Generic OPC syntax

Communication Driver serves as a container for the OPC Groups, which provide the mechanism for containing and logically organizing OPC items. Within each OPC Group, an OPC-compliant client can register OPC items, which represent connections to data sources in the field device. In other words, all access to OPC items is maintained through the OPC Group.

The fully qualified name for an OPC item is called the Item ID (equivalent to the union of Link Name and Item Name). The syntax for specifying a unique Item ID is OI Server-dependent. In OPC data acquisition Communication Drivers, the syntax can be as follows:

AREA10.VESSEL1.TIC1.PLC.N7:11

where each component (delimited by a hint, that is, a period in case of Communication Driver) represents a branch or leaf of the field device’s hierarchy.

In this example:

• AREA10.VESSEL1.TIC1 is the link name for Communication Driver.

• PLC is the name of the target controller.

• N7:11 is the specific data point (Item) desired.

• An item is typically a single value such as an analog, digital, or string value.

Where Item ID describes the syntax for defining the desired data point, OPC provides for another parameter, called Access Path, that defines optional specifications for obtaining that data.

In Communication Drivers, Access Paths are equivalent to Device Groups; it is this parameter that is used to define the update interval between the Communication Driver and the field device for accessing the values of data points in the controller.