[QUOTE=Aramis Wyler;334524]A co-worker of mine tried running pci 3.0 cards in 2.0 slots. The board was solid and could handle 2x16, not one of the boards that can do 1x16 or 2x8. Even so, the PCI 3.0 card wouldn't run in it. They say they are backwards compatible (usually with a *) but the truth is it's really hit and miss if a 3.0 card will run in a 2.0 slot. If it will run, it will almost certainly run slower than it would in a 3.0 slot not because of the channels/bandwidth but because of the clock speeds. Personally I'd reccomend getting a higher end 500 series 2.0 card than a lower end 700 series 3.0 card unless you are looking for a good excuse to replace the motherboard.

Regarding the Chuck's sitation, the 8x or 16x bandwidth won't matter at all, but the board configuration might. Make sure the mboard doesn't mind running 1 and 3 w/o 2. You might be suprised to find out that if you do run 1 and 3 w/o 2, they'll both run at full 16x speed anyway.
edit: looked it up I could get a PCIe 2.0 motherboard
EDIT: For reference, I run an overclocked 480 and and overclocked 580 on an ASUS board with a 750 watt power supply.[/QUOTE]
I suspected as much.
I am not certain I can get a PCIe 2.0 motherboard with LGA 775(Core 2 Quad). I would probably use replacing that as an excuse for the whole system anyway. I am trying to wait for skylake to come out with DDR4. Currently one of my biggest problems with my system is that high density DDR2(currently have 4GB of memory) is expensive and barely available. I don't want to get a long term system with a memory architecture that is getting close to the end of it's life span(plus there has been a spate of new instruction sets that are useful that I would like).
edit: looked it up I could get a PCIe 2.0 motherboard

[QUOTE=Chuck;334503]I was thinking about adding a second GPU; the two 16-lane PCIe slots on my motherboard are next to each other and I would really like to space the GPUs further apart for better heat dissipation.

The third PCIe slot can only run with 8 lanes. With the GPU sieving version of mfaktc, would this make any difference in the performance of the program?[/QUOTE]

My feeling says that PCIe 1.1 x1 is sufficient for a Geforce Titan if you do [B]G[/B]PU sieving.
For [B]C[/B]PU sievings mfaktc needs to transfer 4 bytes per candidate. So if you card is capable of 200M/s you'll need 200M/s*4 = 800MB/s.

Oliver

I may have missed something, but as far as I know you can always put a 2.0 card in a 3.0 board. There's not any problem with getting a 3.0 board regardless of your gpu hardware. The problem would be trying to put a 3.0 card in a 2.0 board.

[QUOTE=Aramis Wyler;334558]I may have missed something, but as far as I know you can always put a 2.0 card in a 3.0 board. There's not any problem with getting a 3.0 board regardless of your gpu hardware. The problem would be trying to put a 3.0 card in a 2.0 board.[/QUOTE]

I have a PCI 3.0 gpu in a PCI 2.0 slot in the motherboard here.

The only problem would be to put a card with a longer connector in a shorter slot, for which you will need a milling machine, and at the end it will not work...

PCIe is a versatile animal. You can put 2.0 cards into 3.0 slots, if they fit physically, and you get all 2.0 performance, as your mobo will always know the deal. You can put 3.0 cards into 2.0 slots, and you get all 2.0 performance, IF the card knows the deal. Not all cards know the deal, and in this case you get lower performance. Worst case you get 1.0a performance (for [URL="https://www.google.com/search?q=pcie+to+pcmcia&hl=en&source=lnms&tbm=isch&sa=X&ei=60RNUZ-wIe-aiAfgo4CwDw&ved=0CAoQ_AUoAQ&biw=1272&bih=806"]some cards[/URL], you always get lower performance, because that is what the "other side" knows, only).

The fun is that you even can put [URL="http://pic.dhe.ibm.com/infocenter/powersys/v3r1m5/index.jsp?topic=/iphcd/iphcdpciexpress.htm"]shorter cards in longer slots[/URL]. The x1 cards end at pin 18, the x4 cards end at pin 32, and the x8 cards end at pin 50, but the x16 cards continue up to pin 82 (you have to check these numbers for yourself, I am not sure), but they [B][U]are[/U] pin-to-pin compatible[/B], and if you take care about the right alignment (the key-notches will help you in this direction) then yes, you can put any card in any slot, if it physically fits.

