Design of remote monitoring system based on CDMA 1X

In existing industrial monitoring systems, commonly used information transmission methods include: digital microwave, digital radio, wired optical fiber, wired cable, etc. Digital microwave and digital radio will be limited by transmission distance and frequency license, while wired optical fiber and The network cost of wired cables is high and is not suitable for use when monitoring points are very scattered and the transmission distance is long.

In view of the characteristics of scattered monitoring objects, lack of organic connection between monitoring nodes, relatively small amount of monitoring information, and low real-time requirements, CDMA 1X provides a good information transmission method.

CDMA 1X refers to the first stage of CDMA 2000 (rate higher than IS-95, lower than 2MB/s), which can support data transmission of 308kb/s. The network part introduces packet switching and can support mobile IP services. It is now There is a new bearer service developed on the CDMA IS-95 system, which aims to provide CDMA users with data services in the form of packets.

Since CDMA 1X adopts new technologies such as reverse coherent demodulation, fast forward power control, transmit diversity, and Turbo coding, its capacity is greatly improved compared to IS-95. From the theoretical analysis results, if it is used to transmit voice services, The total capacity of the CDMA 1X system is twice that of the IS-95 system. If used to transmit data services, the total capacity of the CDMA 1X system is 3.2 times that of the IS-95 system. The theoretical bandwidth of CDMA 1X can reach 300kb/s. The current actual The application bandwidth is about 100kb/s (two-way symmetric transmission). TCP/IP connection is provided on this channel, which can be used for Internet connection, data transmission and other applications.

2 Wireless communication based on CDMA 1X

2.1 Networking scheme

Wireless data transmission systems generally consist of data service centers, communication networks and data terminals. Taking into account factors such as business data volume, security, reliability, network conditions and costs, the following networking method is adopted: the communication server uses a public network fixed IP Or a fixed domain name is connected to the Internet. After the CDMA data terminal unit (DTU) is powered on, it actively establishes a connection with the communication server. If the communication server does not have a fixed IP, you can obtain a fixed domain name by installing the peanut shell software. This solution has the advantages of low cost, It has the characteristics of stable communication quality, moderate security, and reliable operation.

2.2 Principle of wireless data transmission

According to the above networking scheme, the uplink transmission principle of data is as follows:

(1) The lower computer transmits data to the DTU through the RS232 serial port;

(2) DTU packages the data into TCP/IP packets and sends them to the wireless network;

(3) TCP/IP data packets are transmitted to the Internet through the system packet data service node and find a designated data service center (communication server) on the Internet;

(4) The communication server transmits the data to the host computer and stores it in the historical database.

The downlink transmission of data is opposite to the above process and will not be described in detail. The system communication structure diagram is shown in Figure 1.

3 System design

3.1 Host computer design

The host computer includes two parts: real-time monitoring system and historical database.

The real-time monitoring system uses Wonderware's InTouch system, which mainly implements the following functions: system management function; data processing function: including data query function, data storage function, and data timing reporting function; alarm function: including real-time response function, alarm linkage processing function, Alarm setting function, alarm confirmation function, alarm storage function, operation control function, the historical database uses Industrial SQL Server of Wonderware Company, which mainly implements the following functions: data statistics, report function, historical data query function, historical data curve chart, and printing function.

3.2 Lower computer design

The lower computer uses ABB AC31 series PLC, including CPU module and I/O module. The CPU receives the operation control commands of the equipment from one or more designated monitoring centers, performs corresponding operation control actions on the relevant equipment through the I/O module, and sends the The monitoring center returns the operation results, and the lower computer mainly implements the following functions:

Data collection function, data query function, scheduled reporting function, operation control function.

3.3 CDMA data transmission interruption

CDMA data transmission interruption uses the CDMA H7710 DTU of Shenzhen Hongdian Technology Development Co., Ltd. This terminal has the following characteristics:

(1) H7710 DTU directly provides RS232/422/485 interface, providing a transparent, full-duplex, peer-to-peer data transmission channel for users' data equipment.

(2) Ordinary GPRS/CDMA Modem usually needs to be attached to a PC for virtual dial-up Internet access, and uses the resources of the PC for data sending and receiving and protocol conversion. H7710 DTU has built-in automatic network connection and protocol processing modules, without the need for background computer support.

(3) H7710 DTU can realize point-point, point-multipoint, and center-multipoint peer-to-peer data transmission, and the transmission delay is generally less than 1s.

(4) H7710 DTU can automatically attach to the GPRS or CDMA network from the beginning, and establish a communication link with your data center to send and receive data from user data devices at any time.

(5) H7000 mobile data communication can build a virtual mobile data communication dedicated network covering the whole of China anytime and anywhere through the Internet network without relying on the data interface equipment of the operator's switching center.

CDMA H7710 DTU must set event parameters before use: CDMA 1X network login user name and password, communication interface parameters, fixed IP address and port of the communication server or domain name of the communication server, online working mode of the DTU, etc.

