Technical data

88
W&T
3.11.6 Data model for OPC Data Access
From the view of the OPC client an OPC-DA server provides a
collection of named variables which can be read or written. Each
variable is associated which a value, a time stamp and a signal
quality, all of which are continually refreshed. In addition, vari-
ables can have other attributes written to them, item properties
which for example may contain physical units or a general
comment text.
Naming OPC variables
The names of the OPC variables generally consist of several
components, separated by decimal points, whereby each of
these name components stands for a hierarchy level within a
logical tree structure. A typical name would be for example
„Box1.Analog.2“: „Box1“ is the device associated with the vari-
able, „Analog“ refers to the ports on the device, and „2“
represents the second of the (numbered from 1 to 2) ports.
The device names are freely selectable, and if needed the other
name components can also be adapted to your own desires
using the menu point „Change OPC item name“. In addition to
the variables for OPC-DA the dialog window „Edit OPC item
names“ also shows for most devices the names of the event
sources for OPC-A&E.
Abbreviations
The variable names on the following reference pages are shown
abbreviated: The leading name component „Device name“,
which varies in any case and therefore serves at best as an
example, is always omitted. A variable shown as „Analog.1“
would in fact be accessed, depending on what kind of a device
it is located on, as for example „Box1.Analog.1“.
In addition, the following abbreviations are used for access
rights and OLE data types:
R/W: Read and wrote