BECKHOFF EtherCAT Technology Section I - Manual (Page 27)

EtherCAT Protocol
Slave Controller – Technology I-7
2.3.1 Device Addressing
The device can be addressed via Device Position Address (Auto Increment address), by Node
Address (Configured Station Address/Configured Station Alias), or by a Broadcast.

Position Address / Auto Increment Address:
The datagram holds the position address of the addressed slave as a negative value. Each slave
increments the address. The slave which reads the address equal zero is addressed and will
execute the appropriate command at receive.
Position Addressing should only be used during start-up of the EtherCAT system to scan the
fieldbus and later only occasionally to detect newly attached slaves. Using Position addressing is
problematic if loops are closed temporarily due to hot connecting or link problems. Position
addresses are shifted in this case, and e.g., a mapping of error register values to devices
becomes impossible, thus the faulty link cannot be localized.

Node Address / Configured Station Address and Configured Station Alias:
The configured Station Address is assigned by the master during start up and cannot be changed
by the EtherCAT slave. The Configured Station Alias address is stored in the SII EEPROM and
can be changed by the EtherCAT slave. The Configured Station Alias has to be enabled by the
master. The appropriate command action will be executed if Node Address matches with either
Configured Station Address or Configured Station Alias.
Node addressing is typically used for register access to individual and already identified devices.

Broadcast:
Each EtherCAT slave is addressed.
Broadcast addressing is used e.g. for initialization of all slaves and for checking the status of all
slaves if they are expected to be identical.
Each slave device has a 16 bit local address space (address range 0x0000:0x0FFF is dedicated for
EtherCAT registers, address range 0x1000:0xFFFF is used as process memory) which is addressed
via the Offset field of the EtherCAT datagram. The process memory address space is used for
application communication (e.g. mailbox access).
2.3.2 Logical Addressing
All devices read from and write to the same logical 4 Gbyte address space (32 bit address field within
the EtherCAT datagram). A slave uses a mapping unit (FMMU, Fieldbus Memory Management Unit)
to map data from the logical process data image to its local address space. During start up the master
configures the FMMUs of each slave. The slave knows which parts of the logical process data image
have to be mapped to which local address space using the configuration information of the FMMUs.
Logical Addressing supports bit wise mapping. Logical Addressing is a powerful mechanism to reduce
the overhead of process data communication, thus it is typically used for accessing process data.