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  1. #1
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    PCIe specification questions

    Hello,

    As I don't have the PCIe specifications at hand - a few questions:

    1. Are isolation capacitors on the Tx lines a must? What is the motivation behind the DC isolation?
    2. What is the voltage levels of the signals ?
    3. Is there a DC offset ?

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    Re: PCIe specification questions

    1. Are isolation capacitors on the Tx lines a must? What is the motivation behind the DC isolation?
    - Yes, These capacitors are must. In PCIE system, This is few lines from Spec
    "The PCI Express add-in card and system board shall incorporate AC coupling capacitors on the
    Transmitter differential pair. This is to ensure blocking of the DC path between the PCI Express
    add-in card and the system board. The specific capacitance values are specified in the PCI Express
    Base Specification, Revision 2.0. "
    I am exactly not sure about How much is common mode voltage allowed? Some references say AC common mode voltage of 0.3V.
    Bottom line " If you are sure that in Your PCIE system, no common Mode voltages exist b/w TX and RX you can remove them" Ex: DMI interface B/W CPU and Controller HUB(PCH/ICH) of intel Architecture. they will follow PCIe signalling, but they won't use AC coupling. Because they are sure about the interface common mode voltage requirtements.

    For the Other 2 Qtns:
    Even though we can't download the PCIe Spec, there are so many app notes PCI-SIG which gives details abt Electrical signalling, Mecahnical formfactors etc.
    Refer pci-sig.com

    few app notes....

    1. PCIE 1.1 CEM
    2. https://www.pcisig.com/developers/ma...1fdb7017bafda0
    3. https://www.pcisig.com/developers/ma...230f0e6cd8e282
    4. PCI Express System Architecture Book
    5. https://www.pcisig.com/developers/ma...d08b469f57e5f1
    6. https://www.pcisig.com/developers/ma...9f06e10266e6df


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  3. #3
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    Re: PCIe specification questions

    I couldn't find any definitive answer in the links.

    1. What is the voltage for a logic '0', what is the voltage for a logic '1' ?
    2. What is the purpose of the DC offset voltage? Is it a must?



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