Lenze Controller-based Automation - Manual (Page 61)

Lenze · Controller-based Automation · System Manual · DMS 1.4 EN · 04/2014 · TD17
61
8 Appendix
8.1 Communication by means of OPC
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
8.1.2 OPC as universal driver
OPC is a universal driver of the OPC Data Access specification in the automation technology.
• The universal driver encapsulates the original driver and provides the generally understandable
services via a universal interface.
• When accessing the field level, the OPC client comes upon a uniform interface.
8.1.3 OPC items
The process variables are represented by items when an OPC server is used. Here, an item is the
transport medium of a variable. Since in most cases only a part of the items provided by the OPC
server is of interest for the client, they can be summarised in groups.
In addition to the contents of the variables, an item contains various additional information 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.