10.2. The Cascade DataHub
The Cascade DataHub provides information to any node on a network in real time. It allows for isolation of data in separate domains, and can serve multiple clients.
Cogent provides a detailed Application Programming Interface (API) for the Cascade DataHub. This API and supporting documentation is available for download from the Cogent web site (http://www.cogent.ca) or can be sent to you upon request. Also see the Cascade DataHub manual for more information.
10.2.1. Data points
Both the Cascade DataHub and Cascade Connect work without the need for a mapping file to define a relationship between Windows data and server data. We assume that there is a one to one relationship between variables, or data points, on the data server and in Windows.
This relationship is established by simply giving the point the same name in both the data server environment, and in Windows. This simplifies things and means that we avoid the errors that can occur when you are forced to define a mapping file.
When a program wishes to read the value for a point in the datahub, it registers for exceptions for that point. From then on, whenever the value for that point changes in the datahub, the event will be immediately transmitted to all registered clients.
Whenever a datahub client tries to register for exceptions for points that do not exist in the datahub, the point is created in the datahub and any further value changes are reported.
The number of points in the datahub is only limited by data server system resources. It has been tested with over 65,000 points and simply grows in size to meet the need of the application.
10.2.2. Automatic reduction of network traffic
There will often be many more points in the Cascade DataHub than you want to use in your Windows application. So, an option (DB Bandwidth Reduce in Program Launch Options in the Cascade Connect Setup window) allows you to automatically trim down the number of data points sent to Windows. When a point change is sent to a Windows client, the client may refuse the message if the point is irrelevant. If the DB Bandwidth Reduce option is set, Cascade Connect will deregister that point with the Cascade DataHub so that no further changes to it are transmitted.
This mechanism cuts down on network traffic, significantly improving the speed of the Cascade Connect link when using a large number of active data points in the Cascade DataHub (in the order of 15,000 points) and a small number of these points in your Windows client application.
You will probably want to disable this feature (by unsetting DB Bandwidth Reduce) until you have finished developing your application, so you have access to all data points during development.
 | Cascade Connect will not de-register a point that has a DDE ADVISE loop set up for it between the Windows client and the Windows Cascade Connect program. |
10.2.3. Cascade DataHub domain independence
In networked systems, there may be a requirement to keep data from different sources separate. Cascade Connect allows the developer to define separate working domains in which to store data from different sources. Each domain is a separate datahub and multiple domains can exist on a network or single machine.
Because Cascade Connect is licensed on a use/machine basis it is possible for the developer to create multiple domains on a single node. A more effective setup is to place separate domains on different computers connected to the network.
Consider the following:
An application has ten distinct process areas. Each area is to be serviced by an InTouch MMI. Let us say that the total number of data points from all ten InTouch applications reaches 10,000 points, spread almost equally among the ten applications. Let us also assume that at least two of these process areas have a significant amount of data changing on a regular basis.
The first scenario in which the developer could use the Cascade DataHub to network these ten applications would be to launch each InTouch application and define the same domain name for each Cascade Connect instance (the name of the domain is defined in the setup of the Cascade Connect program). Thus a single Cascade DataHub would contain all 10,000 data points and each time a data point changed value, the new value would be broadcast to all ten InTouch stations.
A second approach would be to launch each of the InTouch applications in a separate domain. This would effectively separate the data for each InTouch application and mean that ten Cascade DataHubs would be running. When a point changed value in one of these datahubs, the new value would be broadcast to only the InTouch application that was running in that domain. It would still be possible to have InTouch stations display data from any or all of the process domains, but the network traffic would be significantly reduced because a value change in one InTouch application would not be broadcast to all other applications.
10.2.4. Field device drivers
Cogent has developed a number of field I/O driver programs that interact with the Cascade DataHub. We develop new drivers as and when they are needed. The Cascade DataHub incorporates features specifically designed to accommodate the effective transfer of data between PLC and DCS devices and any client.