Echelon Neuron C - Manual (Page 96)

84 Using Configuration Properties to Configure Device Behavior
Overview
A configuration property is a data item that, like a network variable, is part of
the device’s interoperable interface. A configuration property can be modified by
a network tool. Configuration properties facilitate interoperable installation and
configuration tools by providing a standardized network interface for device
configuration data. Like network variables, configuration properties also provide
a well-defined interface.
Each configuration property type is defined in a resource file that specifies the
data encoding, scaling, units, default value, invalid value, range, and behavior for
configuration properties based on the type. A rich variety of standard
configuration property types (SCPTs) are defined in the standard resource file
set. You can view all currently defined SCPTs online at
types.lonmark.org . You
can also create your own user configuration property types (UCPTs) that are
defined in resource files that you create with the NodeBuilder Resource Editor.
Declaring Configuration Properties
You can implement a configuration property using one of two different
techniques. The first, called a configuration network variable , uses a network
variable to implement the configuration property. This method has the
advantage of enabling the configuration property to be modified by another
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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, and that the number of configuration network
variables is determined by the maximum number of network variables for the
target platform.
To implement a configuration property as a configuration network variable,
declare it using the network … config_prop syntax described in Declaration of
Configuration Network Variables on page 85.
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 in 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, except as constrained by the available memory space on
the device. The disadvantages are that other devices cannot connect to or poll a
configuration property implemented as a configuration file, instead requiring a
network tool to modify a configuration property implemented within a