PCI Express x16 slot compatibility for Non-Graphics Applications?

[mucha]

This is about PCI Express x16 slot compatibility...

Does anyone know if there are any limitations of the PCI Express x16
ports that are shipping on desktop motherboards?

For example, I get a Motherboard that has an Intel 915 chipset that
has:
1 - x16 PCIe slot
1 - x1 PCIe slot
2 - legacy 32-bit PCI slots

The documentation for the Chipset and Motherboards say that the x16
slot is PCI-Express compliant. But it also always puts in the note,
"Intended for Graphics." Is this just so that the mainstream will get
the hint it replaced the AGP slot? Or does it mean: "You'll have a hard
time plugging in other stuff into this port you weird Non-Gamer."

If I get a SCSI raid controller that has a x4 PCI-Express card edge
connector on it. In theory, I can plug it into the x16 slot that is
intended for graphics, Right??

Because I am using the embedded video port (don't need fast 3D video)
and all I care about is the speed from the disks to the CPU, will
everything be ok software/BIOS wise?

In other words one more time, is there any limitation of the PEG port
or PCI Express Graphics Port (x16 PCIe) that prevents other devices
from taking advantage of the bandwidth available?

Thanks,

Jeff

 

[Rob Stow]

This is about PCI Express x16 slot compatibility...

Does anyone know if there are any limitations of the PCI Express x16
ports that are shipping on desktop motherboards?

For example, I get a Motherboard that has an Intel 915 chipset that
has:
1 - x16 PCIe slot
1 - x1 PCIe slot
2 - legacy 32-bit PCI slots

The documentation for the Chipset and Motherboards say that the x16
slot is PCI-Express compliant. But it also always puts in the note,
"Intended for Graphics." Is this just so that the mainstream will get
the hint it replaced the AGP slot? Or does it mean: "You'll have a hard
time plugging in other stuff into this port you weird Non-Gamer."

If I get a SCSI raid controller that has a x4 PCI-Express card edge
connector on it. In theory, I can plug it into the x16 slot that is
intended for graphics, Right??

Because I am using the embedded video port (don't need fast 3D video)
and all I care about is the speed from the disks to the CPU, will
everything be ok software/BIOS wise?

In other words one more time, is there any limitation of the PEG port
or PCI Express Graphics Port (x16 PCIe) that prevents other devices
from taking advantage of the bandwidth available?

Thanks,

Jeff

If a PCI-E slot has more lanes that a device requires, the extra
lanes are simply ignored. So you could, for example,
theoretically put a PCI-E x4 SCSI card into a PCI-E x16 slot.

 

[FredK]

The only devices using 16 lanes are graphics, and some high-speed
cluster interconnects. Most devices are 4 or 8 lane.

The slots are different sizes, and you are supposed to be able to put
an option with a lower lane requirement into a bigger slot - that is,
the 8 lane should be able to plug into a 16 lane slot. You can't go
the other way around. In addition, slots are not supposed to be
underfunded (i.e. a 16 lane slot that only has 8 lanes on it) - with a
notable exception - 8 lane slots can be underfunded with 4 lanes,
and I believe that 8 lane devices are supposed to deal with it.

Now that is assuming that the manufacturer complied with the
standard.

 

[mucha]

Does that mean that it will run at x4?

Some of the x16 slots *hint* at maybe running at x16 or x1 only,
nothing in the middle, if you plugged in a x4 you'd only get x1 speeds.
Does anyone have any experience with this?

 

[daytripper]

Does that mean that it will run at x4?

Some of the x16 slots *hint* at maybe running at x16 or x1 only,
nothing in the middle, if you plugged in a x4 you'd only get x1 speeds.
Does anyone have any experience with this?

A PCI Express link goes through a training sequence to establish the total
number of lanes supported by both ends. So an x8 or x4 or x1 device plugged
into a 16-lane slot will train to x8 or x4 or x1, respectively.

And any PCI Express-compliant device should work in any slot it can physically
be plugged into (the restriction means you can't expect a x16 graphics card to
plug into a slot that uses a physically shorter connector (eg: most x1 and x4
slots)...

/daytripper

 

[Tony Hill]

This is about PCI Express x16 slot compatibility...

Does anyone know if there are any limitations of the PCI Express x16
ports that are shipping on desktop motherboards?

At least in theory, no.

For example, I get a Motherboard that has an Intel 915 chipset that
has:
1 - x16 PCIe slot
1 - x1 PCIe slot
2 - legacy 32-bit PCI slots

The documentation for the Chipset and Motherboards say that the x16
slot is PCI-Express compliant. But it also always puts in the note,
"Intended for Graphics." Is this just so that the mainstream will get
the hint it replaced the AGP slot? Or does it mean: "You'll have a hard
time plugging in other stuff into this port you weird Non-Gamer."

