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Whats the difference between Events, Alarms and Conditions?
March 23, 2020
9:58, EET
rocket science
Forum Posts: 25
Member Since:
March 16, 2017
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Currently I’m working through the OPC UA Specs regarding Events / Alarms / Conditions and I really do not understand exactly the difference between them.

My understanding would be that this are three different things, but somehow I think that Events, Alarms & Condition are more or less the same thing just used for this three different porposes.

So maybe someone can correct and/or comment the following items:

1) All 3 types Events, Alarms & Conditions are accessible e.g. in the Java SDK by adding a MonitoredEventItem to a Subscription ?

2) The Alarm is a MonitoredEventItem but mainly used for some kind of Alarms like ‘Temperatur too high’ or ‘Fluid to low’ – etc ?

3) The Condition is also a Monitored EventItem but mainly used for some kind of conditions – but what conditions? Is it something like machine is in state idle, processing ? Or does it refer to something else?

4) Events – I’m not sure if events are more some ‘umbrella term’ for Alarms & Conditions? What I would expect from an Events would be e.g. something like ‘Material started processing’, ‘Process data is available’, ‘Material finished processing’
As I have seen I can add QualifiedNames as EventFilter on the MonitoredEventItem – but is it also possible to add NodeId’s to the Events – so e.g. with Event ‘Material started processing’ I like to get the values of some nodes (like e.g. Temperature values, Voltage values), with ‘Process data is available’ e.g. some other nodes like ‘Pressure values’ of the machine and e.g. with ‘Material finished processing’ some Nodes like ‘actual piece count’
Would something like this be possible or is my understanding of ‘Events’ wrong in this case?

And … are there any demo servers where I can different usages of Events, Alarms and Conditions ?

March 23, 2020
12:09, EET
Bjarne Boström
Forum Posts: 579
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April 3, 2012
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Good Questions.

However this time I will only give a short answer now. Later me (or someone else) will probably give better one.

Alarms are a subtype of Conditions. Conditions are subtype of Events.

Basic Events do not have a “state” so to say. Conditions and Alarms have states (enabled, acknowledged etc.).

Subtypes of ConditionType are Conditions. Subtypes of AlarmConditionType are “Alarms”. Usually there are not that many Conditions that are not also Alarms, but there is e.g. a DialogConditionType that is not a subtype of the AlarmConditionType and such is not an Alarm, only a Condition. All other types are “basic events”, unless something says otherwise.

Condition state is represented by a node in the address space, for basic events there are no nodes (but you can think of a virtual node right at the moment when it is sent).

All events are is a snapshot of a Node’s sub-structure with the selected fields (and the node must be of course an Event type node). Thus state is also in node values.


1) Yes, since MonitoredEventItem monitors events and all data given by those is events

2) The only thing an Alarm is technically is a series of Events via MonitoredItems + Method Calls from the client to modify the state

3) e.g. the DialogConditionType, not sure if they have been used in practice…

4) The event filter is very complicated and very versatile. However the data is always only the EventType in question with it’s sub-structure. But nothing prevents you from modeling such types that would have a sub-node-structures that have nodes with NodeId DataType and thus Value.

For demo see the SampleConsoleServer, other than that, no.

Also you might need to read the actual specification in order to understand the whole picture, as this is like ~500 pages of information (Part 3,4,5,9).

April 29, 2020
9:11, EET
rocket science
Forum Posts: 25
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March 16, 2017
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Hi Bjarne,
thank you for that explanation!

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