rockbyter :
By example the Asus P5K/Wifi has the following pci express slots
x16
x16(x4 signal)
x1(qty 2)
By default if you use the x16(x4 signal) slot for a second video card, it disables both x1 slots because there simply are no lanes available for them in the PCIe 1.1 standard. You can only use up to 20 lanes for the cards.
PCIe 2.0 with the same slot configuration can share 32 Lanes among the cards, meaning you can do a crossfire configuration and still have lanes left over for sound, network, raid and tv tuner cards. Its enough for most of us, but PCIe 3.0 will bring about enough bandwidth for us to have all PCIe slots, and put the PCI bus to rest alongside ISA, PATA and the floppy drive.
SLI PCIe 1.1 boards are a bit different because the 2 video cards run in x8 mode leaving those extra x1 slots open for use in many cases.
Please correct me if I'm wrong. The combined lane limitation of 20 is the limitation of the board and not PCIe 1.1. Even if this board changed all 4 PCIe slots to 2.0, the limit can still be 20 if ASUS chooses to.
And I'm not quite clear about the second x16 slot (1st is blue, 2nd is black, 3rd and 4th are white). The black one can either operate at x16, x4, or x1. If I can only use up to 20 lanes, the combinations are:
Blue @ 16 + Black @4
Blue @ 16 + Black @ 1 + White @ 1 + White @ 1
Blue @ 8 + Black @ 4 + White @ 1 + White @ 1
Blue @ 4 + Black @ 16
Blue @ 4 + Black @ 4 + White @ 1 + White @ 1
Blue @ 4 + Black @ 4 + White @ 1 + White @ 1
and etc.
You said that if Black was to be used as second video card, it disables both x1 slots. In this case, I'm assuming this is the first case of the above list. Blue is used as the first video card, so Blue takes up x16, which means Black can only use x4. The second video card must be weak enough to not request bandwidth greater than x4. Am I right?
Thanks.