Scan-Based Message Handling

Wonderware's OI Servers are based on the concept of polling a hardware device for information. This polling is driven by a need which is expressed in the form of requests from one or more clients. Once a particular piece of information has been requested by a client, the OI Server formulates its own request and sends that request to the hardware device. The OI Server then waits for a response to its request. Once the information has been received, the OI Server passes that information back to the client, and repeats the process until all clients have ceased requesting information.

The rate at which the OI Server will poll a particular device for a specific piece of information is defined in the device group (topic definition) inside the OI Server, using 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.

Since a fast response is generally wanted, you may be tempted to set the Update Interval to a value close to 0 (zero) seconds. However, if every point is polled at this rate, the entire system will slow down. Therefore, it is recommended that you set the Update Interval to a more reasonable value. You could 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.

Some items, like alarms, change very infrequently but because of their importance require very fast updates. For these items, set the Update Interval at a very small value. For an immediate response, set the Update Interval at 1 (one).