Echelon FTXL - Manual (Page 44)

32 Creating a Model File
Defining a Configuration Property
Like network variables, configuration properties have types, called configuration
property types , that determine the units, scaling, and structure of the data that
they contain. Unlike network variable types, configuration property types also
specify the meaning of the data. For example, standard network variable types
represent temperature values, whereas configuration property types represent
specific types of temperature settings, such as the air temperature weighting
used during daytime control, or the weighting of an air temperature sensor when
calculating an air temperature alarm.
Declaring a Configuration Property
You declare a configuration property in a model file. Similar to network variable
types, there are standard and user-defined configuration property types. You can
use the NodeBuilder Resource Editor to look through the standard configuration
property types, as described in Defining a Resource File on page 40, or you can
browse the standard profiles online at types.lonmark.org . You can also define
your own configuration property type, if needed.
You can implement a configuration property using either of the following
techniques:
• A configuration property network variable
• A configuration file
A configuration network variable (also known as a configuration property
network variable or CPNV) uses a network variable to implement the
configuration property. In this case, a L ON W ORKS device can modify the
configuration property, just like any other network variable. A CPNV can also
provide your application with detailed notification of updates to the configuration
property. However, a CPNV is limited to a maximum of 31 bytes, and an FTXL
application is limited to a maximum of 4096 network variables, including
CPNVs. Use the network … config_prop syntax described in Declaring a
Configuration Network Variable on page 143 to implement a configuration
property as a configuration network variable. By convention, CPNV names start
with an nci prefix, and configuration properties in files start with a cp prefix.
A configuration file implements the configuration properties for a device as one or
two blocks of data called value files, rather than as separate externally exposed
data items. 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. When there are two value files, one contains
writeable configuration properties, and the second contains read-only data. To
allow a network management tool to access the data items in the value file, you
specify a provided template file, which is an array of text characters that
describes the elements in the value files. When you use the Direct Memory Files
feature, the total size of the directory, template file, and value files cannot exceed
65 535 bytes (64 KB -1). When you use FTP, individual files cannot exceed 2 147
483 647 bytes (2 GB -1, or 2
31 -1 bytes).
Other devices cannot connect to or poll a configuration property implemented in a
configuration file. To modify a configuration property implemented in a
configuration file, a network management tool must modify the configuration file,
for which your application must provide an appropriate access method.