Echelon Mini EVK - Manual (Page 72)

Chapter 6, How Devices Communicate Using Application Messages, of the Neuron C
Programmer’s Guide.
Feedback Network Variable Connections
A typical network variable connection uses one output network variable that
provides sensor data, and one input network variable that receives the sensor
reading. The connections may use the acknowledged messaging service, and rely
on the ANSI/EIA/CEA-709.1 (EN14908-1) protocol to ensure delivery of the
related data. This scenario is known as an open loop connection scenario. In this
scenario, the sensor has no feedback about the state of the actuators.
Figure 4.3 Open Loop Connection Scenario
The alternative is known as a closed loop connection scenario, or feedback
connection scenario. A feedback connection contains sensors with an additional
feedback input network variable, and actuators with an additional feedback
output network variable. In this scenario, it is possible for the sensor to get
feedback on the state of any connected actuators.
Figure 4.4 Closed Loop Connection Scenario
Feedback connection scenarios are typically implemented to address one of two
issues:
• Feedback loops allow for synchronization. For example, multiple light
switches implemented as toggling buttons, and a lamp. With the feedback
signal, the switches can be synchronized and always know the lamp’s status.
Switch A can be used to turn the light on at the bottom of the stairs, and
switch B can later be used to turn the light back off at the top of the stairs.
• Feedback loops allow for complete transport control. The acknowledged
protocol service provides a ready-to-use mechanism for delivery supervision,
but, similar to a registered letter, control stops at the point of delivery. The
switch will know from the acknowledgment that the light has received the
command, but it will not know whether the light has actually turned on.
66 Mini EVK User’s Guide