5. CONFIGURATION
ATTENTION
In the Nexto PLC, the events queue is stored in a non-retentive memory area (volatile). This
way, the events present in CPU’s queue, which haven’t been transmitted yet to the control
center, are going to be lost in a CPU’s eventual power off.
The CPU’s event queue is redundant, that means it is synchronized each cycle between both CPUs, when is used CPU’s
redundancy. Further information can be found on the section about CPU redundancy.
The in and out of events in this queue follows the concept of producer/consumer. Producers are those system elements
capable of generate events, adding events in the CPU’s queue, while the consumers are those system elements which receive
and use this events, taking them of the CPU’s queue. The figure below describes this working, including the example of some
events consumers and producers.
Figure 55: CPU’s Event Queue
5.5.3.1. Consumers
The consumers are typically communication drivers that will communicate with SCADA or HMI. After been stored in
CPU’s queue, the consumers receive the events related to communication points mapped in its configuration. These events are
then stored in a consumer’s own events queue, which the size and working are described on the communication driver specific
section.
5.5.3.2. Queue Functioning Principles
Once stored in CPU’s queue, each event is transmitted to the consumer that has this communication point in its data base.
On the figure above, the Event 0 is referred to a communication point mapped to two control centers IEC 60870-5-104 (Client
1 and 2). Thus the Event 1 is referred to a communication point mapped only to one control center IEC 60870-5-104 (Client
2). By its time, the Event 2 is referred to a communication point mapped to another control center IEC 60870-5-104 (Client
3).
The events remain stored in the CPU’s queue until all its consumers acknowledge its receiving. The criteria used to confirm
the receive is specific of each consumer. In case of the IEC 60870-5-104 Server, the acknowledge occurs when the event is
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