OPC data is transmitted via data networks, in particular the Internet. A bi-directional data connection via the Internet in both directions is enabled even across firewalls and starting from an OPC client, which is not visible as a server on the Internet. To achieve this, a first connection request for establishing a first transmission channel is sent from a first data processing device of an OPC client, via a data connection, to an OPC server of an automation system. The OPC server responds to the request using a second transmission channel. The data connection is thus kept open permanently, allowing data to be sent and received bi-permanently, allowing data to be sent and received bi-directionally, the operations being temporally independent of each other, between the OPC client and the OPC server by at least one data network, in particular, the Internet.