Also, the PCIe 4.0 which is yet to appear, featuring another increase in the transfer rate (doubling it again, from 8GT to 16GT - giga-transfers-per-second) will use the same slot, what it will bring new is some "scrambling" algorithm for the data, to avoid the high-frequency interferences between the parallel lines. The limitation is not the connector, but the tracks on the PCB (very fine wires of copper running parallel there and working like antennas relative to each other).

edit: [URL="http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.0"]Wikipedia[/URL] is quite nice in explaining the 2.0/3.0/4.0 differences, and in showing that the bottleneck is in fact the manufacturing process of the silicon, and not the slot/connector. Also [URL="http://www.redbooks.ibm.com/abstracts/tips0456.html"]IBM says[/URL] that "[I]PCI Express uses an embedded clocking technique using 8b/10b encoding. The clock information is encoded directly into the data stream, rather than having the clock as a separate signal[/I]" which is why the card and the mobo negotiate the clock and the encoding "down" until they can understand each other. So, having different cards into different slots will always work at the performance of the "weakest link in the chain" ("no chain is stronger than its weakest link") or worse, but it will work.

That sounds like a great wikipedia entry, but I have actual experience in the matter. We took a pci 3.0 video card and put it in a gaming motherboard with 2.0 slots (that we'd been using for about a year), and it wouldn't work. At all. We put the card in a friend's 3.0 board and it worked fine. We got the best 2.0 card we could and put it in the 2.0 board and it worked fine. Gave the 3.0 card to the friend for christmas.

By the standards, it should be exactly as you said. Unfortunately, not all manufacturers exactly meet spec.

That is exactly why I said "[B]IF the card knows the deal[/B]" in the post above, stressing the IF. Your card would most probably worked in some old compatibility mode, if you set it to SVGA :razz: mode or something like that, hehe, or with the right drivers, or... etc. They still have to negotiate those clocks.

edit: my boss use to make fun of us every time when he can, crying one of his favorite sentences aloud: "the plug and play devices are not plug and play" (well, he says it more plastic)

My observations concerning PCIe bandwidth and GPU throughput (though it's an antidote and the GPUs are very weak).

a) 1 GeForce 8600GT PCIe 16x -> 8.8GHz-d/day /w CPU Sieve
b) 1 GeForce 8600GT PCIe 8x -> 8.8GHz-d/day /w CPU Sieve
c) 2 GeForce 8600GT PCIe 8x in SLi -> each realize 8.6GHz-d/day /w CPU Sieve

I then took one of these cards and put it in a motherboard /w a 16x slot electrically limited to 4x communications:

d) 1 GeForce 8600GT PCIe 4x -> 8.8GHz-d/day /w CPU Sieve

These cards support CUDA 1.1 and are the oldest architecture supported by mfactc. A faster card may be limited by PCIe bandwidth, but these cards seem to be okay with almost any PCIe bus.

Currently on a 8600 GTS. I know how slow they are :smile:

[QUOTE=henryzz;334509]I have a similar question but slightly more extreme. I have been thinking of getting a graphics card at some point something like a 740 when they come out eventually. The card will be designed for 16x with PCIe 3.0 but my motherboard only has PCIe 1.1. I know 2.0 cards should run on my system at an equivalent bus speed of 16x 1.1 or 8x 2.0. Would a PCI 3.0 card run at 4x 3.0 speeds? Would it run at all?
The motherboard is an ASUS P5K-VM if that helps. I will be wanting to use it for a variety of stuff including things like cudalucas, gpu-ecm, gpu P-1 etc.[/QUOTE]
Sorry to bump an old thread but I though here is probably best.
Got a 750 Ti for Christmas. It is working quite happily in my PCIe 1.1 motherboard. Once I get all the programs compiled and working I will do some benchmarks. If someone has a 750 Ti which is in a PCIe 3.0 socket it would be interesting to compare. Does the speed of the cpu make any difference at all for gpu-sieving or cudalucas? If it does my Q6600 probably won't match a more recent cpu.

[QUOTE=henryzz;390990][B]Sorry to bump an old thread but I though here is probably best.[/B]
Got a 750 Ti for Christmas. It is working quite happily in my PCIe 1.1 motherboard. Once I get all the programs compiled and working I will do some benchmarks. If someone has a 750 Ti which is in a PCIe 3.0 socket it would be interesting to compare. Does the speed of the cpu make any difference at all for gpu-sieving or cudalucas? If it does my Q6600 probably won't match a more recent cpu.[/QUOTE]

Did you ever figure out the answer to your question. I just installed a Titan Z in a PCIe 3.0 socket and I'm no longer able to keep the card at 100%, even with several mfaktc instances. I'm wondering if I need to get a fast MB\CPU combo or if it's a limit of the PCIe 3.0 bus at this point?