Schneider Electric 890USE17700 - Manual (Page 161)

Advanced Configuration Features
890USE17700 April 2004 161
Island Fallback Scenarios
Introduction In the event of a communications failure on the island or between the island and the
fieldbus, output data is put into a safe fallback state. In this state, output data is
replaced with pre-configured fallback values, ensuring that a module’s output data
values are known when the system recovers from a communications failure.
Fallback
Scenarios There are several scenarios in which Advantys STB output modules go into their
fallback states:
z loss of fieldbus communications—Communications with the PLC are lost.
z loss of island bus communications—There is an internal island bus
communications error, indicated by a missing heartbeat message from either the
NIM or a module.
z change of operating state—The NIM may command the island I/O modules to
switch from a running to a non-running (stopped or reset) state.
z missing or failed mandatory module—The NIM detects the absence or failure of
a mandatory island module.
In all of these fallback scenarios, the NIM disables the heartbeat message.
Heartbeat
Message The Advantys STB system relies on a heartbeat message to ensure the integrity and
continuity of communications between the NIM and the island modules. The health
of island modules and the overall integrity of the Advantys STB system are
monitored through the transmission and reception of these periodic island bus
messages.
Because island I/O modules are configured to monitor the NIM’s heartbeat
message, output modules will go into their fallback states if they do not receive a
heartbeat message from the NIM within the defined interval.
Note: If a mandatory (or any other) module fails, it needs to be replaced. The
module itself does not go into its fallback state.