PCIe Bifurcation - Page 1
Forum section: electronics / beginners

author: QinX
author_rank: Contributor
timestamp: July 31, 2015, 12:19:47 am
content: Hey everyone,Relevant Datasheets:https://www.idt.com/document/dst/9db233-datasheethttps://www.idt.com/document/apn/808-pci-expresshcsl-terminationhttps://www.idt.com/document/dst/9db433-datasheethttp://hardforum.com/showthread.php?t=1870298Over on ardForum we are trying to figure out how to get PCIe Bifurcation working with the ASRock x99e-itx/ac, a ASRock technical support engineer confirmed that it should work on this board.Right now some guys are trying out different PCIe riser cards, unfortunately without luck, I believe this is mostly because SuperMicro tends to do their cards a little different, because they have control of which servers support this risers.However on of the cards is bases on a PLX chip and should work even with PCIe bifurcation, but that is a different story. I've bought and retraced a SuperMicro card that shows that they made some changes that don't match with the PCIe specificationRight now I'm trying to figure out what is needed to make this PCIe bifurcation work on the PCIe riser side.I think the only thing that is needed is a seperate REFCLK+ and REFCLK- signal for each PCIe slot, so that needs to be duplicatedThis should be achievable with the IDT 9DB233. Now I'm not electrical guru like most of you guys, so bear with me and my noob questions please.Basically this PCIe riser is always going to be used with 2 PCIe cards, so I don't need a function to turn of 1 of the differential signals. I think because of this I could get away with a fairly "dumb" setup.I've got some questions that I can't find an answer to.1) Are the pin connections I've made below correct?2) Do I need a capacitor between each 3.3V input and GND? if so how do I determine which ones?3) I don't need the SMBus I think the default settings I get the are. PLL Bandwidth = LOW and PLL enabled (ZDB mode)What does ZDB mode mean, I'd guess Zero DB Mode?4) On page 9 of the 9DB233 datasheet there is a some info about the differential trace and some resistors, I didn't know what HCSL meant so I searched and found the second link above.Am I correct in saying I have an HCSL signal going from the motherboard to the 9DB233 and then 2 HCSL signals going from the 9DB233 to both PCIe cards?5) Do I need the Rs(33ohm) and Rt(49.9ohm) resistors before and after the 9DB233 or only after?6) In the last image you can see an example of a PCIe riser that seems to have the same type of chip, it's a 28-pin package which I think might be the 9DB433, third link.It looks very simple and that's why I think the 9DB233 might be doable. Or am I missing something?7) Because I'm a noob the obvious question is, did I miss anything?PS: Yes I have a tendency to have an idea and dive right into it without proper base knowledgeThanks for any feedback/input!

author: QinX
author_rank: Contributor
timestamp: August 02, 2015, 05:35:13 pm
content: Small bump!We actually got the PCIe splitters working now, but I'm still looking to building my own PCIe riser to use with a case I'm designing.So any feedback is still very much appreciated.

author: QinX
author_rank: Contributor
timestamp: August 08, 2015, 02:13:14 pm
content: Another bump, nobody interested or able to provide feedback?

author: German_EE
author_rank: Super Contributor
timestamp: August 08, 2015, 05:46:12 pm
content: This is WAY beyond my knowledge and abilities