Echelon ISI - Manual (Page 94)

ISI Programmer’s Guide 92
for the device and all devices that it is connected to using the isiOpen and
isiCreate requests.
To support controlled enrollment for monitoring, a controller must also recreate
monitored connections to a device when the device overwrites on clears any
connections created by the controller. There are two cases where this may
happen. A device that supports connection replacement may overwrite a
controller connection by creating a new connection that replaces the controller
connection. Any device may overwrite a controller connection by resetting the
device to factory defaults, clearing all connections. Either case can occur if the
device supports manual enrollment and the user manually adds the device to a
connection, or if the device supports automatic enrollment and the device
automatically joins a connection. In the first case, the controller can monitor all
CSMO and CSMA messages, and silently add itself to the connection if the
connection will overwrite a monitored connection. A controller silently adds itself
by not issuing a CSMA message, but otherwise doing all the steps described in
Enrolling in a Connection in Chapter 2 to enroll in a connection. In the second
case, the controller repeats the procedure described above to create a monitored
connection using controlled enrollment. The controller can also periodically
recover the connections from all devices in the network to ensure any monitoring
connections are still intact. If the controller discovers any missing monitoring
connections, it can recreate them. This periodic traffic should be minimized to
reduce network overhead. For example, sending one request every 10 seconds to
recover a connection should be sufficient.
Automatic Enrollment
Automatic enrollment, as described in Enrolling in a Connection in Chapter 2,
provides an easy way for a controller and monitored device to work together to
support event-driven updates. This provides a method that requires fewer
network transactions than controlled enrollment, and no wait states, at a cost of
at least a few hundred bytes on the monitored devices. This method requires the
monitored device to support automatic connections and connection replacement,
and also requires the device to be more involved in creating the monitored
connection, since the monitored device will determine which network variables on
the device will be connected to the controller.
For a controller to use automatic enrollment for monitoring, the controller
becomes the automatic connection host for data used by the controller. Standard
ISI profiles define standard functional profiles called monitor points that
controllers can use to request data to be monitored from monitored devices, and
that devices can identify as connections offered for monitoring use.
For a device to support automatic enrollment for monitoring, when a monitored
device receives a monitor point automatic invitation, the device application
follows these steps:
1. If the device supports automatic enrollment for monitoring and also
supports connection extension, the device extends the existing connection
by accepting the connection invitation. If the extension fails, the device
proceeds with the next step.