Echelon FTXL - Manual (Page 45)

FTXL User’s Guide 33
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 an FTXL 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
property array must be shared among all network variables or functional
blocks to which it applies. In this case, the device must implement the
configuration properties within a configuration file.
In addition, you might decide whether to implement configuration properties
within a configuration file for performance reasons. Using the direct memory
files (DMF) feature can be faster than using configuration network variables
(CPNVs) if you have more than a few configuration properties because multiple
configuration properties can be updated during a single write to memory
(especially during device commissioning). However, FTP can be faster than DMF
if there are many configuration properties to be updated.
Use the cp_family syntax described in
The Configuration Property Type on page
140 to implement a configuration property as a part of a configuration file.
When implementing configuration property files, the LonTalk Interface
Developer utility combines all configuration properties declared using the
cp_family keyword, and creates the value files and a number of related data
structures.
However, you must provide one of two supported mechanisms to access these
files:
• An implementation of the L ON W ORKS file transfer protocol
• Support for the direct memory files feature
The LonTalk Interface Developer utility provides most of the required code to
support direct memory files. However, if you use FTP, you must also implement
the L ON W ORKS file transfer protocol within your application program. You would
typically implement the L ON W ORKS file transfer protocol only if the total amount
of related data exceeds (or is likely to exceed) the size of the direct memory file
window.
See the File Transfer engineering bulletin at www.echelon.com for more
information about the L
ON
W
ORKS
file transfer protocol; see Using Direct Memory
Files on page 96 for more information about the direct memory files feature.
To indicate which file access method the application should use, you must declare
the appropriate network variables in your model file:
• For direct memory files, declare an output network variable of type
SNVT_address. If your device implements the SFPTnodeObject
functional profile, you use this network variable to implement the
profile’s nvoFileDirectory member. If your device does not implement the