BECKHOFF EtherCAT Technology Section I - Manual (Page 32)

Frame Processing
I-12 Slave Controller – Technology
3 Frame Processing
The ET1100, ET1200, IP Core, and ESC20 slave controllers only support Direct Mode addressing:
neither a MAC address nor an IP address is assigned to the ESC, they process EtherCAT frames with
any MAC or IP address.
It is not possible to use unmanaged switches between these ESCs or between master and the first
slave, because source and destination MAC addresses are not evaluated or exchanged by the ESCs.
Only the source MAC address is modified when using the default settings, so outgoing and incoming
frames can be distinguished by the master.
NOTE: Attaching an ESC directly to an office network will result in network flooding, since the ESC will reflect any
frame
– especially broadcast frames – back into the network (broadcast storm).
The frames are processed by the ESC on the fly, i.e., they are not stored inside the ESC. Data is read
and written as the bits are passing the ESC. The forwarding delay is minimized to achieve fast cycle
times. The forwarding delay is defined by the receive FIFO size and the EtherCAT Processing Unit
delay. A transmit FIFO is omitted to reduce delay times.
The ESCs support EtherCAT, UDP/IP, and VLAN tags. EtherCAT frames and UDP/IP frames
containing EtherCAT datagrams are processed. Frames with VLAN tags are processed by the ESCs,
the VLAN settings are ignored and the VLAN tag is not modified.
The source MAC address is changed for every frame passing the EtherCAT Processing Unit
(SOURCE_MAC[1] is set to 1
– locally administered address). This helps to distinguish between
frames transmitted by the master and frames received by the master.
3.1 Loop Control and Loop State
Each port of an ESC can be in one of two states: open or closed. If a port is open, frames are
transmitted to other ESCs at this port, and frames from other ESCs are received. A port which is
closed will not exchange frames with other ESCs, instead, the frames are forwarded internally to the
next logical port, until an open port is reached.
The loop state of each port can be controlled by the master (ESC DL Control register 0x0100). The
ESCs supports four loop control settings, two manual configurations, and two automatic modes:
Manual open
The port is open regardless of the link state. If there is no link, outgoing frames will be lost.
Manual close
The port is closed regardless of the link state. No frames will be sent out or received at this port, even
if there is a link with incoming frames.
Auto
The loop state of each port is determined by the link state of the port. The loop is open if there is a
link, and it is closed without a link.