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PCIe specification questions

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shaiko

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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 ?
 

malli_1729

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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/m...c_id=251ecec72f2dd447d967577eb21fdb7017bafda0
3. https://www.pcisig.com/developers/m...c_id=c03837e9bbea4a4e40095eb1d8230f0e6cd8e282
4. PCI Express System Architecture Book
5. https://www.pcisig.com/developers/m...c_id=6d37ec2f8543fc1f9d8ace6264d08b469f57e5f1
6. https://www.pcisig.com/developers/m...c_id=03f17116d36495f5d4360c09299f06e10266e6df
 
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    shaiko

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shaiko

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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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