Client Functionality

The PyOPC framework enables access of OPC XML-DA compliant servers by providing classes that can be used to easily create OPC XML-DA clients.

PyOPC offers two different classes for this task:

• The XDAClient class that implements simple access of OPC servers. This class does not offer concurrent connections.

• PyOPC also provides the more complicated TWXDAClient class, which is based on the Twisted framework. TWXDAClient enables concurrent client connections by utilizing Twisted's event mechanism.

These classes are contained in two different Python modules, namely XDAClient and TWXDAClient) that have to be imported before the client classes can be used. As already shown in Listing 3, global options can be defined during object creation, which then apply to all OPC operations that are handled by this client object.

Most of these options are OPC-specific and are described in [OPCXMLDA]. However, the following options are PyOPC-specific or are automatically handled by the client object:

OPCServerAddress:

This option specifies the address of the OPC XML-DA server, such as http://path/to/server. The OPCServerAddress option is mandatory and can only be applied during client object creation.

ClientRequestHandle/ClientItemHandle:

These options may help the OPC client and server to distinguish between different client requests. If these options are not specified, they will be automatically generated by PyOPC.