Echelon Neuron C - Manual (Page 58)

46 How Devices Communicate Using Network Variables
Figure 6. Sample Development Network with Five Devices
The declarations for these two network variables, which appear in different
programs, are the following:
network output SNVT_switch nvoSwitch;
and
network input SNVT_switch nviLamp;
Behavior of Writer and Reader Devices
A writer device can change the value of a network variable. The connected
network variables in all reader devices are then updated to reflect this change.
In general, a reader device only reads from its copy of the network variable. One
exception is that a reader device can provide an initial value to the network
variable when the variable is declared. Another exception is that a reader device
can modify its local copy of a network variable in its program. However, in
neither case is the new value propagated to any other devices.
A writer device can also read from its last copy of the network variable, but it
only sees the value it wrote last. In other words, two writers of the same network
variable cannot change each other’s value.
When a writer device writes a value to an output network variable, the Neuron
firmware causes a L ON W ORKS message to be sent to all readers of the variable,
informing them of the new value. By default, the message is sent using the
acknowledged (ACKD) service. Not all readers may receive updates
simultaneously. The network application must be designed to handle update
failures and delays.