order to reduce the error code of wireless communication, the original baseband signal is encoded, and has the functions of CRC error detection and FEC error correction, in order to resist burst Error, also adopted interleaving coding; finally, 4-level FSK modulation is carried out on the coded data, which is sent by the radio station FM modulation. In the data receiving mode, the analog signal from the radio is received, after demodulation and decoding, the original baseband signal is restored, and it communicates with the host through the parallel port.
The MX919 modem chip has the following features:
(1) 4-level FSK modulation
(2) Half-duplex working mode
(3) Baud rate up to 19.2kbps
(4) Low voltage (3.3/5V) work
(5) Data Packing Format
(6) Enhanced performance in noisy environments
(7) FEC, CRC and interleaving coding
2 Hardware Design
The block diagram of the system is shown in Figure 1, which is mainly composed of three parts: communication controller, MX919 and voice analog radio. The communication controller is a microcontroller subsystem responsible for the main control of the system, including the RS-232 communication interface with the data source (GPS receiver, computer, etc.), the programming and control of the MX919, and the PTT, SPLINT, etc. necessary for generating the radio. control signal, as shown in Figure 2.
The interface between MX919 and communication controller and radio is shown in Figure 3.
The interface with the communication controller includes the data bus D7~D0, the address lines A0, A1, the read and write signal lines, the interrupt request signal line, and the chip select signal; the interface with the radio station includes the receiving analog modulation signal line RXIN and the sending modulated signal line. Signal line TXOUT.
The radio feeds the FSK signal modulated by the MX919.
High frequency FM modulates and transmits, or receives and demodulates FM modulated signals. Its main interface is the sending signal line TX and the receiving signal line RX.
3 Software Design
Before software design, you must understand the function of each register of MX919, especially the command register, control register and status register. Table 1 shows the contents of these registers.
Table 1 MX919 register content
The whole system software mainly includes some modules as shown in Figure 4.
The main control module is responsible for the control of the entire system, including controlling the timing of the system, detecting the status of data transmission or reception, and so on. When data needs to be sent or received, the data sending and receiving module programs various registers of the MX919. When the IRQ signal of MX919 is valid, the external interrupt module starts to execute, including sending interrupt subroutine and receiving interrupt subroutine. The initialization module completes the initialization of timers, serial ports, interrupt registers, etc.
In the software design, the most important part is the data transmission and data reception module, the data transmission flow chart is shown in Figure 5, and the data reception flow chart is shown in Figure 6.
The wireless MODEM designed with MX919 has the advantages of simple, convenient, practical and flexible, and short development cycle. It has extensive application value and promotion value in real-time monitoring system and remote data acquisition. At present, we have successfully applied it to the GPS vehicle monitoring and dispatching system of oilfield geographic information system, bank cash transport vehicle and bus, and the effect is good.