BECKHOFF EtherCAT Technology Section I - Manual (Page 42)

Ethernet Physical Layer
I-22 Slave Controller – Technology
5.4 RMII Interface
Refer to Section III for ESC specific RMII information.
If an ESC RMII interface is not used, LINK_MII has to be tied to a logic value which indicates no link,
and RXD, RX_ER, and especially CRS_DV have to be tied to GND. The TX signals can be left
unconnected, unless they are used for ESC configuration.
For more details about the RMII interface, refer to the RMII Specification, available from the RMII
consortium.
5.5 RGMII Interface
Refer to Section III for ESC specific RGMII information.
If an ESC RGMII interface is not used, LINK_MII has to be tied to a logic value which indicates no link,
and RX_CLK, RX_CTL, and RXD have to be tied to GND. The TX signals can be left unconnected,
unless they are used for ESC configuration.
For more details about the RGMII interface, refer to the “Reduced Gigabit Media Independent
Interface (RGMII )” specification version 2.0.
5.5.1 RGMII In-Band Link Status
Some ESCs support RGMII PHYs. With these PHYs, it is possible to use the RGMII In-Band Status
instead of the LINK_MII signal. In this case, the LINK_MII input of the ESC has to be tied to a value
which indicates “no link”. The RGMII In-Band status is checked to indicate a 100 Mbit/s Full-Duplex
link. The In-Band status is equivalent to the LINK_MII signal.
NOTE: Some PHYs require that RGMII In-Band Status is enabled by writing to the PHY management registers.
This cannot be done by the ESC itself.
5.6 Link Detection
All ESCs support a LINK_MII signal for fast link detection at each Ethernet MII port. Some ESCs (e.g.,
EtherCAT IP Core) additionally support link detection and configuration via the MII management
interface. Both the LINK_MII signals and the MI Link Detection and Configuration results (if available)
are combined to determine the link state of each port, which is reflected in the ESC DL Status register
(0x0110[15,13,11,9]
– Communication established). Using the LINK_MII signal is mandatory since it is
the only way to achieve fast link loss reaction times.
Table 14: Registers used for Ethernet Link Detection
Register Address Name Description
0x0110:0x0111 ESC DL Status Link Status (Link MII, Communication established)
0x0518:0x051B PHY Port Status MI Link Detection results if available
5.6.1 LINK_MII Signal
The LINK_MII signal used for link detection is typically an LED output signal of the Ethernet PHY. If
available, LINK_MII should be connected to a combined signal indicating a 100 Mbit/s Full Duplex link.
If such a signal is not available, a signal indicating a 100 Mbit/s link (speed LED) might be used. If only
a Link signal is available (link LED), this might be used. Never use (combined) activity signals, e.g.,
Link/Act LED outputs, because the link state will toggle upon activity.
The main advantage of using a dedicated link signal instead of reading out MII management interface
registers is the fast reaction time in case of a link loss. This is crucial for redundancy operation, since
only one lost frame is tolerated. The EtherCAT port of an ESC which loses a link has to be closed as
fast as possible to maintain EtherCAT communication at the other ports and to reduce the number of
lost frames.
The LINK_MII signal state is reflected in the ESC DL Status register (0x0110[7:4]).