What would happen if you put multiple PCIe cards in one PCIe slot?

I recently discovered that you can put a PCIe x4 card in a x8 or x16 slot.

With that said, what would happen if you took multiple PCIe SSDs and shoved them all into a 16x PCIe slot?

FatalSleep

Posted 2016-08-22T22:43:24.560

Reputation: 397

25PCIe slots are keyed in a way that would prevent that. If you forced it, it would result in mechanical damage to the motherboard and device and almost certainly electrical damage to both as well. – Argonauts – 2016-08-22T22:53:01.073

4How exactly would you do that? – user253751 – 2016-08-22T23:15:52.927

@user20574 you wouldn't. – FatalSleep – 2016-08-22T23:23:07.610

5@FatalSleep Your question presupposes you could. – user253751 – 2016-08-22T23:24:27.077

You can plug an x16 card into an x1 slot with some modification. Not sure how negotiation works with PCIe, but I think it's consecutive across the lanes. Lanes can't be freely reassigned though

– Nick T – 2016-08-22T23:43:17.787

2@FatalSleep I was wondering what situation you were thinking of. Since you asked, you must have been imagining a way to do it. – user253751 – 2016-08-23T00:11:27.627

@NickT autonegotiation - could be ether automatic on some motherboards, or driven by port width pin (more common). – BarsMonster – 2016-08-23T11:46:12.143

1Supposing that you could, (by using an adapter?), what the computer does would depend on the operating system and the driver(s) installed. I would expect that, without having drivers specifically designed for this purpose, a modern operating system would realize something was screwy and refuse to do anything related to the purpose of the card other tell the user something is screwy while it continues running any other processes it has running. – Devsman – 2016-08-23T19:36:24.897

Answers

Nothing... The PCIE key slot would prevent a card being inserted anywhere except the front of the bus. Therefore, only one card can be entered in a slot at a time. If you were to remove the key, you may end up damaging your cards and/or your board.

The PCIE key is the first few pins, followed by a gap, and then the rest of the pins.

CConard96

Posted 2016-08-22T22:43:24.560

Reputation: 1 161

Are there adapters for mounting multiple M.2 SSDs? – FatalSleep – 2016-08-22T22:59:19.100

Yes. They do make dual M.2 controller cards. https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=m.2+controller+card

– CConard96 – 2016-08-22T23:01:28.377

1@CConard96 unfortunately that one is for SATA mode only, i.e. it does not combine 2x PCI-E into one. – BarsMonster – 2016-08-23T11:45:02.233

Splitting a PCIe slot into two is a rare, costly affair. http://www.supermicro.com/products/accessories/addon/AOC-SLG3-2E4.cfm this can do it (and you can covert U.2 to M.2). Alternatively you can utilize two slots in one adapter http://www.bplus.com.tw/Adapter/M2P4S.html

– chx – 2016-08-23T20:07:07.593

The computer would most likely not boot. It might short out.

See, a PCI x4 slot is not four as large as an x1 slot.

All PCI-e connectors have a shared layout: The first 18 pairs of connectors are for power supply, SMbus and JTAG communications, clock sync etc. Only then come the data pairs. This means there is only one way to correctly connect them.

Any second card in the same slot would at best connect (and not be able to use) the remaining data pairs. That is at best. At worst you'll connect its pins in an unexpected way, e.g. connecting the possible joined +12 power lines to differential signal paths. That will cause a short.

For more details on its pinout, please see Wikipedia: PCI Express.

Note that PCI-e bridges exist. You could put an x8 bridge card in an x8 (or bigger) slot and (via active hardware on that card) present two x4 slots, each with its own connector.

Hennes

Posted 2016-08-22T22:43:24.560

Reputation: 60 739

2I assume that you meant JTAG not TJAG? – a CVn – 2016-08-23T07:48:41.587