OPC: Frequently Asked Questions

OPC: Frequently Asked Questions

Is the term OPC a synonym for field bus?

OPC is not a field bus, it is a communication standard between two programs and is based on OLE. OPC stands for Open Platform Communication (formerly: OLE for Process Control). Most applications that use this communication standard are in the automation and the process control. You usually use OPC Servers to acquire and to output data in field bus systems, for example, profibus or interbus, with Windows applications.

The type of data exchange with OPC is similar to the DDE data exchange. DDE is a standardized form of data exchange between Windows applications whereby in special cases DDE servers also acquire field bus data. An OPC server fulfills the same function for the Windows application. The OPC server acquires data, for example, in a field bus and transfers this data to the application.

What is the DCOM configuration?

The DCOM configuration in Windows is where you or your system administrator specify how your OLE client applications can access other OLE servers. OPC is based on OLE technology. If you cannot find your OPC server with DIAdem, check the settings of the DCOM configuration. You can find the respective program under DCOMCNFG.EXE.

Why is the OPC block dimmed in the DIAdem DAC function group?

Check whether the GfSOPC-DLL is marked "not loaded" in the GPI-DLL registration although the DLL is in the specified folder. Possible causes are:

• You registered the GfSOPC.DLL under Settings»Extensions»GPI Extensions. You must not register the GfSOPC.DLL as GPI-DLL.

• Copy the GfSOPC.DLL into the DIAdem program folder. This problem can only occur in the GfSOPC.DLL of the 6.00 DIAdem version.

Why does my system get stuck when a certain OPC server is selected?

Windows cannot find the OPC server. If another computer executes the OPC server, the network search might last a while. You can operate DIAdem again after a timeout. Specify the timeout in the Settings for OPC Server dialog box.

What does the message that an OPC server is not accessible mean?

First check with another OPC client on your computer whether you can access the OPC server. You can find clients on the Internet free of charge or on the CDs from the manufacturer of the OPC server. If you cannot establish connection to another client either, you might have registered the server incorrectly or you made incorrect settings in the DCOM configuration.

Which OPC specification does DIAdem support?

The driver supports in its current version the OPC specifications Data Access 1.0A and Data Access 2.0.

The program crashes when DIAdem closes (runtime error)

If DIAdem uses blocks with asynchronous data transfer, the program might crash when DIAdem finishes (runtime error).

1. This problem can occur when the OPC Client (DIAdem) calls functions of the OPC server asynchronously and the OPC server runs on a remote computer. Asynchronous data transfer means that the OPC server sends a message to the OPC client to inform about changes to data or the occurrence of an event. The OPC server requests a so-called callback function of the OPC client. This is only possible if the safety settings of the OPC server are set accordingly in the DCOM configuration. The respective setting is in the DCOM configuration of the computer on which the OPC server is active. Select the Interactive user setting on the Identity tab in the OPC server properties. You also can specify that the OPC server is executed with the same name and same password with which you are logged into the computer with the OPC client.

2. Some OPC servers do not correctly release the OLE interfaces used for asynchronous data transfer. This applies to OPC servers that are not implemented as required by the Microsoft OLE documentation. Contact your OPC server manufacturer or disable the option for asynchronous communication.

The program crashes after the connection to an OPC server was lost

Complete the following steps to prevent the link to the OPC server from failing:

• Ensure that your network is running smoothly.

• Do not close the OPC server manually. DIAdem automatically closes the OPC server when the program no longer requires the server.

• Do not close the OPC server with the Task Manager.