Echelon FTXL - Manual (Page 43)

FTXL User’s Guide 31
• Only a network management tool, such as the LonMaker Integration tool,
can change the type of a changeable-type network variable. The FTXL
device does not initiate type changes.
To create a changeable-type network variable for an FTXL application, perform
the following tasks:
1. Declare the network variable with the changeable_type keyword. You
must declare an initial type for the network variable, and the size of the
initial type must be equal to the largest network variable size that your
application supports. The initial type must be one of the interoperable
standard or user network variable types.
2. Select Has changeable interface in the L ON M ARK Standard Program ID
Calculator (included with the LonTalk Interface Developer utility) to set
the changeable-interface bit in the program ID when you create the
device template.
3. Declare a SCPTnvType configuration property that applies to the
changeable-type network variable. This configuration property is used by
network management tools to notify your application of changes to the
network variable type.
4. You can optionally also declare a SCPTmaxNVLength configuration
property that applies to the changeable-type network variable. This
configuration property informs network management tools of the
maximum type length supported by the changeable-type network
variable. This value is a constant, so declare this configuration property
with the const modifier.
5. Implement code in your FTXL application to process changes to the
SCPTnvType value. This code can accept or reject a type change. Ensure
that your application can process all possible types that the changeable-
type network variable might use at runtime.
6. Implement code to provide information about the current length of the
network variable.
The LonMaker browser provides integrators with a user interface to change
network variable types. However, you might want to provide a custom interface
for integrators to change network variable types on your device. For example,
the custom interface could restrict the available types to those types supported by
your application, thus preventing configuration errors.
The LonMaker Integration tool, Turbo Edition, supports changeable-type
network variables. However, if you use LonMaker 3.0 or earlier to manage an
FTXL device with changeable-type network variables, you must explicitly set the
CP value in the LonMaker browser (or in a device plug-in) to inform the FTXL
device of the type changes in addition to using the “Change Network Variable
Type” facility that is provided with LonMaker 3.0 or earlier to change the type of
a network variable in the LNS database.
See
Handling Changes to Changeable-Type Network Variables on page 88 for
information about how your application should handle changes to changeable-
type network variables.