Control technology| System manual
Appendix
Basics of the OPC standard
52 L DMS 1.3 EN - 02/2011
8.1.2 OPC as universal driver
The OPC-Data Access specification provides a universal driver for the automation techno-
logy. The universal driver encapsulates the original driver and provides the generally un-
derstandable services via a universal interface. The application of the automation
technology comes across a uniform interface when accessing the data on the field level.
8.1.3 OPC items
The process variables are represented by items when an OPC server is used. Thus, an item
is the transport medium of a variable. Since in the most cases, only a part of the items pro-
vided by the OPC server is of interest for the client, they can be summarised in groups. "dif-
ferent
In addition to the contents of the variables, an item contains various additional informati-
on for the data. This additional information is called metadata.
An item is assigned to the following metadata:
Item definition: Unambiguous string of digits to identify an item
Access rights: Access authorisation to the item (reading, writing, or both)
Canonical data type: original data type as in the field device
Time stamp: Time of the last change of an item
OPC quality: State of an item, references to communication errors
An OPC client either receives the values of the items from the PLC or from the item cache
of the OPC server.
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