Articles: Mainboards

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PCB Design and Functionality

Some features of Foxconn ELA required Foxconn engineers to use their creative thinking and non-traditional approach to mainboard building:

Of course, three PCI Express 2.0 x16 slots for graphics cards are something Foxconn engineers are especially proud of. As we know, the technical specifications of Intel P45 Express chipset allow only two slots by default. If only one connector is in use, it works at its full speed of x16; but if there are two graphics cards, the slots witch to x8 mode.

We know situations when manufacturers lay out the third graphics card slot, but it is usually running at what’s left. Since the chipset North Bridge has no extra PCI Express 2.0 lanes, the third slot uses the potential of the chipset South Bridge. So the third graphics card slot automatically loses support of the second generation PCI Express and can use 4 lanes maximum, i.e. it can work at PCI Express x4 speed at best. Foxconn developers couldn’t agree to that. To make sure that all three slots support PCI Express 2.0 they installed an additional IDT controller.

A single graphics card installed in Foxconn ELA still works at full PCI Express 2.0 x16 speed. When we install two or three graphics cards, the slots switch to PCI Express 2.0 x8 mode.

This is all great, but besides three PCI express x16 slots, the board also has two PCI Express x1 slots and two PCI slots. Seven slots take quite a bit of space on the PCB, so it turns out simply impossible to place the chipset North Bridge and processor socket in their traditional spots. So, Foxconn engineers took an unconventional approach: they moved the chipset North Bridge not below the CPU socket, but to the right of it. However, mainboard is a very complex device, its components are all connected and you can’t shuffle them around just like that: the length of signal lines and their route matter a lot for work at hundreds of megahertz frequencies. Therefore, the developers not just moved the North Bridge: they also had to turn the CPU socket. As a result, the “chipset North Bridge + processor socket” knot remained unchanged; it was simply rotated by 90 degrees counterclockwise:

As a result, all changes could be successfully implemented. DDR2 memory slots remained where they were supposed to be and turned out even closer to the chipset than in standard layout. It should have its positive effect on system stability. 8- and 24-pin power connectors are very conveniently located. The 8-phase digital processor voltage regulator circuitry is very compact and easily fit slightly above the socket.

Of course, there certainly exists a cooler that will not fit because of the chipset North Bridge heatsink or the additional heatsink over the processor voltage regulator elements connected to it with a heatpipe. However, we didn’t have any problems with the Zalman CNPS9700 LED cooler we used: there was enough free room around the processor socket.

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