Application and limitations of virtual components

In a NiagaraAX driver that offers virtual components, there are two main applications for “virtual points”:

Px view bindings for simple polling of values, without the station resource overhead of (persisted) proxy points. Upon being unsubscribed, virtual components are simply removed from the driver’s poll scheduler (or subscription mechanism), and also station memory. The only persisted parts are the ords (using virtual syntax) in the Px widget bindings. In some cases, particularly with replicated device applications, this allows a JACE station to graphically monitor many more devices than if using just proxy points. See the next section Virtual components in Px views for more details.
Quick review and (if possible) adjustments to one or more properties in objects in native devices, from the Workbench property sheets of virtual components. Otherwise, you might need to create proxy points, then delete them after reviewing and changing properties. This application applies to both Niagara driver virtual components as well as to BACnet virtual components.

As for limitations, note that virtuals are transient vs. persisted components—they are dynamically created (and subscribed) only when accessed, and are not permanently stored in the station database. This precludes any linking to or from virtuals—as links would be lost. Nor are point extensions (alarm, history) supported under virtual components. These things require use of proxy points, which are persisted in the station database.

To summarize, here are some quick application guidelines for virtual components versus proxy points:

If you need to link station logic into or out of the data item, use a proxy point.
If you need to alarm or trend/log (history) a data item, use a proxy point.
If you only need the data item value while a user is looking at a Px view, use a virtual component.
If you want to configure values in the device for one-time commissioning, use a virtual component.

Note that often proxy points are used for monitoring only, becoming subscribed only when a user is looking at a Px view, then becoming unsubscribed when not being viewed. However, such proxy points persist in the station database always—consuming station resources. The difference with using virtual component in this application is that they not only become unsubscribed, but are removed from the station’s memory. The only persisted part is the “ord” to the virtual components in the Px widget bindings.