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OPC Item Property Set

Every item defined by an OPC server has specific attributes, or properties, that describe that server item in more detail. These properties include the current Value, Quality and TimeStamp for the server item, plus additional properties that a server may require in order to determine the quality of a value, or to decide whether to generate a DataChange event for groups that have a nonzero DeadbandPercent value. Exposure of the server item properties to a client is intended to provide a client with more information on a specific item, and is not intended to provide efficient access to large amounts of data. Rather, you should use the read function to read data from a large number of server items.

Each property is identified by a Property ID, or PropID, which is an integer value. The OPC Data Access Specification defines three sets of these properties, based on their PropID.

OPC Item Property Sets

Set Name

ID Range

Description

OPC Specific

1-99

Information directly related to the OPC server for that item.

OPC Recommended

100-4999

Additional information which is commonly associated with items, such as ranges of valid values, alarm limits, etc.

Vendor Specific

5000 or greater

Specific properties defined by an OPC server vendor. Since these vary from vendor to vendor, the actual descriptions are not presented in this appendix.

Each of the property sets defined by the OPC Foundation is presented in the following sections.

    Note   OPC servers must implement the OPC specific properties. However, the recommended properties are not mandatory, and an OPC server could provide any subset of the recommended properties, or none of them.

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