National Instruments BridgeVIEW How Do You Implement Polled Programming in G?

Chapter 4 Human Machine Interface

How Do You Implement Polled Programming in G?

You do not have to use a separate loop for each Tags or Alarms and Events

VI. This can be cumbersome to program for a large number of tag reads,

although using the HMI G Wizard makes it easy to build separate loops

quickly. The alternative is to poll the database for several tags at regularly

timed intervals. You usually need one While Loop in your diagram to poll

your front panel controls, so you can monitor what the operator is doing.

Using polling, you can combine monitoring of HMI controls with the

reading in of tag values and alarm states.

Figure 4-4 shows an example implementing a more complex user interface

that polls all the input tags as well as the front panel Start Batch button at

100 m/s intervals. When you leave the timeout input unwired, all Read

TagVIs read the BridgeVIEW database immediately by default.

Note In this case, you must explicitly program the loop wait time by using the Wait Until

Next ms Multiple VI. If you do not, the loop operates as often as possible, and

requires most of the CPU time.

This example also illustrates use of the Write Tag VIs. In this case, the

Write Tag (discrete) VI is called only when the front panel button is

pressed. In other cases, you might want to write the tag value at each

iteration. You also can use the Write Tag on Change VI to update the R TDB

only when the value of the front panel control changes. This can improve

your over all application performance.