Why is there no PCIe x16 to 2x x8 adapters?

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razwill

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Jan 24, 2018
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So basically I have PCIe 3.0 x16 slot on my motherboard. And a graphic card. Seems normal, right? But my consern is that any modern GPU uses only half of the PCIe 3.0 x16 bandwidth (8 of 16 lanes (works only on PCIe 3.0, on PCIe 2.0 it uses all lanes)). So why isn't there any adapter that could turn PCIe 3.0 x16 slot into two PCIe x8 slot, so I could plug GPU into one slot and something else into other slot, e.g. M.2 card or extra USB hub. Also, could it be possible to plug another GPU into that second slot (if a motherboard supports CrossFire or SLI, obviously)
From what I understand based on the PCIe pin-out scheme (e.g. this one: http://www.interfacebus.com/Design_PCI_Express_16x_PinOut.html) first few pins in the PCIe connector are responsible only for delivering power and other not very importand data (e.g. SMBus clock and data) to the PCIe card. Can somebody explain to me all my concerns and correct me if I'm wrong?
PS Sorry if I'm posting in the wrong section.
 
the key to this is your statement of this
"first few pins in the PCIe connector are responsible only for delivering power"

the slot is designed to run only so much power (roughly 75w). splitting it and then plugging 2 things into that would require the slot all of a sudden to need to power 2 cards and not one. you'd quickly overdraw the slot and cause a ton of issues.

now that we know it can't power 2 cards, the rest of the lanes are not designed to carry the info for 2 different devices. this would take another chip to maybe make it work. like the chips added at times to get more than 1 3.0 x 16 slot. this adds cost and complexity to any such add-on adapter.

by the time you did this and maybe got it working you may have as well just bought a better mobo that can handle what you want it to do in the first place.

i'm sure there are more technical issues as well that i am not aware of. but hopefully this is a good start for why it would not work. if it was possible, i'm sure someone would have done it by now to cram even more gpu's onto a mobo for mining purposes. this alone tells me it is not possible without even doing any major research.
 
But with extrental power (e.g. AC to DC adapter that could turn power from the wall into the PCIe required power) and some engineering I guess it would work, right? Also, even thogh nobody did it befere, it doesn't mean it is impossible. Maybe just nobody thought of it befere? Anyway, thank you for your answer.
 
Aside from what Math Geek pointed out, note also that OS treats PCIe slot as single device. So to make such splitter work, you would also need to code new device driver - and since PCIe slot is at motherboard level, you would need such driver for each motherboard you want the splitter to work at. Possible? Perhaps. Worth the investment of time and money? I doubt it.
 
Thank you all (both :) ) for your answers. Also, why would be so hard to code it? I mean, could you THEORETICALLY use 13 first pins (those responisble for everything else than data) for both devices (with the use of external PSU for the power responisble pins, as I mentioned before), and then assign half of the remaining pins for the first device and the rest for the second device? Is it really THAT hard? I know programming (mostly C++ and C#), but programming motherboard? Just asking...
Another consern - there are PCIe switches (like this one: https://www.ebay.com/itm/1set-PCI-Express-1X-Expansion-Kit-1-to-4-Port-Switch-Multiplier-Hub-Riser-Card/272999819562?hash=item3f900ea92a:g:RN4AAOSwIJlaRGKL). If you can't plug multiple PCIe devices into one slot, then how do these devices work? Are they some sort of scam or what?
From what I can see PCIe switches are pretty expensive and I don't want to pay that much for them. I'm just curious about it. Also, it looks like they introduce serious bottlenecking (like, PCIe x1 is 16 times slower than x16, right?) and with my idea there would be no bottlenecking.
 
Searching for information about PCIe switches I came up with this article: https://www.diodes.com/design/support/technical-articles/pericoms-articles/top-3-uses-for-pci-express-switches/ explaining why you should consider using PCIe switch. The second reason is pretty intersting. May I quote it: "PCI Express host adapters such as CPUs come configured only one way: say, one x8. But suppose you need two sets of x4? Eight sets of x1 (one lane per device)? A PCIe switch can do this “lane swapping” for you.". So... there are one PCIe x16 to two x8 adapters and they're called PCIe switches and they are just not that popular. Am I correct?
Still, on those PCIe switches sites there is no word about support of the multii-GPU technologies. So?
 
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