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104 •
LON
4.5 Network communication
LON is a decentral bus system, i. e. the individual components can communi-
cate via the bus without requiring a master. In this manner, the information
paths from the sensors via the host to the actuators are drastically shortened,
and the rest of the system is no longer loaded with unwanted data traffic.
The exchange of messages between two nodes in one network is performed by
defining of so-called network variables (NV's).
Network variables are data interfaces for communication via the network
backed with a physical type. There are network variables for instance for cur-
rent, voltage, power, temperature, pressure, date, time, presence, ...etc.
The LON bus coupler writes the input and output data into defined network
variables, which in turn can be linked with network variables of other nodes.
The maximum number of input and output channels is determined by the
maximum number of network variables (62).
With LON, only agreed status changes, elapsing timers and/or limit value
overruns are signalled. Not every signal is subject to a cyclical screening. As a
consequence, the function of the actuators, sensors and controllers is event
controlled and a network variable only transmitted when its value has
changed.
This value is received by all nodes in which this network variable together
with a network input and network output link has been defined.
This means that the programmer need not care about node addressing, data
buffers, message transmission services and other details when using network
variables.
Just one example to clarify the function of a data exchange between network
nodes via a network variable.
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