Alarms and Conditions

This chapter describes the OPC UA Alarms and Conditions (A&C) functionality of the WinCC OA OPC UA server.

Requirements and Restrictions

To send alarms via OPC UA for data point elements of a project, server-sided the following requirements must be fulfilled:

  1. For the data point elements an alert handling config (_alert_hdl) must be configured.
  2. The data point elements must belong to the group OPCUAAlarm or the data point element must be exported using a CNS view with the access level Alarm activated.

Excepting the acknowledgement, an OPC UA client has only read access to the OPC UA alarms. A configuration via the OPC UA client is not provided. Acknowledge() is the only OPC UA method that an OPC UA client is able to call in the OPC UA server to acknowledge alarms. All other method calls return the error code OpcUa_BadNotSupported.

Supported types of alarms and acknowledge

WinCC OA provides a very comprehensive alarm model, which includes various types of alarms and acknowledgement. A consistent mapping of all combinations to OPC UA is currently not possible. Because of this, the WinCC OAOPC UA server supports only a limited number of all possibilities (these apply only for the server - for the matrix for the client see Supported alarm types and restrictions). These are shown in the following matrix. Regarding the description of the types of alarms or acknowledgements see Alert handling, basics and Alert states.

Note: The acknowledgement type CAME and WENT must be acknowledged is not not possible for impulse alarms (see Alert handling, basics) . For that combination N/A (not applicable) is contained in the table below.
Not acknowledge-able Acknowledge deletes CAME is acknowledgeable CAME or WENT must be acknowledged CAME and WENT must be acknowledged
Binary alarm ✔ ✔ ✔ ✔ ✔
Binary alarm (discrete) ❌ ❌ ❌ ❌ ❌
Binary alarm (impulse) ✔ ✔ ✔ ✔ N/A
Binary alarm (discrete, impulse) ❌ ❌ ❌ ❌ N/A
Alarm of continuous values (n ranges) ✔ ✔ ✔ ✔ ✔
Alarm of discrete values (n ranges) ✔ ✔ ✔ ✔ ✔
Alarm of discrete values (impulse, n ranges) ❌ ❌ ❌ ❌ N/A

OPC UA Alarm Model Compliance

WinCC OA has a rather complex and historically grown alarming model, which is not in all configurations compliant to the IEC 62862 based alarm model of OPC UA. In order to get an OPC UA compliant alarm model the following configuration must be considered

  1. Do not use the acknowledgement mode "Acknowledgment deletes", because this is not compliant to IEC62682, what the OPC UA alarming model complies to.
  2. The option "Acknowledge old alarms" must be activated for the acknowledgement modes "Came or Went" and "Came and Went" since older alarms cannot be simply replaced by newer ones within the OPC UA alarm model.
  3. WinCC OA alarms of acknowledgment modes "Not Acknowledgeable" and "Came is Acknowledgeable" are modelled as auto-acknowledged OPC UA alarms because such alarms might have already an "acknowledged" state (AckedState: TRUE) when they are raised.

Mapping to the OPC UA alarming model

The WinCC OA alert handlings are mapped to an instance of an WinCC OA OPC UA alarm type in OPC UA. The instances of the alarm conditions are organized below the OPC UA Server object in the same hierarchy as the corresponding data point elements, and are basically reachable by browsing an OPC UA client. The details of the WinCC OA alert handlings are not written to the alarm types. This means, the alarm limits, alarm classes and alert texts are not visible for OPC UA clients.

In the OPC UA address space EventSource items and Conditions are created for the data point elements with an alert handling which are exported via OPCUAAlarm or the CNS View with Alarm access bit set. These can also be found using browsing.

All changes on WinCC OA alert handlings are updated in the OPC UA server without needed restart.

The WinCC OA alarm types are derived from the OPC UA AlarmConditionType alarm type. According to the A&C specification Shelve and Confirm are optional, have no equivalents in WinCC OA and thus they will not be supported by the OPC UA A&C server.

Depending on the WinCC OA alert handling type the following OPC UA alarm types are created:

WinCC OA alert handling type OPC UA alarm type
Binary alarm OffNormalAlarmType
Multi instance alert of continuous values PvssContinuousAlarmType
Multi instance alert of discrete values PvssDiscreteAlarmType
Summary alarm TripAlarmType

These OPC UA alarm types also are viewed in the type tree in the OPC UA address space.

Mapping of multi instance alerts

Following mapping will be used for the usage of multi instance alerts in WinCC OA:

WinCC OA alert handling type OPC UA alarm type
Binary multi instance alert OffNormalAlarmType
Non-binary multi instance alert PvssContinuousAlarmType