If I have a PCIe x4 slot, am I able to plug a x1 card into it? What
about if I have an x16 slot that is really intended for an SLI card?
Can I plug a x4 card or a x1 card in this? I'm just a little confused
about how all the different ports work with eachother.
If I have a PCIe x4 slot, am I able to plug a x1 card into it?
Yes, a card with fewer lanes, can plug into a slot with more lanes.
What about if I have an x16 slot that is really intended for an SLI card?
Some motherboards have two operating modes. One mode is x16/x1 and the
other mode is x8/x8. If you have an x4 card, then it makes sense to use
the x8/x8 mode, plug video into one slot, and plug your x4 into the
other slot. If the second card was a x1, then the x16/x1 mode should
work, or the x8/x8 mode should work.
Note that using the second video card slot, is subject to the BIOS
being designed for it. On at least one Asus motherboard, the
ability to use the second video card slot, for things like RAID
cards, did not exist until about the fifth BIOS update or so.
If you find it doesn't work, try updating the BIOS.
It may also be possible to plug a bigger card, into a smaller
slot. For example, Tomshardware did an experiment long ago,
where they placed cello tape on the pins of a video card.
They converted the pinout from x16, to x8, x4, x2, etc. Then
inserted the card and benchmarked it. So, that particular
video card was able to operate, with fewer lanes in place.
In theory, if the x1 PCI Express connector was open-ended
on a motherboard, you could plug a x16 video card, and
expect it to work - at least based on the results of the
Tomshardware test.
Some motherboards have an x4 connector, which is "open ended".
The end of the slot is open, so a larger card can "hang out the
end". You can plug a x16 video card into the x4 open ended slot
and enjoy x4 transfer rate performance to the video card. That
would be a 20-30% penalty in 3D games, and virtually invisible
for 2D purposes.
If you look at Table 3 on page 7 here, you can see what the standards
body might have been thinking, when PCI Express was introduced. Without
a copy of the actual standard, it is hard to say what actual combinations
are mandatory and what ones are optional. It would be nice if a x16
card could always be relied on, to work with x8 lanes, x4 lanes, x2 lanes,
x1 lanes, but I don't know if that is mandatory or not from a spec
perspective. Since the video card makers were the first to seriously
make PCi Express cards, they were the ones most likely to try to
do a good job of implementing them (eye diagram excepted).
http://www.dell.com/downloads/global/vectors/2004_pciexpress.pdf
In this white paper, they make it sound like lane negotiation is
handled solely by the agents at the end of the lanes. That is
a pretty complicated function to be handling in hardware, but
I suppose it is necessary since no communication with a card
is going to work, until some lanes have been set up. You'd think
setting up x1 first, enumerating, and building from there, would
also have worked.
http://www.pcisig.com/specifications/pciexpress/resources/PCI_Express_White_Paper.pdf
In terms of practical experience, I've read very few accounts
of people using PCI Express cards, other than video. So their
use still doesn't seem to be too common. I guess not a lot
of people go for the $1K x8 RAID cards
Paul
