The PCBs that make up a GeForce 7900 GX2 are connected to each other by means of metal hexagon poles and screws. We took the screws out to access the bottom PCB and we also removed both the coolers. Although the cooler of the bottom card is completely cut off by the top card, overheat is not likely: there are special holes in the appropriate part of the PCB. So, the GeForce 7900 GT2 takes its air for cooling from both sides at once.
The empty space you could see on the top PCB is occupied by PCI Express x48 switch chip and by two diagonally placed connectors for transferring data. Analogous connectors can be seen on the reverse side of the top PCB, and the PCBs communicate via two textolite contact panels. The reason for this connector position is very simple. Since PCI Express lanes should be of the same length for each point-to-point connection, in case of vertical connector placement they would need either to turn some chips by 45 degrees or to make the cards layout much more complex. So, the diagonal connector placement is a pretty logical move here.
It’s from this connector that the top graphics processors receives data from the switch chip which is directly attached to the PCI Express bus via a standard x16 connector on the bottom PCB. The second output of the x48 chip is directly linked to the bottom G71 processor.
The use of a switch chip allows the GeForce 7900 GX2 to work normally on any mainboard that is based on the nForce4 SLI X16 or any other SLI-compatible chipset from Nvidia. Moreover, as we see from the situation in the professional graphics cards market and in particular solutions like Nvidia Quadro FX 4500 X2, a switch chip like that may allow using the card on platforms that officially do not support SLI technology.
As required by the design of the quad SLI platform, the GeForce 7900 GX2 is equipped with two MIO connectors shifted relative to one another in order not to get in the way of the connecting bridges.
We removed the coolers to have a better view of the design of the PCBs near the graphics processor and memory chips. Each PCB of the GeForce 7900 GX2 carries a G71 D-N chip and 8 chips of GDDR3 SDRAM.
The graphics chips are marked as G71 D-N but they do not differ externally from those installed on GeForce 7900 GTX and 7900 GT, have the same revision A2 and lack a protective frame on the die package. It’s important to note that the graphics processors are clocked at a lower frequency on the GeForce 7900 GS2 than on the GeForce 7900 GTX: 500MHz against 650MHz.
Nvidia just couldn’t help reducing the GPU frequency. Although the G71 chip features low heat dissipation, the cramped two-storied design of the GeForce 7900 GX2 leaves no room for an efficient cooling solution. The overall power consumption of the card would be over 160W whereas reducing the GPU and memory frequencies helped keep within 145W. And after all it would be just much more difficult to ensure stable operation of the complex two-storied solution at high frequencies.
The Samsung K4J52324QC-BC14 chips installed on the GeForce 7900 GX2 have 512Mbit capacity, work at 1.8V voltage and can be clocked at 700 (1400) MHz frequency. At this frequency the power consumption of each chip is about 1.7W, but since they are clocked at 600 (1200) on the GeForce 7900 GX2, their power consumption is lower – 1.55W per each chip. The 16 chips consume a total of about 25W. The chips are placed in the same manner as on other today’s top-end graphics cards; the total amount of graphics memory on the GeForce 7900 GX2 is 1 gigabyte. Two such cards will have a total of 2GB, but you should be aware that the memory amount does not add up in multi-GPU systems, so only 512 megabytes of memory will be available to applications, i.e. as much as each graphics processor from a quad SLI system has access to.
We guess the use of slow memory may harm the GeForce 7900 GX2 in particular and the quad SLI concept at large because it is the memory subsystem performance that’s important in high resolutions and extreme full-screen antialiasing modes. Since the memory of the GeForce 7900 GX2 card works at 600 (1200) MHz, its speed is definitely not high enough to satisfy the growing demand for high memory bandwidth of a system like that. In other words, the performance boost from the four graphics processors may be negated by the slow of each individual graphics processor memory. We’re going to check this supposition in our practical tests soon.