Design of Remote Monitoring System Based on CDMA 1X

1 Introduction

In the existing industrial monitoring system, the commonly used methods of information transmission are: digital microwave, digital radio, cable optical fiber, cable cable, etc. Digital microwave and transmission radio will be limited by transmission distance and frequency licensing. Cable optical fiber and cable have high network costs and are not suitable for use when monitoring points are very dispersed and transmission distance is long.

For 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 way of information transmission.

CDMA 1X is willing to refer to the stage of CDMA 2000 (higher than IS-95, lower than 2MB/s), which can support 308kb/s data transmission. The introduction of packet switching in the network part, which can support mobile IP services, is a new bearing service developed on existing CDMA IS-95 systems. The purpose is to provide data services in the form of grouping for CDMA users.

Because CDMA 1X uses new technologies such as reverse coherence demodulation, fast forward power control, transmit diversity, Turbo encoding and so on, its capacity is much higher than IS-95. From the theoretical analysis, if it is used to transmit voice service, the total capacity of CDMA 1X system is twice that of IS-95 system. If it is used to transmit data service, the total capacity of CDMA 1X system is 3.2 times that of IS-95 system, and the theoretical bandwidth of CDMA 1X can reach 300kb/s. The current practical application bandwidth is about 100 kb/s (two-way symmetric transmission). It provides TCP/IP connection 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 system is generally composed of data service center, communication network and data terminal. Considering factors such as business data volume, security, reliability, network condition and cost, it uses the following networking methods: the communication server connects to the Internet with a fixed IP or a fixed domain name in the public network, and when 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. Fixed domain names can be obtained by installing peanut shell software. This scheme has the features of low cost, stable communication quality, moderate security and reliable operation.

2.2 Principles of Wireless Data Transfer

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

(1) Lower computer transmits data to DTU through RS232 serial port;

(2) DTU sends data as TCP/IP packets to wireless networks;

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

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

The downstream transmission of data is the opposite of the above process, which is no longer described. The system communication architecture is shown in Figure 1.

3 System Design

3.1 PC Design

The upper computer consists of real-time monitoring system and historical database.

The real-time monitoring system uses Wonderware's InTouch system to achieve the following functions: system management function; Data processing functions: including data query function, data storage function, data timer reporting function; Alarm functions: including real-time response function, alarm linkage processing function, alarm setup function, alarm confirmation function, alarm storage function, operation control function. The history database uses Wonderware's Industrial SQL Server, which mainly implements the following functions: data statistics, report function, historical data query function, historical data curve diagram, printing function.

3.2 Lower Computer Design

The lower computer uses ABB AC31 series of PLCs, including CPU module and I/O module. The CPU receives one or more operation control commands from the designated Monitoring Center for the device, performs corresponding operation control actions on the device through I/O module, and returns the operation results to the monitoring center. The lower computer mainly performs the following functions:

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

3.3 Interruption of data transmission in CDMA

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

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

(2) General GPRS/CDMA Modem usually needs to be attached to the virtual dial-up network on the PC, make use of PC resources for data transmission and protocol conversion, H7710 DTU has built-in automatic network connection and protocol processing module, and does not need backend computer support.

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

(4) The H7710 DTU can automatically attach to the GPRS or CDMA network at 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 private network covering China at any time and anywhere through the Internet network, independent of the data interface equipment of the operator exchange center.

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

Design and implementation of 3.4 communication server

Communication server is the hub of the whole system, which mainly implements data transmission between different protocols. The design of communication server is based on the OPC standard. OPC can be understood as OLC for Process Control, which is a technology for application interface developed under Microsoft operating system. Based on Client/Server mode, three software needs to be installed on the communication server in order to achieve the communication of the whole system. OPClink from Wonderware, IOServer from IOServer, CDMA Server. The working process of the communication server and the management between the 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 working on Windows platform. Its main function is to transfer data between OPC server and InTouch Wu Zhijian through protocol conversion. OPCLink can connect with local or remote OPC server-side software (such as IOServer, RSLinx). It converts InTouch's commands into OPC protocols, sends them to the OPC server-side software, reads data from the OPC server-side software, and returns them to InTouch. Similarly, OPCLink can be connected to a local or remote InTouch.

3.4.2 IOServer

The role of IOServer is to achieve 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 consists of the following three parts:

(1) Board (Interface Configuration): IOServer supports multiple interfaces, the most common of which are TCP/IP and serial ports. When configuring an interface, you need to define the protocol of the connected device;

(2) OPC Explorer: Provides a platform for internal testing, as well as data observation and monitoring.

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

3.4.3 CDMA Server

In this system, the software of CDMA Server is written using VisualC++ 6.0. CDMA Server is the communication hub of the whole system, which includes two functions: communication with IOServer and communication with DTU.

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

Communication between CDMA Server and DTU using the dynamic library wcomm_in the development kit Dll.dll, which includes all API functions needed to communicate with DTU, including service startup, data sending, data receiving, service shutdown, etc.

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

Start_ Gprs_ Server: This function is used to start the underlying service. After the service is started, it communicates with the DTU. After the service is started, there is a function in the main window to respond to the message in order to communicate with the underlying service.

Stop_ Gprs_ Server: stop service;

Do_ Read_ Proc: Read the data. When the underlying service receives the data sent by the DTU, it sends a message to the window of the DSC specified in the startup service function, where the message response function 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 bring it offline;

Do_ Close_ All_ User: Close all online DTU terminals and generally execute the API before stopping the service.

Because Windows is message-driven, the underlying service sends a message to the window of the startup function when it receives data from a remote DTU. Therefore, we can define a message response function in the program to handle the data transfer between the center and the DTU. In the message response function, data is transferred by calling the data receive function and the data send function in the dynamic link database. When the data transfer is completed, Call the shutdown service function to end communication.

4 Experiments and Result Analysis

The experimental steps are as follows:

(1) Set up the main parameters of DTU:

DTU ID: 139**********

Online reporting interval: 40s

Local communication port: 5001

Transmission Packet Length: 1000B

Primary DSC IP address: 219.229.3.24

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

(3) Add ports and devices to 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 corresponds to the port provided by the CMA Server, that is, 5002.

Following these steps, communication between IOServer and DTU has been established, and then only specific Groups and Items need to be configured on IOServer and OPCLink, which is not discussed here.

This system has been applied to practical projects. Compared with the wired data collection system, this system has the following advantages:

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

(2) The flexibility of the system is improved, the monitoring range is not limited by region, and the control target of nodes can be expanded arbitrarily.

(3) The communication lines are relatively stable and communication failures are not easy to occur. Even if problems occur, they can be quickly discovered and eliminated.

5 Conclusion

It is believed that with the rapid development of wireless communication in CDMA and the issuance of 3G licenses, wireless communication will surely lead the trend, and remote monitoring system based on wireless communication in CDMA will be more widely used.

Reference:

[1].RS232 Datasheethttp://www.dzsc.com/datasheet/RS232_585128.html.
[2]. PLC Datasheethttp://www.dzsc.com/datasheet/PLC_1248813.html.
[3]. GPRSdatasheethttp://www.dzsc.com/datasheet/GPRS_1594650.html.