Echelon Neuron C - Manual (Page 180)

160 Network Variable, Configuration Property, and Message Tag Declarations
Introduction
The external application interface of a L ON W ORKS device consists of its functional
blocks, network variables, and configuration properties. The network variables
are the device’s means of sending and receiving data using interoperable data
types and using an event-driven programming model. The configuration
properties are the device’s means of providing externally exposed configuration
data, again using interoperable data types. The configuration data items can be
read and written by a network tool. The device interface is organized into
functional blocks , each of which provides a collection of network variables and
configuration properties that are used together to perform one task. These
network variables and configuration properties are called the functional block
members .
Configuration properties can be implemented using two different techniques.
The first, called a configuration network variable , uses a network variable to
implement a configuration property. This has the advantage of enabling the
configuration property to be modified by another L ON W ORKS device, just like any
other network variable. It also has the advantage of having the Neuron C event
mechanism available to provide notification of updates to the configuration
property. The disadvantages of configuration network variables are that they are
limited to a maximum of 31 bytes each.
The second method of implementing configuration properties uses configuration
files to implement the configuration properties for a device. Rather than being
separate externally-exposed data items, all configuration properties implemented
within configuration files are combined into one or two blocks of data called
value
files . A value file consists of configuration property records of varying length
concatenated together. Each value file must fit as contiguous bytes into the
memory space of the device that is accessible by the application. When there are
two value files, one contains writeable configuration properties and the second
contains read-only data. To permit a network tool to access the data items in the
value file, there is also a template file , an array of text characters that describes
the elements in the value files.
The advantages of implementing configuration properties as configuration files is
that there are no limits on configuration property size or the number of
configuration properties other than the limitations on the size of a file. The
disadvantages are that other devices cannot connect to or poll a configuration
property implemented within a configuration file; requiring a network tool to
modify a configuration property implemented within a configuration file; and, no
events are automatically generated upon an update of a configuration property
implemented within a configuration file. The application can force notification of
updates by requiring network tools to disable a functional block or take a device
offline when a configuration property is updated, and then re-enable or put the
device back online.
You can declare functional blocks, network variables, and configuration
properties using the Neuron C Version 2 syntax. You can declare configuration
properties that are implemented within configuration files or configuration
network variables. The Neuron C Version 2 compiler uses these declarations to
generate the value files, template file, all required self-identification and self-
documentation data, and the device interface file (.xif extension) for a Neuron C
application.