introduction
With the comprehensive advancement of China's informatization process, various industries have put forward higher requirements for informatization. The understanding of transmission informatization has also increased from pure digitalization to the integration of digitalization with networking and wireless. In response to current information requirements, although the original wired transmission system can be digitized and networked, the complex wiring and high maintenance costs have greatly restricted the distribution of network nodes, which is largely It hinders the in-depth and popularization of data transmission informatization. The mobile monitoring system can solve the above problems very well. The mobile monitoring system is generally composed of data collection equipment, terminal management computer, and monitoring center. It can install the data collection equipment on a removable carrier, so that the data collected on site can be managed by the terminal After processing by the computer, it is transmitted to the monitoring center through the wireless data transmission channel, so that the monitoring center can understand the status of the scene at any time, so as to realize remote wireless mobile monitoring. Considering the needs of practical application, this paper designs a network data transmission system based on CDMA or GPRS. The system can replace corresponding modules according to different transmission requirements, so as to complete more functions.
1 Wireless data transmission method
At present, the methods of mobile wireless data transmission mainly include GSM short message, CDMA, GPRS and other methods (such as Bluetooth, ZigBee, etc.), each of which has different characteristics.
The GSM short message transmission uses a signaling channel, which uses a store-and-forward approach. The transmission delay is not fixed, depending on the current use of the short message service. The disadvantage of this method is that the data transmission rate is low, it is delayed and the time is uncertain.
CDMA (Code Division MulTIple Access), also known as code division multiple access, is a wireless communication technology. The system can assign each user a specific address code. The address codes have mutual quasi-orthogonality, so that they can overlap in time, space and frequency. The theoretical transmission rate of CDMA 1X can reach 300 kbps, and the current actual transmission rate is about 100 kbps. It can support mobile IP services and can be used for Internet connection and data transmission.
GPRS (General Packet Radio Service) is the abbreviation of General Packet Radio Service, it is often described as "2.5G", that is, this technology is located in the second generation (2G) and third generation (3G) mobile communication technology between. GPRS uses packet switching technology. It can efficiently transmit high-speed or low-speed data and signaling. The theoretical transmission rate of GPRS can reach 171.2 kbps, and the actual transmission rate is about 40 kbps.
Bluetooth technology is actually a short-range wireless communication technology and an open global specification for wireless data and voice communication. The Bluetooth system is generally composed of four functional units: a wireless unit, a link control (firmware) unit, a link management (software) unit, and a Bluetooth software (protocol stack) unit. The transmission distance of Bluetooth is 10 cm ~ 10 m. If you increase the power or add some peripherals, you can achieve a transmission distance of 100 m. It uses the 2.4 GHz ISM frequency band and FM and frequency hopping technologies, and uses weighted error correction coding, ARQ, IDD and baseband protocols.
ZigBee is a set of technical standards related to networking, security, and application software developed based on the 802.15.4 wireless standard approved by IEEE. Its effective coverage is 10 ~ 75 m. When the network rate is reduced to 28 kbps, the transmission range can even be expanded to 134 m, thus having higher reliability.
This article uses a high-performance microprocessor LPC2210, the device can be widely used in industrial and civilian wireless data transmission field, especially suitable for some areas difficult to wire and remote areas. The CDMA wireless data transmission module used in the design uses Huawei EM200 and is connected to the processor through a standard RS232 interface. This article describes how to transfer user data information. In fact, user data information is only an application of the entire information transmission management system, and different information transmission can be realized by replacing different data source modules, thus forming different information transmission management systems, such as remote meter reading system, hydrological data collection system Wait.
2 Networking solution
The M2M (machine-to-machine) data transmission method is currently widely used in many industries. It can collect data distributed in various places through data collection terminals, and link the collected data to CDMA wireless transmission through a common interface. The module then transparently transmits the data through it and sends the corresponding data to the corresponding data center. Figure 1 shows the structure principle diagram of CDMA-based mobile wireless data transmission system. The data connection between the wireless data transmission system and the user's data center can generally be established via TCP / UDP. The user's data center serves as the communication server, has a public fixed IP, and specifies the listening port. The data transmission module in the wireless data transmission system can be set to automatically dial upon power-on. If the wireless data transmission system and the user's data center use the TCP protocol to transmit data. Then automatically establish a TCP connection with the server.
3 Hardware composition of wireless data transmission terminal
The hardware part of the wireless data transmission terminal mainly includes three parts: ARM CPU control module, CDMA Modem module and power supply. Its hardware connection relationship and structure block diagram are shown in Figure 2.
