To supervise a specific process with a SCADA system, usually an application containing the definition of variables involved is built up, with names and paths, Screens, definitions of alarms and others, which is called an Application Database.
When this process requires using two or more computers, users must make sure that each application on each computer exchange data with the others. Most traditional SCADA systems were based on a common structure to accomplish this task:
•Each SCADA server must have a copy (partial or not) of the application configured on the local database
•Each SCADA server has and executes only one database at a time
This lead to some management problems, such as applying changes to all servers, controlling application versions, or else working with different software and hardware manufacturers.
E3 solves this problem by using the concept of Domain, which includes in one single environment the definition of the computers executing real-time tasks (servers) and the project databases that must be executed in these servers, with the possibility of executing many projects in each server. Users can also add, delete, or modify projects at run time, without affecting other parts of the executing Domain.
Each project can contain any type of object, such as Screens, I/O Drivers, Alarms, Historics, Reports, Formulas, Databases, among others. When two or more projects are within the same Domain, they can access properties and objects among themselves as they reside on a single database. This is possible by using Links (connections) that an object can perform with any other. If both objects exist and are executing, the connection is active and any value change is asynchronously sent among the objects (depending on the connection type). If one of the objects is destroyed or stopped, the connection is broken, then the application is notified about it and can then indicate that status in a user-defined way.
The Domain structure is restricted to servers and similar, such as server machines, projects, users, and passwords. The client interface for operation and visualization, called E3 Viewer, can connect to any E3 Server directly (with proper E3 Viewer licenses). An E3 Viewer contains three special features:
•Application projects reside exclusively on the server
•Internet Explorer can be used as an operating interface, without any change
•The client interface can alternate from a switched off server or a failed one to the next available server, without interrupting the monitoring process
One alternative for using an E3 Viewer on the client machine is using the Terminal Service technology. This is a service that began with Windows NT 4.0, and it is intended to allow remote access among computers via a protocol called RDP (Remote Desktop Protocol). This protocol allows interaction between a client station and a server machine, which is accessed remotely. An E3 Viewer is executed in a new user session created on the server machine which, by its turn, transfers video data to the client machine, and receives back mouse and keyboard events.
E3 architecture
In all cases, a E3 Viewer works as the operation and visualization interface. The only difference resides in the place of execution, which could be either the client machine (E3 Viewer's regular operation) or the server computer, with the user operating it physically next to the computer or through Terminal Services. Since a E3 Viewer is executing directly on the client computer, there is a higher initial traffic for downloading Screens and objects. This traffic decreases substantially as data changes are reported from the server to the E3 Viewer, which brings advantages in terms of performance and graphical presentation quality.
Terminal Services, on the other hand, contains a broader range and demands less client computer's processing capacity when there are few graphical and visual effects, in addition to the possibility of operating E3 in other software platforms not supported by E3 Viewer, such as Windows CE and Linux or Unix. However, RDP's drawback is that it shows only 256 colors, which increases considerably the traffic when there are too many animations or blinking effects on a Screen. To check the best way to implement multiple accesses to an E3 Server, it is important to study each case individually, to define the best alternative for the application. To use RDP with multiple users, it is necessary to have a licensed Windows 2003 Server SP2, in addition to the same number of available E3 Viewers on the Server.