Buffer Alarms - Allen-Bradley Logix5000 Reference Manual

Chapter 1 FactoryTalk Alarms and Events Logix-based Instructions (ALMD, ALMA)

Buffer Alarms

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In order to receive controller-based alarm messages, alarm clients (such as an
RSLinx Enterprise server) must establish a subscription to the alarms in the Logix
controller. The controller maintains a connection to each subscriber and
monitors the status of that connection.
As alarm state changes occur, the alarm instructions in the controller cache the
necessary information (such as timestamps and associated tag values) and request
the transmission of the alarm message to all of the subscribers. The publisher
mechanism delivers the alarm messages to each subscriber as quickly as possible.
If any subscriber fails to confirm receipt of the alarm message, or if the connection
to a known subscriber is not good, the publisher mechanism stores the
undelivered alarm messages in a 100 KB buffer. Each subscriber has its own buffer
so communication problems with one subscriber do not interfere with alarm
delivery to other subscribers. When the buffer is full, newer alarm messages are
discarded. The buffer is created when the subscriber establishes its initial
connection and is maintained for a configurable length of time (0...120 minutes,
default is 20 minutes) after a subscriber loses its connection.
When the subscriber re-establishes a connection within the buffer time-out
interval, it obtains the current state of all alarms, begins to receive current alarm
messages, and also uploads any buffered messages that may have accumulated.
Even if the buffer was full, and messages were discarded, the subscribers accurately
synchronize to the current state of the alarms (including the most recent
InAlarmTime, RetToNormalTime, and AckTime timestamps).
The buffer continues until is filled. Once filled, the buffer stops adding alarm
transitions until space is made available in the buffer by the subscriber.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011