Echelon LNS - Manual (Page 232)

LNS Programmer's Guide 218
To recap this discussion, make sure that you follow these steps when using the polled
network variable monitoring scenario:
1. When you create your monitor points, you should consider setting the Tag
property to a value you can use to identify the source device and network
variable.
2. When you set the monitoring options for the monitor set, make sure that
the PollInterval property is set to the interval at which you want to
poll each network variable. In addition, make sure that the
ThottleInterval property is not set to a greater value than the
PollInterval property. The ThottleInterval property should be set
to 0 if you want to be assured of receiving an event for every network
variable update.
See the next section, Setting the Poll Interval, for guidelines you should
follow when setting the poll interval for your application.
3. If you only want your application to be informed when the value of a
network variable monitor point changes, set the
ReportByException
property to True when you set the monitoring options for the monitor set.
4. When you enable the monitor set, or when you enable any of the monitor
points in the set, make sure that the doPoll element is set to True. Your
application will be informed of the result of each poll via the
OnNvMonitorPointUpdateEvent
event. A sample
OnNvMonitorPointUpdateEvent
event handler is shown later in the
Example of a Network Variable Event Handler section on page 219.
Setting the Poll Interval
When setting the poll interval for your LNS application, you should note the following:
1. The total number of data points being polled, combined with the
PollInterval chosen, results in a specific number of network variable
polls per second. The maximum number of polls that your network can
sustain is subject to the channel types being used, network topology, use
of authentication, size of the data being polled, performance of the devices
that contain the polled network variables, and network interface you are
using. You should make sure that your poll interval does not cause your
network resources to be exhausted.
2. The poll interval that you specify is not a precise value. LNS tries to poll
the data at the specified interval, but some network conditions may
prevent polling at the exact, desired moment. For example, if too many
messages and polling requests are using priority slots, it could cause
collisions on the priority slots. Regular network traffic, or transient
partial network outages may prevent successful polling at times. Such
failure will be reported by the OnMsgMonitorPointErrorEvent or
OnNvMonitorPointErrorEvent events.
3. Typical L ON W ORKS networks implement a distributed control algorithm
by means of distributed devices, and data exchange with network
variables. In many of those systems, such as a typical building control
system, monitoring applications are used to oversee network operation.