Echelon LNS - Manual (Page 204)

LNS Programmer's Guide 190
Introduction to Monitor and Control
By definition, device applications only have a local view of the network. They know what
data they receive from the network (usually without regard to which devices produce the
data), and they know what data they produce (usually without regard to which devices
will consume the data).
However, in most control systems there is a need for an application that provides an
overall view of the system. For example, in a process control system, the operator
interface provides users with supervisory control over the entire process, and displays
the current status of the system. Or, in a building control system, there might be
multiple control panels from which users can view the current setting of any point in the
system, and change the value of any set point in the system. You can use the monitor and
control services provided with LNS for these purposes.
Monitoring is the process of fetching and receiving data from devices on the network,
while controlling is the process of writing data to those devices. Both involve subordinate
tasks such as data formatting, connecting devices, and address change tracking (to
ensure that data is not lost due to address changes). In a L ON W ORKS network, data can
be retrieved from application devices using network variable connections, or fetched from
devices with regularly scheduled polling messages and explicit polls. Basic formatting is
accomplished automatically by processing network variable types according to predefined
formatting files. Additional formatting or processing can be performed by the network
application.
You can use monitor sets to implement monitor and control operations in your LNS
application. A monitor set is a collection of monitor points. A network variable monitor
point represents a single network variable on a device that is being monitored by an LNS
application, and you can use the monitor point to read and write that network variable’s
value. You can use a message monitor point to send an explicit message to a device with
your LNS application, or to receive messages sent by application devices to your LNS
application.
As of LNS Turbo Edition, there are two kinds of monitor sets:
• Permanent monitor sets. Permanent monitor sets are stored persistently in the
LNS database, and can be used in multiple client sessions.
• Temporary monitor sets. Temporary monitor sets are not stored persistently in
the LNS database, and are only used in a single client session.
Depending on how you need to monitor devices, you may want to use either type of
monitor set. If you are writing a device plug-in application, and only want to monitor the
values of the network variables on the device while you configure it, a temporary monitor
set will suffice, as you will only need to monitor the device for a short period of time.
On the other hand, you might be managing a very large network with hundreds or even
thousands of devices, and you need an application to monitor and control those devices
persistently. In that case, you should use permanent monitor sets. You could define a
group of monitor sets as you install the network, and then use them to monitor and
control your devices when your network begins operation.
At some point, you may also want to read or write the value of single network variable
without monitoring its value beforehand. You could do this by directly reading or writing