Echelon LNS - Manual (Page 238)

LNS Programmer's Guide 224
4. You can now control the input network variables on the network by
writing to the host network variable on the PC. Or, you can monitor value
changes on the device network variables though the
OnNvMonitorPointUpdateEvent
event.
Defining Host Network Variables
An LNS application can dynamically create and delete local host network variables (i.e.
network variables on the Network Service Device being used by the LNS application). All
host network variables become part of the local Network Service Device’s main interface,
which is accessible by reading the
Interface
property contained in the
NetworkServiceDevice
object’s
AppDevice
object. It is not possible to add network
variables directly to the main interface.
However, you can add network variables to custom
Interface
objects created by LNS
applications. To do so, use the
Interfaces
collection returned by the
NetworkServiceDevice
object’s
Interfaces
property. For more information on
custom interfaces, including how to add and remove network variables from them, see
Using Dynamic Device Interfaces on page 179.
Using Message Monitor Points
LNS supports the transmission and receipt of application messages and foreign frame
messages. Application messages and foreign frame messages are network messages that
do not relate to network variables, network management or diagnostic services.
Application messages use a 0x00...0x3E message code range, and are sometimes used for
non-interoperable data exchange. Foreign frame messages use the 0x40...0x4E message
code range and are sometimes used when tunneling packets related to other
communication protocols through a L
ON W ORKS network. This allows you to use your
application to monitor and control devices that use application messaging.
Application messages are explicitly constructed in the transmitting device, unlike
network variable messages, which are implicitly constructed by writing to a network
variable or by calling library functions. Message monitoring is implemented by
connecting an output message tag on a device on the network to an input message tag on
the LNS application’s Network Service Device. Note that application messages cannot be
polled, and therefore application message monitoring always requires a bound
connection.
Message control is implemented with message monitor points. You can use the message
monitor points to send explicit messages to the device synchronously or asynchronously.
When using message monitor points, you should note that the maximum size of any
application message is usually 40 bytes. However, this varies depending on the network
buffers and the application buffers in the network interface you are using, as well as on
any network or application buffers in any intervening routers along the message’s path,
and in the application device that is receiving the message. If a message fails as a result
of being too large, the failure will appear to be the same as any communication failure,
and the device receiving the message will not respond. As a result, you should be careful
not to exceed your available network resources when creating an application to send
application messages.