3.4 Design and implementation of communication server

The communication server is the hub of the entire system, mainly realizing data transmission between different protocols. The design of the communication server is based on the OPC standard. OPC can be understood as object connection and embedding for process control (OLC for Process Control), which is developed under the Microsoft operating system A technology used for application program interfaces, based on Client/Server mode. In order to realize communication of the entire system, three pieces of software need to be installed on the communication server: OPClink of Wonderware Company, IOServer of IOServer Company, and CDMA Server. The working process of the communication server and the management between various software are shown in Figure 2.

The following describes the functions and applications of the three communication software on the communication server.

3.4.1 OPClink

OPClink is an application software that works under the Windows platform. Its main function is to realize data transmission between the OPC server and InTouch Wu Zhijian through protocol conversion. OPCLink can connect to local or remote OPC server software (such as IOServer, RSLinx). It converts InTouch commands into OPC protocols and then sends them to the OPC server software, and reads data from the OPC server software and returns it to InTouch. Likewise, OPCLink can connect to local or remote InTouch.

3.4.2 IOServer

The role of IOServer is to realize data transmission between different protocols. IOServer supports the following 14 protocols: Modbus protocol, AB protocol, ASCII protocol, Courier protocol, DNP protocol, GE protocol, HR6000 protocol, IEC103 protocol, KingFisher protocol, LoopBack protocol, Melsec protocol, Omron protocol, TI505 protocol, UCA2 protocol. In this system, ABB PLC uses Modbus protocol. Therefore, IOServer reads ABB PLC data from CDMA Server, converts it into TCP/IP protocol, and transmits it to OPCLink.

The configuration of IOServer mainly includes the following three parts:

(1) Board (interface configuration): IOServer supports a variety of interfaces, the most commonly used of which are TCP/IP and serial ports. When configuring the interface, you need to define the protocol of the connected device;

(2) OPC Explorer: Provides a platform for internal testing and can also perform data observation and monitoring.

(3) OPC Gateway: allows data transmission between OPC servers based on different protocols, as well as data transmission between IOServer and a certain control unit.

3.4.3 CDMA Server

In this system, Visual C++6.0 is used to write CDMA Server software. CDMA Server serves as the communication hub of the entire system and includes two functions: communicating with IOServer and communicating with DTU.

The communication between CDMA Server and IOServer is based on SOCKET programming and uses TCP/IP protocol for transmission. The port and IP address need to be set. IOServer establishes communication with CDMA Server through this port and IP address.

The communication between CDMA Server and DTU uses the dynamic library wcomm_dll.dll in the development package. This file includes all API functions needed to communicate with DTU, including service startup, data sending, data receiving, closing services, etc.

The communication design between OPC Server and DTU is based on the user program interface of the development tool kit, namely the dynamic link library cdmagprs.dll. This library includes all API functions needed to communicate with DTU, such as:

start_gprs_server: This function is used to start the underlying service. After the service is started, it communicates with DTU. After starting the service, there must be a function that responds to the message in the main window to communicate with the underlying service.

stop_gprs_server: stop service;

do_read_proc: Read data. After the underlying service receives the data sent by the DTU, it will send a message to the window of the DSC specified in the startup service function. The message response function in the window should immediately call the read data function to read the data sent by the DBU;

do_send_user_data: Send data to DTU;

da_close_one_user: Close a DTU terminal and take it offline;

do_close_all_user: Close all online DTU terminals. This API is generally executed before stopping the service.

Since Windows is message-driven, after the underlying service receives the data from the remote DTU, it will send a message to the window that starts the function. Therefore, we can define a message response function in the program to handle the data transmission between the center and the DTU. The message In the response function, data is transmitted by calling the data receiving function and data sending function in the dynamic link library. When the data transmission is completed, the closing service function is called to end the communication.

4 Experiments and result analysis

The experimental steps are as follows:

(1) Set the main parameters of DTU:

DTU identification code: 139********

Online reporting interval: 40s

Local communication port: 5001

Maximum transmission packet length: 1000B

Primary DSC IP address: 219.229.3.24

(2) Add users on the CDMA Server, as shown in Figure 3, where the terminal port is consistent with the setting on the DTU, which is 5001. The gateway exit port, that is, the CDMA Server and IOServer connection port is set to 5002.

(3) Add ports and devices on IOServer, as shown in Figure 4. In this system, since IOServer and CDMA Server are installed on the same machine, the IP address is set to 127.0.0.1, and the port number is the same as that provided by CDMA Server. The port corresponds to 5002.

After the above steps, the communication between IOServer and DTU has been established. Next, you only need to configure the specific Group and Item on IOServer and OPCLink. I will not go into details here.

This system has been applied to actual projects. Compared with wired data acquisition systems, this system has the following advantages:

(1) Reduces the construction and maintenance costs of communication lines;

(2) The flexibility of the system is improved, the monitoring scope is not restricted by geography, and the node control targets can be expanded arbitrarily.

(3) The communication line is relatively stable and is not prone to communication failures. Even if a problem occurs, it can be quickly discovered and eliminated.

5 Conclusion

It is believed that with the rapid development of CDMA wireless communication and the issuance of 3G licenses, it will surely lead the trend of wireless communication, and remote monitoring systems based on CDMA wireless communication will have more extensive applications.