PCI express for board-to-board link.

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Discussion Overview

The discussion revolves around interfacing two processing boards using PCI Express x8 connections via cables. Participants are exploring the compatibility of different connector types, specifically a 68-pin off-board connector versus a 98-pin edge connector, and the associated technical challenges such as impedance control and signal integrity.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks guidance on how to match the 98-pin edge connector to the 68-pin off-board connector and questions the standardization of the 68-pin connector.
  • Another participant inquires about the availability of application notes from Molex regarding the 68-pin connector.
  • Concerns are raised about the length of the cables and the implications for impedance control, terminations, and signal integrity.
  • A participant notes the existence of a pinout description on Molex's website for cable assemblies and mentions that impedance control information is primarily available for edge connectors.
  • There is a reference to the PCI Express External Cabling specification, which is noted to be incomplete in its draft form.
  • Another participant expresses interest in the outcome of similar attempts to interface the boards, indicating a shared challenge.

Areas of Agreement / Disagreement

Participants appear to share common challenges regarding the technical aspects of the PCI Express connection, but there is no consensus on the specifics of the connector standardization or the completeness of available documentation.

Contextual Notes

Limitations include the lack of comprehensive information on the 68-pin connector's standardization and the incomplete draft of the PCI Express External Cabling specification, which may affect the discussion on impedance control and signal integrity.

Who May Find This Useful

Individuals working on hardware interfacing, particularly those involved in PCI Express implementations and cable assembly design, may find this discussion relevant.

fiedel
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I want to interface two processing boards with PCI express x8 and connect them with cables. I find 68-pin off-board connectors for x8 connection (by molex) where as there're 98 pins for edge connectors. How do I match these 98 to 68? Are there some documents talking about this? I read PCI Express Card Electromechanical Specification, but there's only edge connector definition inside. Is this 68-pin connector also standard?
 
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fiedel said:
I want to interface two processing boards with PCI express x8 and connect them with cables. I find 68-pin off-board connectors for x8 connection (by molex) where as there're 98 pins for edge connectors. How do I match these 98 to 68? Are there some documents talking about this? I read PCI Express Card Electromechanical Specification, but there's only edge connector definition inside. Is this 68-pin connector also standard?

Does Molex have any application notes for that connector?

Beyond the electromechanical issues, how long are these cables? What is your plan for impedance control, terminations, and signal integrity?
 
Well, I see the pinout description on their website, for cable assemblies.

Molex provides several selection for cable length. Impedance control is another problem for me. I can only find such information for edge connectors. It must have been defined in PCI Express External Cabling specification. I can only find a draft of it, in which several chapters are missing...
 
I am trying to do the exact same thing. How did this work out for you?
 

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