EtherCAT coupler port allocation

According to the EtherCAT specification, an ESC (EtherCAT Slave Controller, hardware processing unit of the EtherCAT protocol) can have 1 to 4 ports, which it controls itself. Via an open port it can handle outgoing and incoming Ethernet traffic.
The direction of data flow in a fully wired EK1100 is shown in Fig. Internal and external port assignment for Bus Coupler EK1100:

The port assignment in the case of the EK1101, EK1501 and EK1501-0010, EK1814 applies accordingly.

EtherCAT coupler port allocation 2: — Internal and external port assignment EK1100 Bus Coupler

Frame processing sequence

The EtherCAT frame arriving at the EtherCAT signal input is passed on by Port 0 (A) to the EtherCAT processing unit.
The EtherCAT frame arrives at Port 1 (B) and the data frame departs via Port 1 (B) to the following slave in the EtherCAT terminal network (if a slave is connected there and reports "Link").
After the arrival of the data frame at Port 1 (B) from the terminal network, this is passed on to Port 2 (C) and leaves the coupler at the following EtherCAT output (if a slave is connected there and reports "Link").
The data frame arrives at Port 2 (C). This is now passed on to Port 0 (A) and leaves the EK1100 via the EtherCAT input.

	Processing of the data The data in the EtherCAT datagrams are processed only between Ports 0 (A) and 3 (D) in the EtherCAT processing unit. The non-implemented (internal) Port 3 (D) is considered to be closed and passes on the datagram to Port 1 (B).