OPC HDA (OPC Historical Data Access) standardizes
communication for retrieving archived
OPC HDA is the solution for accessing historical data across multiple vendors.
OPC HDA will do for real-time databases what Microsoft's ODBC
(Open Database Connectivity) did for relational databases. Recall that even as
recently as 1995, ODBC was only available for a select group of relational
database products. But today, it is rare to find a large-scale Microsoft
Windows-based deployment of a non-ODBC compliant database. Similarly, with
OPC HDA, today's standards-savvy engineers will be much better prepared to
escape the limitations of proprietary process historian solutions.
Eliminate custom driver development costs
Adapt rapidly to changing information management needs
Connect to any Process Historian
Seamlessly change data sources using off-the-shelf software
In the past, each Process Historian had its own proprietary
communication method. Applications that made use of historical
process data were always tightly coupled to their respective Process
Historian, or at the very least, they required a proprietary
driver to make the connection. When someone needed data from a
different Process Historian, a new driver would have to be written.
This limited the effectiveness and reach of the applications because
the necessary drivers were usually unavailable.
The problem affected everyone:
Process Historian vendors could not make their data available to all applications
Application vendors could not access data from all process historians
Because of the above, users could not select a best-of-breed solution
OPC HDA standardizes the communication between the Process Historian and its
associated applications. Consequently:
HDA enables any application to connect to any data source.
Any application that makes use of historical data can seamlessly change its data source without any reprogramming
Users have the flexibility they need to rapidly adapt to changing information management needs.
Thus, like OPC DA, OPC HDA enables smooth interoperability between the
applications that archive data and the applications that require it.
Relationship of OPC HDA with other standard specifications
OPC HDA is similar to other well-known and accepted communication standards.
The following is a brief comparison to the most popular standards.
Relationship with OPC DA: past and present OPC HDA and OPC DA are very
close cousins and widely complementary protocols. Both have the same data
types, and both transfer process values. In fact, the biggest difference
between the two is that DA transfers real-time values, whereas HDA transfers
archived values. For instance, a typical Process Historian would have an
OPC DA client to access real-time values from a control system. The Process
Historian would then make this data available to everyone using an OPC HDA
server. In other words, all values archived using the DA client can be
accessed using HDA. So DA is used to find the current value of a point
(or tag), while HDA is used to find the historical values of that point.
Relationship with OPC XML: viewing over the Internet Although OPC XML
and HDA are intended for different uses, they are in fact complementary.
XML brings process control data to the Intranet and Internet in general,
and to the web specifically. Once users move their applications from the
Intranet to the Internet they quickly find that their applications can no
longer be used for real-time control and display of data that changes rapidly.
After all, Internet access times and bandwidth can rarely be guaranteed.
Rapid access and retrieval of historical data is more important than ever,
and an OPC XML server can get its data source directly and quickly from an HDA
server so that historical data can be accessed from anywhere on the Internet.
Relationship with ODBC: relational versus process data ODBC (Open
Database Connectivity) is a highly accepted standard for Microsoft
Windows-based RDBMS (Relational Database Management Systems). Therefore,
both ODBC and OPC HDA are used for retrieval of historical data. The
difference is that ODBC is optimized for relational data, whereas OPC
HDA is optimized for process data. (Although a discussion on the
differences between an RDBMS and a Process Historian is an interesting
one, it is the subject for another day.) While ODBC can be used in place
of OPC HDA, the implementation time will be much greater, and the
resulting performance will be poor in comparison with a pure OPC HDA
Data Access Specification 1.2 [PDF 6,425 kb].
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HDA products section.