Each OpcDaClient has a Points device extension (OpcPointDeviceExt). Note that unlike some other drivers, other device extensions such as Schedules, Histories, and Alarms are not used.
The Points extension represents one “OPC Group,” where all Opc proxy points contained in the tree rooted by Points are members of this OPC Group. The primary interface to the Points extension is via its default view, the Opc Point Manager.
However, the property sheet of the Points extension is significant too, as shown in Figure 7.
The following properties are unique or have special importance to the OpcPointDeviceExt:
-
Read Delay
Amount of time after “device up” before the client creates a subscription and reads any values from the server.
-
Write Delay
Amount of time after “device up” before the client attempts to write any values to the server.
-
Group Name
Name of the group to create on the server for all points contained under this OpcPointDeviceExt.
-
Percent Deadband
Percentage of change in a (numeric) data item value that causes a server-initiated change-of-value message to be sent to the client (Niagara). Note this applies only to numeric Opc proxy points.
-
Update Rate
Client requested update rate in milliseconds, this specifies the fastest rate at which data changes may be sent by the server. For very large integrations, this value may need to be increased. Also see the “Server Band Width” property, one of the Status properties.
-
Revised Update Rate
(Read only) The server-returned value for the update rate.
-
Batch Limit
Specifies the maximum number of items that can be batched together into a single subscribe, unsubscribe, or synchronous read operation.
-
Message / Message Time
(Read only) If anything is wrong, check here.
-
Server Handle
(Read only) The server-generated handle for this group.
-
State
(Read only) Client group state, either
Attached,Attaching,Detached, orDetaching. -
Enable Async Io
If true, asynchronous change-of-value messages from the server are enabled. Note that Async Io is the most efficient and scalable way to integrate with an OPC server.
-
Async Io
(Read only) Whether or not async messaging is being used; some OPC servers do not support it.
-
Default Point Facets
Null, by default. If set to a non-null value, these facets are applied by the OpcPointManager to the proxy extension device facets first—they would then be overridden by any facets coming from the OPC server. These facets also affect the control point facets, since they are overridden by the proxy extensions device facets.