LPC2210 is a 16 / 32-bit ARM7TDMI-S CPU microcontroller from Philips. It is a RISC microprocessor based on ARM general architecture, which has the advantages of low cost, low power consumption, high performance and so on. Usually after receiving user data through RS232 / 485 port and putting the data into IP packets, you can use the EM200 module to access the CDMA network, and then send the data to an external network or data processing center through various gateways and routes. The embedded CPU chip is the core of the entire data collection terminal. It must not only support the embedded operating system, but also provide a Modem interface for controlling CDMA. The EM200 module mainly completes the wireless Internet access function. Modem interface to connect to complete the control of the module (such as dialing and switching modes, etc.). Once connected to the Internet through the module, the collected data can be sent to any host with a public network IP address using TCP / IP transmission to achieve wireless transmission of data.
3.1 UIM card interface design
The EM200 CDMA wireless data transmission module integrates a UIM card interface that conforms to the IS07816-3 standard, and can be connected to the module B2B connector through the PCB trace. Thus, the UIM card interface signal is provided for the external UIM card socket. The EM200 type CDMA wireless data transmission module can support and can automatically detect the 3.0V and 1.8V UIM cards. This design uses 3.0V UIM cards. Table 1 lists the interface definition of the UIM card pin on the system connector.
Taking into account the requirements of EMC (ElectromagneTIc CompaTIbility), the UIM card holder can be designed as close as possible to the interface of the EMC200 wireless data transmission module in the design, so as to avoid waveform distortion caused by too long traces. Reduce the impact on signal communication and data transmission. In actual design, a 0.1μF capacitor can be connected in parallel between UIM-VCC and UIM-GND, and a 33p capacitor can be connected in parallel between UIM-CLK, UIM-IO, UIM-RST and UIM-GND. Filter out the interference of radio frequency signals. At the same time, a 33Ω resistor should be connected in series on the UIMCL-K, UIM-IO, and UIM-RST traces. The connection diagram of the interface circuit between UIM and EM200 module is shown in Figure 3.
3.2 System audio part design
The EM200 module provides a complete audio interface, and only a small number of peripheral auxiliary components need to be added during design. The audio is divided into main channel and auxiliary channel. In addition, the audio design should also be as far away as possible from the radio frequency part of the module to reduce radio frequency interference to the audio. Because the first audio input and output channels (MICI-P, MICI-N; EARl-P, EARl-N) are all differential signals, so in the specific PCB wiring, must meet the requirements of differential signal routing, and routing It should be as short as possible to avoid interference of radio frequency signals on audio. If only one audio channel is used, it is best to use the first main audio channel. The design circuit of its main audio input and output channel is shown in Figure 4.
3.3 System status indicator design
In order to understand the working status of the wireless data transmission system more intuitively. This system has designed an LED light controlled by the LPG pin of the EM200 CDMA wireless data transmission module as a sign to indicate the network status. And through the different blinking patterns of the turntable indicator to indicate different network status. In actual design, the LPG pin cannot directly drive the LED, but needs to be used with the transistor. The specific status indicator circuit is shown in Figure 5.
4 Mobile terminal software design
The software of this system is composed of upper computer and lower computer software, server application interface and database. The lower computer software is responsible for serial communication between ARM microcontroller and CDMA module. The flow chart is shown in Figure 6.
To enable the wireless data transmission system to transmit data correctly, it must access the network smoothly. The wireless data transmission system needs to log into the CDMA network and negotiate with the PDSN gateway through LCP, PA-P, and IPCP protocols. The frame structures of LCP, PAP, and IPCP protocols are similar, and the most commonly used are REQ, ACK, and NAK. The LPC2210 microcontroller and the PDSN each negotiate with one party. Either party can send a REQ frame to request a certain configuration. The other party thinks that the configuration cannot be accepted and will respond with a NAK frame; if it can be accepted, respond with an ACK frame.
5 Conclusion
According to the development trend of monitoring systems and CDMA data communication technologies in recent years, this paper introduces a method for wireless data transmission between mobile terminals and computers. On this basis, if the data receiving software and database are established on the monitoring end to receive data from the CDMA network and store it for querying, data transmission and historical querying can be easily achieved. If you use the ARM controller to directly read the data of the monitor or sensor to complete the data collection, you can achieve data collection and wireless transmission control. At present, the system has been widely used in rental, weather, public security, transportation, water conservancy, etc.
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