Beckhoff EL2124 Documentation page 66

Basics communication
Boot
In the Boot state the slave firmware can be updated. The Boot state can only be reached via the Init state.
In the Boot state mailbox communication via the file access over EtherCAT (FoE) protocol is possible, but no
other mailbox communication and no process data communication.
66 Version: 5.2 EL20xx, EL2124