What size is a typical OPC tag structure byte

This question is usually asked by users trying to determine bandwidth requirements or memory allocation considerations.  In OPC DA, an OPC item definition consists of several parameters including:

 

[string] LPWSTR            Access Path

[string] LPWSTR            ItemID

BOOL                           Active State

OPCHANDLE                Client Handle

DWORD                        Blob Size

[size_is(size)] BYTE *    Blob – optional data handling area

VARTYPE                     Requested Data Type

WORD                          Reserved Area

 

This structure is used to manage OPC items, including adding items and validation.  This structure is different from the ‘data’ value that is passed between the OPC Server and Client whenever a data value updates.  OPC tag ‘updates’ are represented by the following structure:

 

OPCHANDLE                Client Handle

FILETIME                      TimeStamp

WORD                          Quality

WORD                          Reserved Area

VARIANT                      DataValue

 

These structures are shown to provide a general ‘rule of thumb’ for typical OPC tag structure sizes.  A full understanding of the OPC specifications would be needed to discuss the real use of these structures and the many other associated parameters.  An end-user of OPC products should generally not be concerned with this level of detail, as a well written OPC server will optimize data transfer and other programmatic concerns.