Opc data Access Automation Specification

9

 

Appendix D- Notes On Automation Data Types 

The OPC Custom Interface allows servers to support data types including VT_I1, VT_UI2, VT_UI4, as well as 

arrays of these same data types. For a client that is developed in C++, using these data types is very straight 

forward, but for an automation client application, these data types are not natively supported.  Therefore, we have 

chosen to provide a logical mapping and conversion to those data types which are more native to automation 

client applications. 

The automation interface shall provide the standard automation data types therefore the requested data types 

that the automation client requests will be those that are natively supported by the automation applications.  The 

problem comes in, when the automation client either does not specify a requested data type, or the server 

application rejects the requested data type, and the data is then returned in the servers native canonical data 

type. 

The following is the conversion approach that the automation interface (and corresponding implementation) 

should provide to facilitate providing data values in the data type representation most suitable for automation 

applications.  A value in the canonical data types representation will be converted to the automation data types 

according to the table below.    

CANONICAL DATA TYPE 

 AUTOMATION DATA TYPE 

 VT_I1   

 

 VT_I2 

 VT_UI2 

 

 

 VT_I4 

 VT_UI4 

 

 

 VT_R8 (or VT_CY) 

 VT_ARRAY | VT_I1 

 

 VT_ARRAY | VT_I2 

 VT_ARRAY | VT_UI2   

 VT_ARRAY | VT_I4 

 VT_ARRAY | VT_UI4   

 VT_ARRAY | VT_R8 (or VT_CY) 

  

These conversions rules are only applicable when the client either has not specified a requested data type, or the 

requested data type conversion are rejected by the server application.  Note: The canonical data type that is 

returned by the automation methods, will indicate the data type natively supported by the server, and not the 

automation data type (from the table above) that the value will be converted to.  (This is for consistency with 

Custom interface client applications) 

  

 

---