An EtherCAT fieldbus system has an EtherCAT master and a number of EtherCAT slaves. The EtherCAT master and the number of EtherCAT slaves are coupled together by an EtherCAT fieldbus in order to exchange data. A method ascertains active EtherCAT slaves by way of the EtherCAT master; requests respective product codes of the active EtherCAT slaves by way of the master; ascertains a respective device identification of the active EtherCAT slaves from the respective product codes by way of the EtherCAT master, and switches the state of the EtherCAT fieldbus system into the operational state by way of the EtherCAT master if the respective device identification of the active EtherCAT slaves matches a specified device identification.