Method and system for coupling data networks

A method for transmitting data of a first user of a first data network being an isochronous real-time Ethernet to a machine coupled with a second data network having a communication protocol different from that of the first data network, comprising the steps of:

• monitoring a loop control using real-time-critical data in the machine coupled with the second data network by transmitting data from the first data network in at least one first transmission cycle which is subdivided into a first area for transmitting real-time-critical data at defined times between the first user and the machine and a second area for transmitting non-real-time-critical data, to the second data network having means for transmitting data in at least one second transmission cycle which is subdivided into a third area for transmitting real-time-critical data and into a fourth area for transmifting non-real-time-critical data,
• transmitting real-time-critical data of the first area of the first transmission cycle to the machine in the third area of a second transmission cycle following the first transmission cycle, and
• synchronizing the second data network by means of a first master clock generated within the first data network, wherein the first data network sends out real-time-critical cycle synchronization messages for synchronizing local relative clocks of users of the first data network, and wherein the switching unit transmits the real-time-critical cycle synchronization messages from the first area of the one transmission cycle into the third area of the second transmission cycle.