BECKHOFF EtherCAT Technology Section I - Manual (Page 40)

Ethernet Physical Layer
I-20 Slave Controller – Technology
5 Ethernet Physical Layer
ESCs with Ethernet Physical Layer support use the MII interface, some do also support the RMII or
RGMII interface. Since RMII/RGMII PHYs include FIFOs, they increase the forwarding delay of an
EtherCAT slave device as well as the jitter. MII is recommended due to these reasons.
5.1 Requirements to Ethernet PHYs
EtherCAT and Beckhoff ESCs have some general requirements to Ethernet PHYs, which are typically
fulfilled by state-of-the-art Ethernet PHYs.
The MII interfaces of Beckhoff ESCs are optimized for low processing/forwarding delays by
omitting a transmit FIFO. To allow this, the Beckhoff ESCs have additional requirements to
Ethernet PHYs, which are easily accomplished by several PHY vendors.
Refer to the EtherCAT Slave Controller application note “PHY Selection Guide” for Ethernet PHY
requirements and example Ethernet PHYs.
Refer to Section III for ESC specific information about supported features.
5.2 PHY reset and Link partner notification/loop closing
The main principle of EtherCAT operation in case of link errors is disabling unreliable links by closing
loops. This is automatically performed by the ESCs. The ESCs rely on the LINK_MII signal from the
PHYs for detecting the link state.
It is crucial that a PHY does not establish a link while the ESC is not operating. Otherwise, the
communication partner would also detect a physical link, causing him to open the communication link.
Subsequently, all frames will get lost because the ESC is not operating.
So at least the following requirements have to be fulfilled, otherwise frames will be lost:

ESC in reset state → PHY disabled
The recommended solution for this issue is to enable the PHY together with the ESC by using the
ESC’s reset signal for the PHY, too. If the ESC has individual PHY reset outputs, they should be used
instead. This solution prevents the PHYs from establishing a link while the ESCs are not operating.