The DA Items category provides status information about each OPC DA tag to which an OPC DA client is subscribed. Status information for OPC DA tags is shown when OPC is used to access the database. Each OPC DA tag represents a path to a value.
The status information for each OPC tag is categorized as:
- Id—The Id is a number that ClearSCADA uses to identify each OPC tag and the item to which it relates.
- Tag—Indicates the path and name of the OPC tag to which the OPC DA client is subscribed.
- Canonical Datatype—Shows the original type of the OPC data that is being requested. This can be:
- VT_BSTR—a string value
- VT_BOOL—a Boolean value
- VT_VARIANT—the type varies (and can be any of the other data types)
- VT_I1—a 1 byte signed integer value
- VT_I2—a 2 byte signed integer value
- VT_I4—a 4 byte signed integer value
- VT_UI1—a 1 byte unsigned integer value
- VT_UI2—a 2 byte unsigned integer value
- VT_UI4—a 4 byte unsigned integer value
- VT_R4—a single precision floating value
- VT_R8—a double precision floating value
- VT_DATE—a date and time value.
NOTE: These datatypes can also be arrays, in which case, the datatype is appended with |VT_ARRAY, for example, VT_UI4|VT_ARRAY.
- Requested Datatype—Shows the data type that has been requested. Some OPC clients can request data in a different type to that in which it is stored in the ClearSCADA database, for example, a real value could be requested as a string. If there is more than one client,
The datatypes that can be requested are:
- VT_EMPTY—A value has not been specified. This means that the client has not requested for the value to be converted from its original datatype. The datatype of the value that is returned will be the same as the value’s Canonical Datatype.
- VT_I1—a 1 byte signed integer value
- VT_I2—a 2 byte signed integer value
- VT_I4—a 4 byte signed integer value
- VT_R4—a single precision floating value
- VT_R8—a double precision floating value
- VT_CY—a currency (monetary) value
- VT_DATE—a date and time value
- VT_BSTR—a string value
- VT_BOOL—a Boolean value (True =-1, False=0)
- VT_UI1—a 1 byte unsigned integer value
- VT_UI2—a 2 byte unsigned integer value
- VT_UI4—a 4 byte unsigned integer value
- <Multiple Clients>—There are two or more clients requesting the same OPC data (and could be requesting different data types).
NOTE: These datatypes can also be arrays, in which case, the datatype is appended with |VT_ARRAY, for example, VT_UI4|VT_ARRAY.
- Value—Indicates the current value of the OPC tag to which the OPC DA client is subscribed.
- Value Datatype—Shows the type of the OPC data after it has been converted into the requested datatype. The possible value data types are exactly the same as those for Canonical Datatype.
- Quality—Indicates the current quality of the value. This provides an indication as to the reliability of the data.
- Time Stamp—Indicates the time at which the Value and Quality statuses were recorded.
You can use the DA Items status information to investigate Mimic Update Problems.