Echelon Mini EVK - Manual (Page 74)

To achieve the same level of transport control in a star-shaped feedback
connection, the initiator needs to know the number of expected feedback network
variable updates. The initiator device might be able to determine that number
dynamically by inspecting the appropriate system tables, but this information is
not always available. If the star-shaped feedback connection is made using the
acknowledged service and group addressing, the group’s member count is not
known to the device by default.
An advantage of the star-shaped feedback connection is that it allows for quick
propagation of new values, and oscillation is easier to control.
Oscillation
Like any looping construct, feedback loops can start to oscillate. This is most
obvious in a daisy-chain scenario: the loop starts to oscillate if the sensor
propagates revised data before the previous loop has been completed. Consider a
series of On/Off/On commands issued by a switch in quick succession. If these
commands propagate quickly enough, the loop will carry three different sets of
data. If the initiator device was designed with a feedback loop for transport
control in mind, this might have no negative effects. If, however, the initiator was
designed with synchronization in mind yet connected in a daisy-chain, the loop
can oscillate.
Oscillation is less likely with a star-shaped connection, but in both cases,
manufacturers should document recommended connection scenarios for their
devices. When designing closed loop applications, the following is recommended:
• Feedback loops that serve the purpose of synchronization should use a star-
shaped feedback connection if possible. When a daisy-chain feedback
connection is supported, sensors should only issue new values once the loop
has been completed, or once a sufficient timeout has expired, whichever
comes first.
• Closed loop sensors should ignore feedback (input) network variable updates
if the update contains a network variable value that equals the value of the
corresponding output network variable.
• Feedback loops that serve the purpose of tied transport control should be
created under control of specialized software. A configuration tool should use
knowledge of the exact connection shape to set configuration properties with
each network variable, indicating the expected number of feedback updates.
• Due to its superior stability and speed of propagation, star-shaped feedback
connections are preferred. Daisy-chained feedback connections are possible
where a specialized connection tool (as described in the previous
recommendation) is not available, and where oscillation is not possible due to
sufficiently slow sensors.
Detecting First Application Start
Sometimes applications need to identify the first reset after a new application
image has been loaded. This can be used to initialize data in the uninitialized
EEPROM memory area supported by some Neuron Chip and Smart Transceiver
models, where regular initializers are not supported. The same feature may also
be used to set other defaults, such as enabling the ISI engine. This can be easily
accomplished by taking advantage of the fact that Neuron C variables declared
68 Mini EVK User’s Guide