Handle SIDirect scan-based messages

AVEVA Communication Drivers poll hardware devices for information. This polling is requested by one or more clients.

After a client requests a particular piece of information, the SIDirect Communication Driver creates its own request and sends that request to the hardware device. The SIDirect Communication Driver then waits for a response to its request. After the SIDirect Communication Driver receives the information, it passes that information back to the client and repeats the process until all clients stop requesting information.

You define the rate at which the SIDirect Communication Driver polls a particular device for a specific piece of information in the device group (topic definition) inside the SIDirect Communication Driver. You use a parameter called the Update Interval. When setting this parameter, there is always a trade-off between the update speed of the device group and the resulting decrease in system responsiveness.

Because you more than likely want very fast response, the temptation is to set the Update Interval to a value close to 0 (zero) seconds. However, if every item is polled at this rate, the entire system suffers due to slow response time. Therefore, compromise and set the Update Interval to a more reasonable value.

You can also create multiple device groups for each device, setting the Update Interval to different values, then assigning different items to different device groups, depending on how quickly the values change and how quickly you want to see an update of those changes.