Time-tagged data via OPC ?


Thread Starter

D. Zink

Dear List, basically I'm clear about the function of OPC connections although I did not try it yet.
I found hundreds of OPC servers for many different protocols on the WEB. Among them are DNP3.0 and IEC 60870-5-101. Both protocols can carry time-tagged data. Is it possible to transmit data from one protocol to the other using OPC while keeping the original source time-stamp ?
Any hint or experience is welcome.
To allow the transfer of time stamp from one OPC Server to another OPC Server you need a gateway (basically an OPC Client) to read the VQT (Value, Quality and time stamp) of data from one OPC Server and write that VQT to another OPC Server.

Although the Standard OPC Interface allows the reading of the time stamp of data, It does not allow the writing of time stamp data (i.e specify the time stamp of the data).

To support the transfer of time stamp information, the destination OPC Server have to add a non standard interface to allow the writing of QTV information.

interface IOPCAsyncIOQVT : IUnknown
HRESULT Write([in] DWORD dwConnection,
[in] DWORD dwCount,
[in, size_is(dwCount)] OPCHANDLE * phServer,
[in, size_is(dwCount)] OPCITEMSTATE * pItemValues,
[out] DWORD * pTransactionID,
[out, size_is(,dwCount)] HRESULT ** ppErrors);

Our DNP 3.0 OPC Server and Gateway supports the above interface. For example, our Gateway can read the QTV data from an IEC 60870-5-101 OPC Server and transfer that information to our DNP 3.0 slave. A remote DNP 3.0 Master accessing the DNP 3.0 slave will get the unmodified QVT data from the IEC 60870-5-101 OPC Server.

[email protected]
There are products available that offer bridging between OPC Servers, but OPC clients are not provided a mechanism to specify the time stamp when writing a value to an OPC server. Further, the OPC Item Properties (where data quality is stored) are read only to OPC Clients. This would seem to prevent the preservation of timestamps by an application that uses OPC as it's only bridging mechanism.

Hope this helps,
Jeff Dean
[email protected]