The ItemID is the completely eligible definition of an item of data in the server, generally submitted to as the WHAT. There is no other information is needed to categorize the data item for the client to be capable to read and write values.
The Item definition called as ItemID, applied in the OPCITEMDEF and somewhere else is a nil-terminated string that exclusively recognizes an item of OPC data. The identifier syntax is server dependent and it presents a reference or key to an item in the source of data. The item is the thing that can be characterized by a VARIANT even though it is usually a single value for example an string value, digital or analog.
For instance, an item FIC101 may stand for an entire record for example a Fieldbus, ProfiBus or Hart Foundation data structure. Such behavior is particularly enabled but not needed by OPC - the go back of such structures is measured to be vendor particular behavior. Alternately FIC101.PV may correspond to one aspect of a record for example the process value. This would possibly get the double form which could be utilized by any client.
Since the itemID syntax is server specific, added information for instance Signal conditioning, Engineering Units Scaling and Counts information could be inserted in the string definition.
An OPC is defaulting to the server of data type for all items. This has run so far so it hasn’t set any items to something other than server. Currently that they are having a problem with some tags of digital output not corresponding to the PLC, the support of GE is asking to locate the data type to Boolean.
The Server default setting lets the server of OPC to transfer the data type of the item to be converting and they accept it. If you desire to override that data type you can complete the field of data type manually. The Data type can be consideration of as a characteristics definition of the data. It will state how the SCADA handles and what it anticipates for the data properties. The definitions of data type in the driver of OPC client help can be set up under the help topic Item properties and are obviously stated. You can also visit to the opcfoundation.org for detailed info about specifications of OPC in general. What other detailed information are you interested in, you can ask for a specific question about the data types.