Does a slower card pull down the speed of the PCI-E bus?

[MichaelD]

Say a motherboard has two 16x PCI-E slots. Both slots are also 16x electrical. If you have a 16x card in one slot (GPU) and a 4x card (RAID card) in the other, does the whole bus get pulled down to 4x speed?

 

[tcsenter]

Not without something seriously wrong in the BIOS.

 

[Fullmetal Chocobo]

No. You can use a 1X PCIe card in a 16x PCIe slot (albeit it's a waste of a x16 slot), and the rest of the PCIe subsystem will not suffer any speed loss.

 

[MichaelD]

Thanks much, FMC and tcsenter. :beer:

 

[Peter]

... and the actual technical reason behind all this is:

PCI Express is not a bus.

All connections are individual, independent, point-to-point.

 

[MichaelD]

Originally posted by: Peter
... and the actual technical reason behind all this is:

PCI Express is not a bus.

All connections are individual, independent, point-to-point.

Thank you, Peter. I have a question though. In motherboard block diagrams, I always see the two 16x slots connected to the north bridge on the same line. Is that just for simplicity's sake? I always thought it was a bus, albiet a much faster one than PCI or PCI-X.

 

[Peter]

It's still point to point in that special case.

What happens here is, the chipset detects whether there are one or two cards, and configures itself to offer one 16x link for one card, or two independent 8x when both slots are in use.

 

[MichaelD]

Thanks again, Peter. Now I've just gotta decide on a new motherboard! I really dislike this part...it's like shopping for a new car. Gotta read 1,000 reviews and micro-analyze every detail.