SCADA Web Pages

Scada installations traditionally display on the computer screens on which they run, or in the case of central server based systems, on their client screens. With the advent of the Internet and the World Wide Web, a different means of display becomes possible. By having the Scada / HMI system render web pages, it is possible to view displays from any computer appropriately configured.

How this functionality is implemented and what it can offer, depends on the individual Scada / HMI system. A number of factors need to be considered such as:

How easy is it to configure?
What security needs to be implemented?
What can, or cannot, be done from a web display?
Can the displays be integrated into other websites?
Is a specific browser required or not?
Do the client browsers require any special software?

Scada / HMI page displayed in browser window

Saturn Page displayed in FireFox Browser

Hexatec 'SCAN1000' and 'Saturn' Web Page Functionality

SCAN1000 and Hexatec Saturn software includes fully integrated facilities to enable display pages to be viewed on standard web browsers.

No special additional software needs to be added, the facility is fully available 'out of the box'.

Designed to work without the need to install any special client software, web pages can be viewed with any standard browser.

Hexatec SCADA software includes a built-in web server
No additional software is required to generate web pages
Pages can also be generated in file space and transferred directly to other internet locations using the ftp file transfer facilities, included in the software
Firewall support facilities are included

An example of this can be found at our Centre Desk Case Study.

We now describe the various options available in Scada / HMI web page creation, along with details on how Saturn implements these options.

Methods of Page Generation

Web pages produced by Scada / HMI systems can be created in a number of different ways. Typically, a representative image needs to be produced which is then displayed on the client screen:

Client display object is notified of display details from server. It creates an image based on this information using some local software such as an ActiveX object.
Client is sent an image which it then displays through a local display object.
Image is created on the server and the client browser refreshes its display routinely, causing the display to update.
Client requests server for information to fill in part created page which it then generates.

Saturn uses a combination of the third and fourth options. When Saturn first starts, it creates a standard html page which can be viewed by any standard browser.

This page includes a reference to an image file which will be the actual display on the client screen. The html page also includes 'Refresh' information in order that the display is routinely refreshed. The image file, which can be in a number of formats, is then regenerated routinely by Saturn in order that client refresh requests will view a new image.

The regeneration process only occurs when a request is made by a client, so no unnecessary overhead is imposed when the page is not being viewed by client browsers. Multiple clients accessing the same page, on the other hand, are supported by a single image - it is not necessary to repeat the process for each client in turn.

This process is designed to be as efficient as possible in terms of both the server (Saturn) and the network. Large numbers of clients can be served with up-to-date pages without a heavy penalty being imposed on the server. No special software is required to be installed or licensed on the client computers - the pages displayed are standard, simple html which include a single image.

Saturn includes a web server which removes the need to install and maintain any separate software. As the server is only generating specific, previously designed pages, it is not possible for this process to be externally 'hacked' or modified.

User Interaction

Web pages normally require some ability to react to user requests, typically to allow navigation around related pages. The traditional mechanism for this process is the hyperlink which will cause the browser to change to a different location.

In order that Saturn web displays mimic the HMI display pages as much as possible, hyperlinks are created at page design time for any Saturn objects or display areas. This is done in such a way that the action taken on a browser display can be different to that taken on the actual HMI display. This includes the ability to navigate away from the HMI pages to other sites as necessary.

Careful thought was given to what actions might be taken on browser displays and it was decided to limit user interaction to display changes only. No control actions are taken on browser screens - these need to be done on the actual HMI display directly. This is to avoid the complex issues associated with Security, reliability of remote access and control / design of multiple user based systems.

Other Options

In most cases, the internal web server of Saturn offers an easy to use solution to remote browser access. However, a number of other options are included to cover installations where this approach is either not possible, or possibly unsuitable. These are:

Image format. The image produced can be one of three common formats: jpeg, png and bmp with jpeg being the default.
Dynamic Capability. Png format files can include a transparency option if desired. Saturn can make use of this option to allow separation of foreground (dynamic) and background (static) content. This reduces the routine page rendering process even further, although, at the moment, Windows Internet Explorer (the most common browser) does not support this option!
Simple file access. Instead of having pages served to a browser by the integrated web server, it is possible to simply generate files and directly access them from browsers. Although this process is very simple, it does have a number of disadvantages. Security of access has to be considered. Saturn does not know when files are accessed by browsers, so it will have to routinely recreate the files even though no client accesses are active.
A modified version of file access above is also supported. This uses ftp (file transfer protocol) to transfer pages routinely to different locations, such as a folder in existing web space on a different server.
Firewall negotiation. To support remote ftp transfer, Saturn includes the necessary mechanisms to allow transfer through a range of commonly used firewalls.
Dial-Up support. If the HMI is at a remote, non-network connected location, it is possible for it to use a modem connection to remote access file space elsewhere and transfer pages as required.