Network tuning policies
Tuning policies are important for providing network-wide controls for a particular network driver. Figure 4-6 shows network level tuning options that are available from a driver property sheet view in workbench.
A network’s Tuning Policies view holds one or more collections of “rules” for evaluating both write requests (e.g. to writable proxy points) as well as the acceptable “freshness” of read requests from polling. Some drivers also support different poll frequency groups (Slow, Normal, Fast) within their network tuning policies or under their poll scheduler component.
Figure 4-6 Network tuning policies – NiagaraNetwork
Note: Some driver networks do not have Tuning Policies. For example, an RdbmsNetwork for a database driver. Also, the NiagaraNetwork has greatly simplified Tuning Policies.
In the network’s property sheet, expand the Tuning Policies (Map) slot to see one or more contained Tuning Policies. Expand a Tuning Policy to see its configuration properties as shown in Figure 4-7.
Figure 4-7 Example tuning policies map (Bacnet)
By default, a driver’s Tuning Policy Map contains just a single TuningPolicy (“Default Policy”). However, you typically create multiple tuning policies, changing those items needed differently in each one. Then, when you create proxy points under a device in that network, you can assign each point (as needed) to the proper set of “rules” by associating it with a specific tuning policy.
As a simple example (under a BacnetNetwork), you could duplicate the default tuning policy twice, naming the first copy “Slow Policy” and the second copy “Fast Policy.” Then, in each copy change the “Poll Frequency” property from “Normal” to “Slow” or “Fast,” respectively. You would then have 3 available (and different) poll frequency groups to pick from when you create and edit proxy points.
Tuning Policy properties
Tuning Policy properties for typical field bus drivers are as follows:
Min Write Time
Applies to writable proxy points, especially ones that have one or more linked inputs. Specifies the minimum amount of time allowed between writes so that it provides a method to throttle rapidly changing value so that only the latest value is written. If the property value is 0 (default), this rule is disabled (all value changes attempt to write). In this case, if you have a rapidly changing value and set a zero value for min write time, this single point can create frequent message traffic.
Max Write Time
Applies to writable proxy points. Specifies the maximum “wait time” before rewriting the value, in case nothing else has triggered a write. Any write action resets this timer. If the property value is 0 (default), this rule is disabled (no timed rewrites). Larger values in this property allow for less message traffic by not forcing a write but they do not cause the write to occur.
Write On Start
Applies to writable proxy points. Determines behavior at station startup.
Write On Up
Applies to writable proxy points. Determines behavior when proxy point (and parent device) transitions from “down” to “up.”
Write On Enabled
Applies to writable proxy points. Determines behavior when a proxy point’s status transitions from “disabled” to normal (enabled). Note this status can be inherited globally if the parent device was set to disabled (or network-wide if driver network was set to disabled).
Stale Time
Applies to all proxy points.
- If set to a non-zero value, points become “stale” (status stale) if the configured time elapses without a successful read, indicated by Read Status “ok.”
- If set to zero (default), the stale timer is disabled, and points become stale immediately when unsubscribed.
Note: By default, proxy point status “stale” is indicated by tan background color. In addition, stale status is considered “invalid” for any downstream-linked control logic.
Poll Frequency
In the BacnetNetwork (only) TuningPolicy. Applies to all proxy points. Provides a method to associate the tuning policy with one of 3 Poll Rates available in the network’s Poll Service: Fast Rate, Normal Rate, or Slow Rate. The default poll frequency is “Normal.”
Depending on the driver, there may be a single “Poll Service” (or “Poll Scheduler”) slot under the network, or as in the case of a BacnetNetwork, a separate “Poll Service” for each configured port (IP, Ethernet, Mstp) under its BacnetComm > Network container. The NiagaraNetwork uses subscriptions instead of polling.
Cov Subscription Lifetime
In the BacnetNetwork (only) TuningPolicy. Applies to all proxy points under this driver. Provides a method to limit the time that a Cov will maintain a Subscription state. The default time is 15 minutes.
Note: Depending on the driver, there may be a single “Poll Service” (or “Poll Scheduler”) slot under the network, or as in the case of a BacnetNetwork, a separate “Poll Service” for each configured port (IP, Ethernet, Mstp) under its BacnetComm > Network container. The NiagaraNetwork uses subscriptions instead of polling.
Tuning policies used by a specific driver may have unique characteristics. For example, under a NiagaraNetwork, TuningPolicy has only two properties: Min Update Time and Max Update Time.
Other drivers may have specific considerations for tuning policies. For more information, refer to the “Tuning Policy Notes” section within any NiagaraAX driver document.