Echelon LonTal Stack - Manual (Page 73)

LonTalk Stack Developer’s Guide 61
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 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 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/docs for more
information about the L
ON
W
ORKS
file transfer protocol; see Using Direct Memory
Files 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 SFPTnodeObject functional profile, simply add this network
variable to the model file. You do not need to initialize this network
variable (any initial value is ignored – the LonTalk Interface Developer
utility calculates the correct value).
• For FTP, declare at least two mandatory network variables, an input
network variable of type SNVT_file_req, and an output network variable
of type SNVT_file_status. You also need to define a message tag for the
transfer of the data. In addition, you need an input network variable of
type SNVT_file_pos to support random access to the various files. You
must also implement the L ON W ORKS file transfer protocol within your
application program.