Echelon LNS - Manual (Page 229)

LNS Programmer's Guide 215
method is most efficient if network variable values need to controlled
individually, or if they need to be written infrequently and unpredictably.
For more information on this, see Explicitly Reading and Writing
Network Variable Monitor Points on page 215.
• Explicit Bound Monitoring and Control. This method should only be used
for fan-out connections, which is the process of a single host output
network variable controlling two or more input device network variables.
This is most efficient when updating many network variables to the same
value at once. For more information, see The Explicit Bound Network
Variable Monitoring and Control Scenario on page 221.
Explicitly Reading and Writing Network Variable Monitor Points
In this scenario, the application explicitly reads and writes network variable values. This
method is most efficient if network variable values need to be read infrequently and
unpredictably, or written individually.
To explicitly read or write a network variable monitor point, you must first create a
monitor set and add monitor points for the network variables you want to monitor. For
more information on these tasks, see Creating Monitor Sets on page 194, and Managing
Monitor Sets on page 195.
Following that, you can open and enable the monitor set, and begin explicitly reading
and writing the values of the network variable monitor points in the monitor set. To do
so, you need to acquire a
DataPoint
object representing the network variable monitor
point. You can access such a
DataPoint
object by reading the
NvMonitorPoint
object's
DataPoint
property.
The
DataPoint
object contains the value data for the monitor point. The value data is
stored in three properties on the
DataPoint
object:
Value
,
FormattedValue
, and
RawValue
. All three of these properties point to the same piece of data in memory, but
they format the data in different ways:
• The
Value
property formats the data as a raw, scaled numeric value.
• The
FormattedValue
property formats the data based on the LNS
Object Server’s
CurrentFormatLocale
setting. The data stored in the
FormattedValue
property is scaled and unit-converted based on the
FormatSpec
the data point is using. For more information on
FormatSpec
objects, see Data Formatting in page 241.
• The
RawValue
property formats the data as a raw byte array.
Whenever one of these values is set, the values of the other two properties will be
updated to reflect this change. However, you should note that reading and writing any of
the three value properties does not automatically access or update the network variable
on the source device.
Each
DataPoint
object contains a
Read()
method and a
Write()
method. The
Read()
method copies the monitor point value data from the device into the
DataPoint
object.
The
Write()
method copies the monitor point value data from the
DataPoint
object to
the device. It is possible to automate this process with the
AutoRead
and
AutoWrite
properties. Set the
AutoRead
property to
True
to cause the
Read()
method to be called
automatically before any of the three value properties are read. Set the
AutoWrite
property to
True
to cause the
Write()
method to be called automatically after any of the
three value properties are set.