3 REDS REDUNDANCY SYSTEMS
3.1 DIAGNOSTIC PROCEDURES
The messages exchanged by the controllers over the redundancy cable are of two different types:
1. Notification of the redundancy role
2. Diagnostic status (presence sensor and encoder status)
Notifications are used both at initialization time and runtime (cyclic notifications).
Diagnostic status exchange (cyclic) conveys diagnostic data and is used only by the Standby
controller to validate the status of the active controller.
In order to enhance system reliability, in the event the response to the diagnostic polling is missing
over the serial channel, the request is redirected to the Lonworks network.
3.1.1 Startup
The initial status is determined from a lookup table that summarizes the description of all the
possible startup scenarios. The status assigned at startup depends on the assigned topology role,
redundancy role notifications, status of each controller at the moment of the role negotiation
(startup, initialization, runtime).
If the controllers are rebooted, the role negotiation algorithm has no memory about the role each
controller played before being powered down.
3.1.2 Runtime
During normal operations the controllers cyclically exchange data over the redundancy cable
(cyclic role notifications, cyclic encoder and presence sensor diagnostics).
The Standby controller compares its own presence sensor and encoder diagnostics to validate the
role of the Active controller.
Also the Active controller monitors the status of the Standby controller in order to raise an alarm
that alerts about the unavailability of the back up system.
3.1.3 Switch Over
The decision to switch over is taken by the Standby controller when it is sure that taking over
improves the overall performance of the whole system.
3.1.4 Role Conflicts
A role conflict may occur under various circumstances. In the event of a role conflict an automatic
negotiation will lead to the automatic correction of the configuration and definition of the correct
roles.
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