Kontron COMe Starterkit Eval T2 - Manual (Page 75)

COM Express Interfaces
2.7.3. Routing Considerations
The 8-wire PHY / MDI circuit is required to meet a specific waveform template and associated
signal integrity requirements defined in the IEEE 802.3-2005 specification. In order to meet
these requirements, the routing rules in Section 6.5.7. 'LAN Trace Routing Guidelines' on page
187 should be observed on the Carrier Board.
The four status signals driven by the COM Express Module to the Carrier Board are low
frequency signals that do not have any signal integrity or trace routing requirements beyond
generally accepted design practices for such signals.
2.7.3.1. Reference Ground Isolation and Coupling
The Carrier Board should maintain a well-designed analog ground plane around the components
on the primary side of the transformer between the transformer and the RJ-45 receptacle. The
analog ground plane is bonded to the shield of the external cable through the RJ-45 connector
housing.
The analog ground plane should be coupled to the carrier’s digital logic ground plane using a
capacitive coupling circuit that meets the ground plane isolation requirements defined in the
802.3-2005 specification. It is recommended that the Carrier Board PCB design maintain a
minimum 30 mil gap between the digital logic ground plane and the analog ground plane.
It's recommended to place an optional GND to SHIELDGND connection near the RJ-45
connector to improve EMI and ESD capabilities.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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