To provide a bi-directional data connection via the Internet, even across firewalls and starting from a client which is not visible as a server on the Internet, a first connection request for establishing a first transmission channel is sent from a first data processing device of a client via a data connection to a server of an automation system. The 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-directionally. The operations between the client and the server are temporally independent of each other by at least one data network, in particular, the Internet. To guarantee a temporally unlimited service life of a data connection, dummy data is transmitted, for example, at regular intervals from the server to the client, even if no useful data is present, to maintain at least one data connection.