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How to Use OPC Data Access (OPC DA)

When should OPC DA used?

The short answer is: anytime real-time data needs to be transferred. While there are many scenarios to consider, the most common categories are identified below with brief explanations of the challenges involved and recommendations on how to best address them using standard OPC components:

Data Source Data Sink (User) Resolved Issues Recommendation
Controller (ex. PLC) Application (Ex. HMI) Different vendors’ controllers use their own protocols. OPC eliminates the need for the HMI to have a ‘custom driver’ for each Controller in question. - Controller: use an OPC Server for Controller X
- Application: usually has an OPC Client built in. If one is not included may an OPC DA Client for Application Y
Controller or Device Controller or Device Controller/Controller communications are different than Controller/Application communications because both products typically use their own, vendor specific protocols. - Use OPC DA Servers for both Controllers
- Use OPC Data Manager to facilitate intelligently moving relevant real-time data between the two controllers.
Controller or Device Relational Database Relational databases communicate using Structured Query Language (SQL) over the Open DataBase Connectivity (ODBC) protocol while controllers and devices use their own custom protocols. Finding a data-bridges is difficult and often requires technical experience to set up. - Use an OPC DA Client for ODBC to capture real-time OPC Data and correctly transfer it to the database via SQL/ODBC.
- se an OPC DA Server for Device X to expose

NOTE: OPC DA is bidirectional so if needed, real-time data may be written from the database to the device, controller, or application.its data to OPC clients.
Controller/Device Process Historian Process historians are designed to capture real-time data so they often have their own native protocols and custom drivers for collecting data from various devices and applications. Here again the challenge is finding a Historian that supports existing equipment as well as future (and legacy) data sources. - Historians typically have their own protocols, almost all of them also have OPC DA Clients built in. An example of such an OPC enabled Historian is the OPC Desktop Historian.
- For the data source: use an OPC DA Server for Data Source X
Redundant Devices Controller/Application Traditionally, if the controller or application did not support device-level redundancy, additional hardware was required to make two (or more) devices redundant. - Devices require: OPC Server for Device X
- Different devices can use different OPC Servers.
- Use OPC Redundancy Broker (ORB) to implement redundancy.
- For an Application (ex. HMI or Historian): use its OPC Client for Application X
- If a controller (such as a PLC) only has an OPC Server rather than an OPC Client, use that OPC Server along with OPC Data Manager to allow ORB and the controller to exchange real-time data (bi-directionally).
Remote Devices (SCADA): ex. RTUs Application/Controller Communicating with distant devices and data sources in general is more complex due to communication breakdowns, lower bandwidth, and is also more expensive. Custom drivers address these challenges with varying degrees of reliability. Use a SCADA Class OPC Server for the Remote device. Unlike regular ‘plant edition’ OPC servers this class of OPC servers is designed to work with complex SCADA environments. (ex. MatrikonOPC Server for SCADA Modbus)

Can OPC DA be used to transfer historical data?

No. OPC DA is specifically geared to transferring current values. Once the current value is passed and the next on is read it, the OPC DA specifications have no defined interfaces for providing OPC DA Clients with past values. If you need to transfer historical data, you will need to use OPC Historical Data Access (OPC HDA) based OPC Clients and OPC Servers which, are geared specifically for that purpose.

Can an OPC DA Client from one vendor be used with an OPC DA Server from a different vendor?

Usually yes. This assumes the OPC DA Client and the OPC DA Server support the same revision of the OPC DA Specification (see above) or at least one is backward compatible.



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