Echelon LonTal Stack - Manual (Page 72)

60 Creating a Model File
use the NodeBuilder Resource Editor to look through the standard configuration
property types, as described in Defining a Resource File, 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 configuration NV) 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 configuration NV
can also provide your application with detailed notification of updates to the
configuration property. However, a configuration NV is limited to a maximum of
31 bytes, and a LonTalk Stack application is limited to a maximum of 4096
network variables, including configuration NV s. Use the network …
config_prop syntax described in to implement a configuration property as a
configuration network variable. By convention, configuration NV 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.
You must implement configuration properties within a configuration file if any of
the following apply to your application:
• The total number of network variables (including configuration network
variables and dynamic network variables) exceeds the total number of
available network variables (a maximum of 4096 for a LonTalk Stack
device, but potentially fewer than 4096 depending on the resources
available).
• The size of a single configuration property exceeds the maximum size of a
configuration network variable (31 bytes).
• Your device cannot use a configuration network variable (CPNV). For
example, for a device that uses a configuration property array that
applies to several network variables or functional blocks with one
instance of the configuration property array each, the configuration