The AGP Answer

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I asked, and a lot of you responded. Thank you all for doing so!

Fortunately, there were very few “answers” in the answers. This is a good thing for this kind of question, you don’t want fifty creative answers to 2 + 2, either. 🙂

The Official Reasons

Well a big part of the reason goes to the fact that AGP was developed from the PCI specification and, in fact, the slot is meant to replace one of your PCI slots on the mainboard. The reason that PCI cards are “upside down” can be found in the PCI specification.

http://kbs.cs.tu-berlin.de/~stewe/pct/PCI_21.pdf


“The component side of a PCI expansion card is the opposite of ISA/EISA and MC cards. The PCI card is a mirror image of ISA/EISA and MC cards. A goal of PCI is to enable its implementation in systems with a limited number of expansion card slots. In these systems, the PCI expansion board connector can coexist, within a single slot, with an ISA/EISA or MC expansion connector. These slots are referred to as shared slots. Shared slots allow the end user to install a PCI, ISA/EISA, or MC card. However, only one expansion board can be installed in a shared slot at a time. For example, shared slots in PCI systems with an ISA expansion bus can accomodate an ISA or a PCI expansion board; shared slots in PCI systems with an MC expansion bus can accomodate an MC or a PCI expansion board; etc.”

So there you go, that’s your answer. It’s because PCI cards are upside down and AGP is an extension of PCI and replaces one of the PCI slots.

(A number of people pointed that ISA cards had the components facing upward, and having them point downward for PCI cards was another means people could distinguish between the two.)


I had an AGP Spec PDF on my drive which can be had here:

ftp://download.intel.com/technology/agp/downloads/agp20.pdf

It doesn’t actually say why its ‘upside down’ but does mention this (Page 209)

The maximum component height on the primary side of the A.G.P. ATX expansion card is not to exceed 14.47 mm (0.570 in.). The maximum component height on the backside of the card is not to exceed 2.667 mm (0.105 in.) unless otherwise specified.
So I imagine when they designed it they envisioned something pretty damn close to it on the top side, why else would they specify such a small component height.

The GPU is on that side of the card because the spec for AGP/PCI cards has height tolerances that are much tighter on the back. This to because nowhere does it say that the AGP slot has to be the first slot or that the space behind the card isn’t going to be full of something else (eg PSU).

(Some pointed out that in the days of slot 1 cards, there wasn’t as much room north of the AGP slot as there is today).


The components facing down was originally done to allow for the card and
its mounted components to occupy that area taken by the bookplate. Take
a good look at a PCI card and compare it to an AGP card….the PCI card
circuit board sits at the bottom (in a vertical case configuration) and
the components occupy the space above the board. When AGP was
introduced, they (Intel) decided flipping the circuit board design would
prevent users from attempting to insert an AGP card into a PCI slot.
Since the circuit board was at the top of the slot area, they needed to
have the components mounted on the opposite side in order for the device
to occupy the area within a single slot. I still remember the few combo
cards, which were BOTH PCI & AGP in a single product. (You would
physically remove the metal slot carrier and ‘FLIP’ the card 180 degrees
to change it from PCI to AGP and back again).

Now you also need to take into consideration that the AGP design was also
done around the same time as the ATX standard was being heavily pushed by
Intel…..Remember that Intel’s original standard required that the Power
Supply blow air onto the CPU to cool it! – Another ingenious thought by
the same engineers no doubt! (Take air which has been already warmed in
an effort to cool the power supply and push it down to cool a hot CPU).


A Manufacturing Reason

You can give the guy an easy answer, it just
is the way it is. Just like the great purple/greeen PC98 spec that intel
added to its motherboards, the AGP hardware layout specification is defined
by a group of people here: http://www.agpforum.org/.

Just like PCI and ISA have their own electronic and mechanical
specifications, AGP has a specific hardware layout listed here:
http://www.agpforum.org/downloads/apro_r10.pdf

Look for the mechanical specification, specifically on Page 22.

Simple Double-sided surfaced mount PCBoards are laid out as follows:

1).____Component_Side__________________________________
2)|_____Through-hole connections also known as “Vias”__
3) Solder Side.

The components are glued to the Component side of the PCB with a substance
known as “solder-paste.”

The components are then placed on the board in their proper location.

The board is then passed through what’s called a “wave-solder” machine, or
something like it, basicaly it’s a large molten solder bath where all of
your solder connections are made at once.

And you’re done–now inspect the boards and see if you need to touch-up any
“cold-solder” joints.

Multi-layered boards, like RAM and video cards are very complex to design,
but basically are 2 x 2 layered boards stamped together with many TONS of
force (yes, I know a ton is a measurement of weight, not of force, but work
with me here.)

The point: if you don’t adhere to the AGP spec, you can’t call your product AGP 4x or
8x or PRO, samething if you violate the PCI spec (ie: having traces that are
far too long.)

Every single slot connector in a PC was designed against a spec, and in many
cases you’ll find that a specific layout is actually required in order to
meet it (perhaps UL or FCC have something to do with it, (remember ASUS
losing the mounting holes on their A7M266-D ?)

It’s a boring read.


Consider the “tour” of Gainward’s manufacturing process that was floating around a while ago. The components such as the GPU, RAM, and other small items were ‘baked’ onto the board, but the larger components like caps and connectors are placed through the board and soldered after. This soldering was done by ‘floating’ the card across a bed of molten solder which then adhered to the components sticking through. This would surely damage a GPU.

From what I can figure out, most likely the RAM on the back of a video card is placed after all of this is accomplished in what is probbably a ‘mini-process’ similar to the beginning of the assembly line. Placing the GPU on the back of the card would probabaly neccesitate passing the card through the whole assembly line twice, doubling the time needed to make one and therefore increasing costs.


If This Weren’t A Reason, It Ought To Have Been

I’ve thought long and hard about this one over the years. I believe it
all comes down to dust build-up. Most people never crack open their PCs
to blow the dust off. I’ve opened up IBM XTs that have literally inches
of dust build-up.

You can imagine that this would become a fire hazard on today’s latest
video cards. Upside down cards don’t collect dust on the hottest areas,
and the dust can not affect the active coolers. (though the fact that the
fans usually die within a year or two is ironic).

I think that pretty much sums it up. Again, thanks to all who responded!

Email Ed

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