Design of Remote Data Acquisition System for Web Network Sensors

introduction

With the development of network technology and embedded technology, combining embedded systems with the Internet to monitor and manage system data and images has become one of the important research directions. Embedded Web network sensor technology is a cross-integration of related technologies such as sensors, embedded, distributed information processing and the Internet. It uses the TCP/IP protocol as an embedded application based on intelligent sensors and embeds it into the ROM of existing intelligent servers. Using the built-in Web server in the sensor, users can collect information about remote monitoring objects through a browser.

1 Working Principle of Web Network Sensor Data Acquisition System

Users access data on the Web server through a browser, and display the remotely collected data on an Internet web page in real time. Users need to see data changes in real time, but HTML web pages can usually only be static data, so it is necessary to apply dynamic web page technology on the Web server. The implementation technologies of dynamic web pages include CGI and HTML script embedding technology (such as ASr, PHP, JSP, etc.). Since the network sensor embedded system is implemented by a single-chip microcomputer, its resources are limited, so CGI technology is used to implement dynamic web pages.

1.1 CGI Principles

CGI (Common Gateway Interface) provides a channel for external programs to the Web server, enabling communication between the processor and the browser. CGI is an interface specification that can process data input by the client (usually a browser) and complete interactive operations between the browser and the server. CGI is a program running on the Web server, triggered by the browser's input, and serves as a bridge between the server and other programs in the system. The CGI program is an external program and an executable file running on the server.

The schematic diagram of the working principle of the CGI program is shown in Figure 1.

After the client's web browser browses to a homepage, it submits data in a certain way and sends a request to the web server through the HTTP protocol. The HTTP Daemon on the server side passes the homepage information described to the CGI program specified by the homepage through the standard input (STDIN) and environment variables, and starts this application for processing (including processing of the database). The processing results are returned to the HTTP Daemon through the standard output (STDOUT), and then returned to the client's browser through the HTTP protocol. The browser is responsible for interpreting and executing, and displays the final results to the user.

1.2 Data collection process based on CGI

The data collection process of CGI is shown in Figure 2. When using CGI to collect temperature data from remote sensors, the CGI program can obtain data by directly accessing the hardware or calling the driver. After the data collection is completed, the CGI program organizes the data into an HTTP stream and sends it to the Web server, which is responsible for sending it to the client.

2 Real-time data collection technology

2.1 Implementation method of continuous data acquisition

Since HTTP is based on a request/response model, an interaction between a client and a server begins with a request from the client and ends with a response from the server. However, only one set of data can be obtained from one interaction. So how can we achieve continuous data collection? Generally, there are two methods: client traction and establishing a TCP connection.

Client-side traction is to insert a timed refresh function into the web page. Its order of magnitude is s, which allows users to see the changes in data, but it cannot meet the requirements of strong real-time performance, and some intermediate data will be missed.

Establishing a TCP connection is to enhance the client's functions by embedding ActiveX controls or Java Applets in the web page. Among them, ActiveX controls can only run in the Windows environment, while Java Applets are highly versatile and can achieve cross-platform operations. Therefore, this system uses Java Applet to collect remote data in real time.

2．2 Java Applet

Applet is a small program written in Java, containing visual content, and embedded in Web pages to produce special page effects. It is often used to increase the multimedia effects of Web pages and make them interactive.

When a user visits a web page with a Java Applet, the Applet is downloaded to the user's computer and executed, but the prerequisite is that the user is using a web browser that supports Java. Since the Applet is executed on the user's computer, its execution speed is not limited by the network bandwidth or MODEM access speed, and the user can better view the multimedia effects generated by the Applet on the web page.

Applets must be embedded into a web page using a pair of HTML tags and or to run in a browser. In IE, the