A bit of both. The system is designed so that it SHOULD be able to
use any compatible PCI-Express card in the PCI-E 16x slot. However
there are VERY few (none that I know of) PCI-E 16x cards out there
that aren't video cards. To go along with that, the motherboard
companies (and to a lesser extent the chipset companies) probably
don't do much in the way of testing other PCI-E cards in these slots.

If I get a SCSI raid controller that has a x4 PCI-Express card edge
connector on it. In theory, I can plug it into the x16 slot that is
intended for graphics, Right??

In theory yes, that is correct.

Because I am using the embedded video port (don't need fast 3D video)
and all I care about is the speed from the disks to the CPU, will
everything be ok software/BIOS wise?

Now THAT brings up a bit of an interesting question that you might not
have thought of. When you add a PCI-E 16x graphics card into ALL
boards using the i915 or i925 chipsets, it will *DISABLE* the
integrated video (no way around this). So, the interesting question
is, does putting a non-video card into that same 16x PCI-E slot ALSO
disable the integrated video? ie is it just the fact that there's a
video card in there or do the integrated video and 16x PCI-E slot
itself share some resources? I honestly haven't got a clue about this
one, though digging through Intel's datasheets might shed some light
on this matter.

Another option might be to simply look for a board with a PCI-Express
4x slot. They do exist, though they are quite rare. Or maybe
consider a board using an ATI chipset, as they do NOT disable the
integrated graphics when plugging in a PCI-E 16x video card.

 

[Paul]

This is about PCI Express x16 slot compatibility...

At least in theory, no.

Can't speak for the OP, but I have x8 Infiniband cards that I'd love to plug
in.

Because I had seen dire warnings about x16 intended for graphics only, I had
assumed my x8 card would not work in that configuration.

Problem is that many desktop motherboards support x16 and x1 or maybe x4,
very few support x8. Maybe the SLI boards may be different. (I'd love x8 for
graphics + x8 for Infiniband network)

I was hoping for some dual core desktops with IB to replace more expensive
server boards.

Tom's hardware did some tests a while back with configuring x16 slots at x8,
x4 etc. It might be a useful read. I think his system required some
unreleased BIOS update to unlock other speeds on the x16 slot. It didn't
give me hope that BIOSes would support what I wanted to do. Probably a
chipset manufacturer restriction to protect their server chipsets?


So I haven't seen anyone tell me that physically the slot is different, but
two things are unknown:

1) Would plugging something into the x16 slot mean the motherboard insisted
it was used for graphics (rather than continuing to use the onboard graphics
or say a slower PCI adapter)

2) Would attempting to use the x16 for a slower device be prevented by BIOS
incompatabilities.

Paul

 

[Alan Charlesworth]

Now THAT brings up a bit of an interesting question that you might not

have thought of. When you add a PCI-E 16x graphics card into ALL
boards using the i915 or i925 chipsets, it will *DISABLE* the
integrated video (no way around this). So, the interesting question
is, does putting a non-video card into that same 16x PCI-E slot ALSO
disable the integrated video? ie is it just the fact that there's a
video card in there or do the integrated video and 16x PCI-E slot
itself share some resources? I honestly haven't got a clue about this
one, though digging through Intel's datasheets might shed some light
on this matter.

The BIOS on Dell boxes with these chip sets (Dell 4700 & 8400) gives one
the choice of using the integrated graphics even with a graphics card in
the x16 slot. Perhaps this would do what the original poster desired:
enable the integrated graphics even with a card in the x16 slot. Note: I
did not actually test this BIOS option.

 

[daytripper]

Tom's hardware did some tests a while back with configuring x16 slots at x8,
x4 etc. It might be a useful read. I think his system required some
unreleased BIOS update to unlock other speeds on the x16 slot. It didn't
give me hope that BIOSes would support what I wanted to do. Probably a
chipset manufacturer restriction to protect their server chipsets?


So I haven't seen anyone tell me that physically the slot is different, but
two things are unknown:

1) Would plugging something into the x16 slot mean the motherboard insisted
it was used for graphics (rather than continuing to use the onboard graphics
or say a slower PCI adapter)

There's no accounting for stupid bios implementations. A smart one would
realize whether a plugged-in agent had a video rom or not, before doing
something stoopid like disabling embedded video.

2) Would attempting to use the x16 for a slower device be prevented by BIOS
incompatabilities.

See above.

/daytripper