Data Acquisition & Telemetry


Embedded web servers- a new technology trend

May 2000 Data Acquisition & Telemetry

The internet has impacted ont the modern world to such an extent that the abilityto browse websites and download information has become the norm. This user expectation to be able to access virtually any device via the web browser on his PC, has lead to many manufacturers adding a web capability to their products.

In the past, getting data from an embedded device or application has often proved to be a difficult exercise as traditionally data has been transferred serially, using a protocol derived from the RS232 and RS485 formats. To try and up the data transfer rate, raw data would be transferred across the connection. A dumb terminal, connected to the embedded device, could then collect the data for the user to interpret and use.

In the case of multiple embedded systems, each one would require its own serial link with obvious strain on the host system resources. Furthermore, should graphics be involved, this would require the writing of a specific graphics interface, further loading the system resources.

The widespread use of TCP/IP and the expansion of Ethernet-based local and wide area networks has brought a solution as many of these problems can be solved by including a web server as part of the embedded device. Now, by using HTML as development language, the embedded device can display the same data with any commercial web browser.

The user can now comfortably sit in his office, and using a TCP/IP connection over the Internet, monitor and modify the configuration of his embedded device anywhere in the world.

Communication

For an embedded device to be web enabled it has to have a web server running that may be accessed by an external web browser connected to it. The protocol used by a browser connected to a web server is HTTP. (Hypertext Transfer Protocol). As HTTP runs on top of TCP/IP, the engine of a web server is the TCP/IP stack, which is responsible for the transfer of data from the application, across a physical Ethernet (or serial) link.

User accessible web pages are stored within the web server. The web server thus handles requests from the browser to access these pages and passes data back to the browser. The browser is the interface between the web server and the user and presents text, graphics, video and audio in a format acceptable to the user.

When a web browser retrieves a file from a web server, the server provides the MIME (multipurpose Internet mail extension) type of the file. The browser uses the MIME type to establish whether the file format can be read or, if not, whether a suitable application is available to read the file. As web browsers such as Internet Explorer and Netscape can resolve many MIME types, the web server approach offers virtually limitless possibilities for data access.

Typical web enabled embedded devices will consist of the web server as well as an application running on the device. The web server thus provides an interface between the web browser, or the user, and the application running on the embedded device. The application still controls and interfaces with any external hardware while providing an interface to other devices.

A cost-effective solution

Unfortunately there exist many embedded devices without an embedded web capability. The cost of upgrading all these devices is obviously prohibitive and not practical. The limitations found in embedded systems impose specific and significant challenges for the use of web server technology in these devices. Traditional web technologies were developed for general applications and have been proven to be costly and ineffective when implemented in embedded networks. Realtime embedded systems, using web-based software for device access and control, have specific cost and system resource limitations, prohibiting the use of traditional methods. However, the desire is still there to bring these devices up to speed by web enablement - bringing with it all of the advantages offered by this interface.

SAN People has brought an innovative solution to this problem by its development of the EtherPAD range of thin servers. EtherPAD enables the connection of any RS232 serial device with an Ethernet network. The EtherPAD is programmed with an IP address, Subnet Mask and Gateway Address compatible with the Ethernet Network it is planned to use the device on. When the EtherPAD is running, it becomes 'transparent' passing data to and from the serial device to the application.

The EtherPAD also contains a web server. The EtherPAD can thus be remotely fully configured using any commercial web browser. Parameters such as serial port configuration, Ethernet interface and DNS configuration can be easily changed from anywhere in the world.

The EtherPAD also has the ability to manipulate the serial device it is providing connectivity for. A web page can be browsed allowing custom commands to be sent to the serial device or data to be read from it. EtherPAD also comes with serial port redirector software. This software enables an application, not able to communicate with a TCP socket, running on a PC, to transparently redirect data addressed to a serial port, to the IP address of an EtherPAD on the network.

The World Wide Web has brought a powerful, vendor independent user interface to the world of embedded applications. SAN People have brought this interface to all embedded serial devices.




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