Not a bad idea however you have hard limits on how physically long the PCIe lanes can be. We had problems making sure we hit signal budget for a PCIe gen 4 slot on an ATX motherboard. The problem (PCIe lane length) gets worse as the speeds increase.

My hope was it could similar length to today, just on the other side of the CPU.

Thinking about it though isn’t that where a lot of motherboards put a lot of their power regulation circuitry? I’m sure something would have to move.

But excellent point. That could easily bet a much bigger sticking point that I didn’t really consider too much.

On a further note, why does it even have to be inside the case? Make a slit in the case on the top, so that the PCIe slot is sticking out. Stick a GPU in that slot, supported by the case. The GPUs these days look much cooler anyways.

Briefly, EMI.

> Only downside is the case has to be taller

Not necessarily. A flexible interconnect would allow the GPU to be planar with the MB; just bend it 180 degrees. Now your GPU and CPU can have coolers with good airflow instead of the farcical CPU (125mm+ tall heatsinks...) and GPU cooling designs (three fans blowing into the PCB and exhausting through little holes...) prevailing today.

My idea is to separate the whole CPU complex (the CPU and heatsink, RAM slots, VR, etc.) from everything else and use short, flexible interconnects to attach GPUs, NVMe and backplane slots to the CPU PCB, arranged in whatever scheme you wish.

I was kind of hoping doing it that way would let you put big CPU style coolers on the GPU parts with a lot more height than a 1x or 2x expansion slot.

If you “folded” the GPU over the CPU to save height I would think that would be worse than today for heat.

Maybe I’ve got this backwards. Give up on PCIe, or put it above the rest of the motherboard. The one GPU slot, planar to the motherboard, stays below. Basically my previous idea flipped vertically.

The other PCIe slots don’t need to run as fast and may be able to take the extra signal distance needed. The GPU could secure to the backplane (like my original idea) but would have tons of room for cooling like the CPU.

> If you “folded” the GPU over the CPU to save height I would think that would be worse than today for heat.

Why? Cooling would be far better: the CPU and GPU heatsinks would both face outward from the center and receive clean front-to-rear airflow. Thus, looking down from above:

        intake air -->  [cpu heatsink]    exhaust --> 
                     [cpu+vrm+ram+chipset]
                    ---------pcb----------
                    | <-- CPU/GPU interconnect
                    ---------pcb----------
                        [gpu+ram+etc]
        intake air --> [gpu  heatsink]    exhaust -->

The power supply and power connectors are on the bottom. Another PCB lays flat on top to host open PCI-E slots, NVMe, whatever, connected 90 deg. to the CPU PCB with one PCI-E slot interconnect. All interconnects are short. Air flow is simple and linear. The CPU/GPU Heatsinks are large and passive: you only need intake fans.

Upvote for ascii-visual-aids

Thanks. :)

I've been refining this. I'm actually learning FreeCAD to knock out a realistic 3D model.

One obvious change: run the CPU/GPU interconnect across the bottom: existing GPU designs could be used unmodified (or enhanced with only a new heatsink) and the 16x PCI-E lanes for the GPU would be easier to route off the CPU PCB.

There are virtually no significant differences between the motherboard layout IBM promulgated with original IBM PC (model 5150) in 1981 and what we have today. That machine had a 63W power supply and no heatsinks or fans outside the power supply. The solution to all existing problems with full featured, high power desktop machines is replacing the obsolescent motherboard design with something that accommodates what people have been building for at least 20 years now (approximately since the Prescott era and the rise of high power GPUs.)

>This doesn’t really help dual GPU setups

They are with the people who spend the most money on GPUs

Are they?

Seems with the last GPU generation most enthusiast websites recommend to go for a bigger card instead of going SLI.

GPGPU workflows scale in a linear fashion so the more you buy the faster your workloads run