Scan-Based Message Handling
- Last UpdatedOct 23, 2017
- 2 minute read
Wonderware's DAServers 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.
For DDE/SuiteLink, once a particular piece of information has been requested by a client, the DAServer formulates its own request and sends that request to the hardware device. The DAServer then waits for a response to its request. Once the information has been received, the DAServer passes that information back to the client, and repeats the process until all clients have ceased requesting information.
The rate at which the DAServer will poll a particular device for a specific piece of information is defined in the device group (topic definition) inside the DAServer, 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.
If you use OPC interface, in addition to the capabilities described in the preceding paragraph, the OPC Client also has additional capabilities on Update Interval control.
Since very fast response is usually desired, the temptation is to set the Update Interval to a value close to 0 seconds. However, if every point is polled at this rate, the entire system will suffer due to slow response time. Therefore, you should compromise, and 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 those kinds of items, you should set the Update Interval at a very small value. If you desire an immediate response, set the Update Interval at 1 (one). See the Unsolicited Message Handling section.