Data Exchange

General

The read of input values from an OPC UA server is carried out either via subscriptions (spontaneous) or via poll groups (periodically). The preferred variant is the spontaneous transfer as here the transfer of values that have not changed is omitted and so the unnecessary load is prevented.

Subscriptions

The peripheral address defines to which subscription an address is assigned. The parameters of a subscription are defined at the corresponding data point of the type _OPCUASubscription (see Internal Data points of the OPC UA Client). This data point can be partly (some of the settings are only configurable via internal data points) configured via the subscription panel.

If all addresses of a subscription are deactivated or have been deleted, the client removes automatically the subscription on the server.

Monitored Items

The parameters of the monitored items are partly fix-coded or predefined in the subscription. That means that it is assumed that a lot of monitored items are summarized to groups with the same parameters.

Mapping of WinCC OA <-> OPC UA Attributes

Each node in the address space has attributes. These are mapped to their correspondents in the WinCC OA address space.

OPC UA status codes (see OPC UA Status Codes) have to be mapped to WinCC OA user bits in each project.

Note

With the config entry setInvalidForConnLoss all input and in/output addresses of the UPC OA client can be set invalid when a connection loss between client and server occurs. The invalid bit is reset when the next valid value is received.

If the client is shut down properly or the connection to the server is deactivated inside the configuration panel, the invalid bit is not set. It is also not set for alerts and output addresses.

General Query (GQ)

GQ for data values

The general query for data values can be triggered either for all connected servers, for one specific server or for one specific subscription.

For all servers:

Internal data point element: _Driver1.GQ.._original

For one server:

Internal data point element: _Server4.Command.GQ.._original

For one subscription:

Internal data point element: _Subs23.Command.GQ.._original

Automatic GQ

During an automatic general query on connection establishment only those data is queried, which are not in a subscription as the data in any case are sent when the subscription is created. With the aid of the autoGQ config entry the different variants of the automatic query can be set.

GQ for alarms (not supported yet)

The general query for alarms like the GQ for values can be triggered either for all connected servers, for one specific server or for one specific subscription. The data points for triggering the GQ are the same as those for the values (see above). Which GQ is triggered can be determined by the value written on the corresponding data point element.

0 -> values and alarms

1 -> values only

2 -> alarms only

Acknowledge of alarms (not supported yet)

Alarms are not acknowledged directly in WinCC OA, but the acknowledgement is passed over the A&C client to the OPC UA server. The alarm is not marked as acknowledged until the OPC server reports the acknowledge as an event to the A&C client.