Echelon LNS DDE Server - Manual (Page 12)

Monitoring and Controlling a L
ON
W
ORKS
Network
Due to their high reliability, low cost, and flexibility, L ON W ORKS networks are used
in many ways. For example, they can be used to instrument relay-controlled
systems, saving the time, complexity, and cost associated with running wire from
each point back to the monitoring station. They can also be used in conjunction with
a central controller or PLC, for example, to provide a subsystem of smart sensors.
L
ON
W
ORKS
networks are frequently used to create peer-to-peer systems where each
device controls its own actions and shares information with its neighbors as needed
to control the entire system.
When monitoring a L
ON
W
ORKS
network, the monitoring system can request status
directly from a device, since each device is intelligent and knows its own status.
Similarly, when controlling a L
ON W ORKS network, a control device can send data
directly to a controlled device. No master needs to be involved and no additional
wires need to be run. Also, using the information in the LNS Server, devices can be
addressed by the names assigned during installation. These names are much easier
to remember than the complex addressing schemes used by hard-wired systems, and,
since they are assigned in lock-step with the installation, they match what is
actually contained in the system.
There are several methods for exchanging data between L ON W ORKS devices. These
are network variables, configuration properties, and application and foreign frame
messages. These are described in the following sections. L ON M ARK objects are also
described since they provide a higher-level description of the interfaces to a device.
Network Variables
The most common way for L ON W ORKS devices to communicate with one another is
through network variables. Network variables are the logical inputs and outputs of a
device. The types, functions, and number of network variables in each device are
determined by the application code within the device. For example, a temperature
sensor might have two network variables — a temperature reading output and an
over-temperature alarm output. Some devices may also support dynamic network
variables that can be added and deleted by an LNS network tool such as the
LonMaker Integration tool. The host PC running the LNS DDE Server also supports
dynamic network variables.
The actual data representation of a network variable is defined by its type. The type
may either be a user defined network variable type (UNVT), or a standard network
variable type (SNVT). SNVTs ensure common data representation and
interoperation for all L ON W ORKS devices; using SNVTs is recommended whenever
possible. A master list of standard network variable types is contained in the LNS
Utilities and LonMark Reference help file included with the LNS DDE Server.
Configuration Properties
Application configuration data is specified using configuration properties.
Configuration properties may be implemented on a device as configuration network
variables or as configuration parameters. Any device can use configuration network
variables. Devices that comply with the LONMARK Application Layer
Interoperability Guidelines, version 3.0 or later, may use configuration parameters.
1-4 LNS DDE Server Overview