The EL6688 EtherCAT Terminal presents itself as a device in the IEEE 1588 synchronisation system with support for PTPv1 (IEEE 1588-2002) and PTPv2 (IEEE 1588-2008) based on Ethernet.

On the one hand, the EL6688 is an IEEE 1588 clock (master or slave) that is synchronised based on the protocol precision. On the other hand, it is synchronised as an EtherCAT Terminal by the distributed clock system. The following operating modes can be selected via the TwinCAT System Manager: "SlaveOnly", "MasterOnly" and "Best Master Clock". In this way, a consistent timebase can be created across applications for any number of spatially separated EtherCAT systems and machine sections, e.g. for application with axes or measurement technology. The compact EtherCAT Terminal enables flexible deployment depending on the application requirements.

https://www.beckhoff.com/vi-vn/products/i-o/ethercat-terminals/el6xxx-communication/el6688.html