As a container slot under the LonNetwork, the LonPollService operates as in most other drivers. See “About poll components” in the Drivers Guide for general information. In a LonNetwork, polling provides one means to update station database values from transient nv data in networked Lon devices.
Polling is used when proxy points are first created, before you bind nvs to the station (LocalDev) using the Lon Link Manager.
Typically, you bind nvs so that most updates occur using nv updates, vs. polling requests. As necessary, polling also occurs
to update nv values of LonComponents (visible as nvis and nvos in
the property sheet of a LonDevice). These exist even if proxy points are not created (see LonComponents).
The LonPollService sequentially poles all the devices on the network, and reads nv data per the nv update rules. See the next section, Network variable (nv) poll/update rules
General rules for polling nv (nvi and nvo) values are as follows:
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An nv is subscribed if it is being viewed or has a proxy point which is subscribed. A proxy point is subscribed if it is viewed, or if it is linked to control logic (or has an alarm or history extension).
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When an nv is first subscribed, its value is read (if device is in a valid state). Further reads are handled by the poll thread. (The poll service must be enabled for polling to function normally.)
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Adding a proxy point creates a local connection to the associated nv. If a proxy point for an nvo, or a writable proxy point for an nvi, each appears as one entry (row) in the Lon Link Manager.
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There is a “Bound To Local” property on nvProps which is set when an output nv with a proxy is bound. An output nv is not polled when it is bound locally. Instead nv updates will be received from the device.
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The “Poll Enable” flag is set if there are unbound links and the nv is not bound locally. To prevent polling on an nv, the user may set its Poll Enable flag to
false. The initial read is still performed when the nv is first subscribed, but subsequent polling by the poll service does not occur.
Polling rules for nvs with specific connection scenarios are provided in Table 1.
Table 1. Polling rules
| Type of nv, connection | Update rules |
|---|---|
| nvi with writable proxy | Poll when nv or proxy point subscribes. If proxy value changes update nv. |
| nvi with read-only proxy | Poll when nv or proxy point subscribes. |
| nvo with writable proxy | <Not allowed> |
| nvo with read-only proxy (unbound) | Poll when nv or proxy point subscribes. |
| nvo with read-only proxy (bound) | Receive nv updates (No polling). |
| polled nvo* with read-only proxy bound (*see Polled network variables) | Poll when nv or proxy subscribed. |
| nvi linked | Poll when subscribed. |
| nvo linked unbound | Poll when subscribed. If nvo changes push value to linked nvi. Write nvi value to device. |
| nvo linked bound | Poll when subscribed. |
Input nvs (nvi) with read-only proxy points are never bound, and do not show up in the Lon Link Manager.
Some Lonworks devices include one or more nvos that are “polled” types, meaning that the nvo does not send nv updates upon a value change. Such an nvo must be polled. Typically, any such nv represents a data item that does not frequently change.
In the Lon device object, any such nvo LonComponent appears with its read-only “Polled” property set to “True” (as found under its “Nv Props” container). See Figure 9 for an example.
If you create a proxy point for such an nvo and bind to it in the Lon Link Manager, it still polls to receive value updates. Also, when creating a link (bind) between two Lon devices, the Link Editor only allows “polled” outputs to be linked to “polled” inputs.