In the Lab
A comparative look at SLI configurators
A Deck of
Daughtercards
A behind-the-scenes look at
Maximum PC testingF
or consumers, the most confusing ele-
ment of nVidia’s SLI technology is the
little daughterboard that informs the
motherboard how to configure the two x16
PCI Express slots.
PCI-E lane-shifting shenanigans are neces-
sary because today’s consumer chipsets don’t
support more than 20 or 22 PCI Express
lanes in the north bridge, falling short of the
32 lanes of a true dual x16 configuration.
(Adding more lanes would necessitate a more
complex physical interface between the chip-
set and the motherboard.)
For SLI, nVidia takes the 20 lanes in the
nForce4 chipset and dedicates 16 to the
graphics slot when running in single-card
mode. The second x16 slot is then config-
ured to run in either x1, x2, or x4 mode (the
choice is left up to the motherboard maker).
SLI actually runs best when both slots are
running at the same speed; so when the
daughterboard is set to SLI mode, both x16
physical slots are instructed to run at x8.
Because today’s graphics cards don’t even
touch the 8GB/s speed that’s available in x16
mode, you’re probably wondering why nVidia
didn’t simply leave both slots in x8 mode at
all times. The reason is marketing: To have a
chipset that doesn’t offer a x16 mode, even
if it isn’t used for years, would grant a huge
spec-war advantage to the competition.
Several motherboard companies have
sought to improve upon nVidia’s SLI refer-
ence design, and we’ve seen no fewer than
four different motherboard implementations.
Of them all, MSI’s K8N Neo 4 Platinum/SLI
board follows nVidia’s pattern the closest.
Although it works just fine, the little plastic
arm that holds the daughtercard in place feels
weak enough to snap off.
The daughtercard implementation on
Asus’ A8N SLI Deluxe was hooptie, as well,
in its original iteration. The first card we
received was affixed with a sticker that
made it difficult to lock into place on the
mobo. Production versions of the A8N,however, use a silk-screened label on the
card, which makes it far easier to install,
and once in, it seems quite stable.
DFI’s solution, featured on the LAN Party
nForce4 SLI-DR board, rates highest on
the annoyance meter. This design uses six
blocks of jumpers—as opposed to a daugh-
tercard—all of which must be switched.
Although it’s scary looking, the jumper
blocks really aren’t that difficult to change,
and we’ve since warmed to the design
because there’s no card that might shift or
pop out over time.
The best solution, however, could be the
one found on Asus’ new A8N SLI Premium
board: Asus uses a Pericom Semiconductor
PCI Express integrated circuit to change
PCI-E modes with a mouse click. Asus hasn’t
released all the details yet, but if you’re
switching modes with your mouse, it’s pre-
sumably happening from within the OS.
That presents a whole new way to think
about SLI: You could go from running mul-
tiple monitors to running SLI gaming on a
single display with a mouse click. Existing
hardware solutions require you to crack
open the box, pull out the second video-
card, and flip the daughterboard. And for
all intents and purposes, that means you’ll
probably never switch modes.66 MA XIMUMPC MAY 200 5
MSI’s SLI
daughterboard is
patterned after
nVidia’s reference
design; a small
plastic arm holds
the card in place.As annoying as it
is effective, DFI’s
connector requires that
you switch six jumper
blocks to go from SLI to
single-card mode.Asus uses a connector similar
to a notebook PC’s SO-DIMM
module; it snaps in place with
two arms on the side.In this design, Asus eliminates the card
altogether by using an IC to switch modes.