Communication between nodes on a network takes place using the network variables that are defined in each node. The systems integrator designer defines the network variables when the application program is created as part of the Application layer of the protocol. Network variables are shared by multiple nodes. Some nodes may send a network variable while others may receive. By only allowing links between inputs and outputs of the same type, network variables enforce an object-oriented approach to product development. This greatly simplifies the process of developing and managing distributed systems.
Whenever a node program writes a new value into one of its output variables, the new value is propagated across the network to all nodes with input network variables connected to that output network variable. This action is handled by the protocol within the Neuron Chip. The user defines the network variable connections when the nodes are installed on the network using a network configuration tool.
The use of Standard Network Variable Types (SNVTs, pronounced "snivets") contributes to the interoperability of LONWORKS products from different manufacturers. Echelon maintains a growing list of over 100 SNVTs for nearly all physical measurement types including the type of variable such as integer or floating point. For example, a SNVT for continuous level is defined as SNVT_lev_contin.
If all manufacturers use this variable type in their application when a network variable for continuous level is defined, any device reading a continuous level can communicate with other devices on the network that may be using the variable as a sensor output to initiate an actuator. As long as a network input variable and a network output variable are defined with the same SNVT when the developer creates the applications, they can be connected together on the network through a process called binding.
Binding is defined at the time of installation using a Windows based graphical installation tool. When you install a node, you specify which network variables are to be connected between nodes. This is easily done by highlighting the output network variable on one node and the input network variable on the node or nodes to be connected. This easy since the important information in each node is presented graphically through a standard Windows interface. Only network variables of the same SNVT type can be bound together. In other words, a temperature type could not be bound to a pressure type.
The following are examples of SNVTs. A complete list of SNVTs is available from Echelon.
|Day of Week||Enumerated List (Mon-Sun)|
|Elapsed Time||Second, Milliseconds, Days or Hours|
|% of Full Scale||Percent|
|Phone State||Enumerated List (On-hook, off-hook, busy, ringing, etc.)|
|Voltage||DC or AC RMS|
|Current||Amps AC, RMS|
|Volume||Gallons, CCF, litres|
|Flow||Gallons or litres/hour|
|Speed||Miles or Km/hour|
|Pressure||Lbs./sq.in, Pascal's, Inches-Hg|
We hope this has been a useful introduction to Echelon Corporation’s LONWORKS® technology, and not too technical.
Davmark Group make things work. We do the hard work, not you.
Davmark Group provides advanced solutions using LONWORKS.
Extensive portions of this article were quoted verbatim
from Motorola document BR1108/D,
LONWORKS Product Line Brief
LonTalk Protocol. Davmark Group wishes
to express their grateful acknowledgment to both Echelon and
Motorola Semiconductor for these reference documents.