General Information on OPC DA


General Information

What is OPC DA?

OPC DA (Open Platform Communications Data Access) is a standardized interface for access to process data. It is based on the Microsoft standard COM/DCOM2 (Component Object Model / Distributed COM) that has been extended due to the requirements of data access in automation, where the interface is used to read data from or write data to the controller.

Whenever the short form OPC is used throughout this document, it is referring to OPC DA.

Examples of OPC DA clients


oPrograms collecting operating data

Examples of providers of OPC servers

oController systems

oFieldbus interface cards

What is an OPC Server?

The OPC server is an executable component that you install on your local PC. Whenever you establish a connection between an OPC DA client and a controller, the OPC server is started automatically. It provides information to the OPC DA client about variable values or states that have changed.

The OPC server provides the variables (referred to as Items in OPC) that are available on the controller (Item Pool or Address Space). These items are managed within a Data Cache that helps to provide fast access to their values. Also possible is a direct, non-cached access to the items of the controller.

In the OPC server, the items can be organized in Groups (Private and Public).

The private groups can be composed in the client arbitrarily from particular items. Initially, they do not affect the groupings in the OPC server, but if necessary can be transformed to Public Groups. Working with Private Groups, for example, is useful in order to be able to activate or deactivate certain groups of variables with just one single command, depending on whether they are to be accessible or not.

Grouped data is normally read from the OPC server coherently, i.e., the variables are to be read concurrently. However, this is not always possible in case of target systems with restricted communication buffers.

Due to the characteristics of COM / DCOM, it is possible to access an OPC server running on another computer. More than one client can access the data source simultaneously.

The OPC server is based on the PLCHandler of 3S - Smart Software Solutions GmbH. This communication module permits a direct communication to those controllers that are programmable with CoDeSys.

The OPC server V3 or later supports the following OPC specifications:

oOPC Common Definitions and Interfaces Version 1.0

oData Access Custom Interface Standard Version 1.0

oData Access Custom Interface Standard Version 2.05A

oData Access Custom Interface Standard Version 3.0

oData Access Automation Interface Standard Version 2.0

Communication between an OPC server and the EcoStruxure Machine Expert controller can be carried out via the following interface:

oGateway V3 (parameter Interface > GATEWAY3 in the OPC configuration tool)

NOTE: You can configure the OPC server for simulation with the parameter Interface > SIMULATION3, but the values of the configured variables cannot be read or written.

For information on PacDrive M controllers, refer to the Appendix.

Basic Architecture




Symbol File

The symbol file is the basis for exchanging data via the CoDeSys OPC server. If a project from Logic Builder is loaded to the controller, a symbol file is generated at the same time (also refer to, Declaring a Variable with EcoStruxure Machine Expert Logic Builder).

In the EcoStruxure Machine Expert environment, the symbol file on the controller is integrated into the application. Additionally, it is saved to the data system as an XML file with the extension *.xml in the project directory in the computer on which the project has been created. You can open the XML file to display the generated symbols.

The symbol file contains variables that are referred to as items in OPC. Use these items to address variable values in the controller.