Echelon LonMaker - Manual (Page 32)

16 Introduction
Applications in devices are divided into one or more functional blocks. A functional block is a
collection of network variables and configuration properties that are used together to perform one task.
For example, a four-port digital input module may have four digital input functional blocks that
contain the configuration properties and output network variable for each of the four hardware digital
inputs on the device.
Network Variables
Network variables allow a device to send and receive data over the network to and from other devices.
Network variables are data items (such as temperature, the state of a switch, or actuator position
setting) that a particular device application program expects to receive from other devices on the
network (an input network variable) or expects to make available to other devices on the network (an
output network variable).
Configuration Properties
Configuration properties define how an application device behaves by determining the manner in
which data is manipulated and when it is transmitted. Configuration properties determine the functions
to be performed on the values stored in the network variables. For example, a configuration property
may specify a minimum change that must occur on a physical input to a device before the
corresponding output network variable is updated.
Standard Network Variable and Configuration Property Types
Every network variable and configuration property has a type, which determines the content and
structure of the data. To enable devices from different manufacturers to interoperate, the following
standard types are defined:
• Standard Network Variable Types (SNVTs, pronounced snivits). SNVTs contain many common
operational data types. For example, SNVT_temp_p is a network variable type for network
variables containing temperature as a fixed-point number.
• Standard Configuration Property Types (SCPTs, pronounced skipits). SCPTs contain many
common configuration data types. For example, SCPT_location is a configuration property type for
configuration properties containing the device location as a text string.
See types.lonmark.org/index for a list and description of all SNVTs and SCPTs.
User-defined Standard Network Variable and Configuration Property Types
Applications may use manufacturer-defined non-standard types—user network variable types
(UNVTs) and user configuration property types (UCPTs)—which are defined in user resource files.
Device manufacturers may provide additional resource files that define these types. See the
NodeBuilder Resource Editor User’s Guide for additional information on creating or using these files.
Network Variable Connections
The application program in a device does not need to know anything about where input network
variables come from or where output network variables go. When the application program has a
changed value for an output network variable, it simply passes the new value to the device firmware.
Through a process called binding that takes place during network design and installation, the device
firmware is configured to know the logical address of the other device or group of devices in the
network expecting that network variable. It assembles and sends the appropriate packets to these
devices. Similarly, when the device firmware receives an updated value for an input network variable
required by its application program, it passes the data to the application program. The binding process
thus creates logical connections between output network variables on one or more devices and input
network variables on one or more devices. Connections may be thought of as “virtual wires.”