Echelon LonTal Stack - Manual (Page 66)

54 Creating a Model File
Example: The following declaration defines an input network variable of type
“SNVT_type” with the name “nviAmpere”.
network input SNVT_amp nviAmpere;
You define a simple output network variable using the same syntax, but with the
output modifier:
network output type name;
Example: The following declaration defines an output network variable of type
“SNVT_type” with the name “nvoAmpere”.
network output SNVT_amp nvoAmpere;
By convention, input network variable names have an nvi prefix and output
network variables have an nvo prefix.
See Network Variable Syntax for the full network variable declaration syntax.
The LonTalk Interface Developer utility reads the network variable declarations
in the model file to generate device-specific code. For the example of the
nviAmpere and nvoAmpere pair of network variables above, the utility generates
a standard ANSI C type definition for the SNVT_amp network variable type and
implements two global C-language variables:
typedef ncsLong SNVT_amp;
…
volatile SNVT_amp nviAmpere;
SNVT_amp nvoAmpere;
The ncsLong data type defines the host equivalent of a Neuron C signed long
variable. This type is defined in the LonPlatform.h file.
Your LonTalk Stack application can simply read the nviAmpere global C variable
to retrieve the most recently received value from that input network variable.
Likewise, your application can write the result of a calculation to the nvoAmpere
global C variable, and call the appropriate LonTalk API function to propagate the
network variable to the L ON W ORKS network.
Choosing the Data Type
Many functional profiles define the exact type of each member network variable.
The SNVT_amp type used in the previous section is such a type. Using a
different network variable type within a functional profile that requires this
network variable type renders the implementation of the profile not valid.
Other profiles are generic profiles that allow various network variable types to
implement a member. The SFPTopenLoopSensor functional block (described in
Defining a Functional Block ) is an example for such a generic functional profile.
This profile defines the nvoValue member to be of type SNVT_xxx, which means
“any standard network variable type.”
Implementing a generic profile allows you to choose the standard network
variable type from a range of allowed types when you create the model file.
For added flexibility, if the specific functional profile allows it, your application
can implement changeable-type network variables. A changeable-type network
variable is network variable that is initially declared with a distinct default type
(for example, SNVT_volt), but can be changed during device installation to a
different type (for example, SNVT_volt_mil).