I will attempt to once and for all solve this controversial question about whether PCIe slots are 'REALLY' backward compatible or not.
All the information I am providing are from official sources and by that I do not mean some random 'So called' local experts. They were collected from official various motherboard and graphic card manufacture's technical staff and personal testing, experimenting and experience.
Let me start by telling all of you how I came about doing all this research and finally got a valid answer to it.
It all started when I was researching for a new graphic card for myself. I was on a budget so I needed a good mid ranged card providing the performance I need. After comparing and doing lot of research I settled down for a GTX 560 (Non ti) but it was still pushing the limits of my budget!
Then I started looking on the AMD/ATI side for a card that suits me the best and came across the HD 7770. It just seemed perfect for my needs.
This was all until I realized it needed a PCIe x16 v3.0 slot. I had a motherboard with PCIe x16 v1.1 slot.
Now we all know that PCI SIG is famous for designing the newer versions of PCI express slots which were backward and forward compatible with all the versions previously made. Until they introduced PCIe x16 v2.1, after which they chain of backward compatibility was reported broken.
On various forums there were instances of people having trouble with their PCIe v2.1 graphic cards not running on PCIe v1.1 slots.
Now was this true? Why was the backward compatibility for PCIe slots broken?
Reading the article on Wikipedia and after having a chat with the PCI SIG's tenchical staff I was explained how the thermal and electrical designs were changed in PCIe v2.1 slot apart from the higher bandwidth speeds.
PCIe v2.1 had all the features of the latest PCIe v3.0, like the thermal and electrical designs, power usage protocols etc. but featured the same bandwidth speed of PCIe 2.0.
Now most of you people out there never need to worry about your card having compatibility issues with your motherboard because almost all motherboard released after 2010 have PCIe x16 v2.0 slots. So this article is not aimed at these users as all PCIe v3.0, v2.1 and for that matter of fact any PCIe version of card is compatible with PCIe v2.0 slots.
This article is for those few people like me who still use PCIe v1.1 slots and have a doubt about the compatibility with the newer cards like Nvidia GTX 600 series or AMD/ATI Radeon 7000 series.
So I started researching various forums to find an answer to one of controversial question on Internet regarding computer components, "Is PCIe x16 v3.0 compatible with PCIe x v1.1 slots?"
Some sites and forums and the members in them claimed that all of them are backward and forward compatible while some claimed that they are not.
What was to be believed?
Off course it is hard to believe what people generally say on forums and you really need a solid source to make your decision. This is when I came across this little review on techpower.com: http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/24.html
Reading that article pretty much made me sure that yes they are all backward and forward compatible and I dont have to worry about which version of PCI Express the card uses.
I decided to get the Sapphire Radeon HD 7770 OC edition but just before I made my purchase I thought checking with the vendor (i.e Sapphire) once just so that I could have some peace of mind. That is when my hopes were shattered when Sapphire's online technical staff told me that their HD 7770 needs at least PCIe x16 v2.0 slot to function properly.
Off course I don't mean to say that the article on techpowerup.com was false, it was perfectly legit but what I did not realize is that they tested those cards all of PCIe x16 v3.0 slot!
Just at different bandwidths and by scaling down the verions of PCIe slots to test those cards. At the end of the day they still used a perfectly build PCIe x16 v3.0 slot which featured all the newer thermal and electrical designs and the technology called 'SCRAMBLING' for encoding which was absent with the older PCIe version like the 1.1 which uses 8b/10b encoding scheme for that purpose.
Now on one side the sources like Wikipedia (http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.1) and the technical staff of Saphhire told me that PCIe x16 v3.0 is not compatible with PCIe v1.1 slot.
While on the other side various forum members, the PCI SIG (http://www.pcisig.com/news_room/faqs/pcie3.0_faq/#EQ6) stated that they are all backward and forward compatible.
So what is the truth? ARE THE PCIe SLOTS BACKWARD AND FORWARD COMPATIBLE?
The answer is 'YES' and even 'NO'. Confused? Keep reading...
While it is true that PCI SIG while developing PCI express slots made sure that they remain compatible with each other, it is the GRAPHIC CARD manufactures (i.e Nvidia & AMD) and the sub vendors (Saffire, Zotac, EVGA etc) that really make the difference!
We all know that the AMD Radeon HD 7000 series is extremely power efficient. Having said that it comes at a cost. This power efficiency comes due to the new electrical interface featured in PCIe v3.0 and 2.0 slots and is ABSENT in PCIe v1.1 slots. Hence they are NOT COMPATIBLE.
Sources of proof: Online interaction with Sapphire technical staff
Online interaction with AMD technical staff
Personal interaction with technical staff from MSI
They all clearly stated that that PCIe v3.0 cards are not compatible with v1.1
Further to prove their point MSI was even kind enough to provide a reference card they had with them, i.e the HD 7850 and it did not work on my computer's motherboard featuring PCIe x16 v1.1 slot. Just to make sure I installed the card correctly I placed another card from Nvidia i.e GTX 560 onto the board that I got from a friend and everything worked flawlessly.
Does that mean the compatibly of PCIe slots is broken? NO!
This whole scenario where the cards did not seem compatible featured only AMD cards. I did not tell you anything from Nvidia.
Here is the twist. Most of us would agree when I say that Nvidia cards like GTX 600 series use much more power than the HD 7000 series.
The question is why does it consume so much power?
This was explained to me carefully by a head of technical staff of Nvidia (India) by stating "Our cards seem to draw more power because we have tried to maintain THE COMPATIBILITY OF PCIe slots of newer generation with the older ones"
That is to say that the Nvidia cards featuring PCIe v3.0 are fully backward compatible with PCIe v1.1
This explains why Nvidia cards are so power hungry. It is because the 'PCIe interface' in these cards is similar to that of PCIe 1.1 slots and provides higher bandwidth of the PCIe v.30 slots.
Even all the reference cards from Nvidia of the 600 line up were able to run on PCIe v1.1 slots.
This even answers why the GTX 500 series never used PCIe 2.1 interface like the HD 6000 series. Because remember the PCIe 2.1 is exactly similar to PCIe 3.0 and the only difference is the 'Bandwidth'
Sources of proof: Personal Interaction with Nvidia's head of tenchincal staff (India)
Zotac Website (http://www.zotac.com/index.php?option=com_virtuemart&Itemid=100313&lang=en) and an online interaction with them.
I also personally tested the Nvidia GT 640 on the same rig the AMD card was tested on and I did not have any problems what so ever.
So to most of the people who own the motherboards having chipset X38 and latter you have both AMD and Nvidia choices open to you.
But to the few people like myself owning a motherboards with G41 and previous chipsets the only solution seems to be Nvidia.
Note: Read this info about which Northbridge chipset supports what version of PCI Express (http://en.wikipedia.org/wiki/List_of_Intel_chipsets)
Now at the end I am sure many of you out there would be disappointed and even disagree or even claim that this is a lie but I am sorry to say it is the TRUTH.
If you wish you go through the whole process that I went through you can. The purpose of this article was to end the chapter of PCIe compatibility question once and for all.
I hope most of the controversies and question regarding this whole 'PCIe compatibility' issue has been solved.
If you further have any doubts please leave your question in comments below.
UPDATE: (After reading the comments I went back to researching again over this question and I must partly decline my statement of the PCIe configuration not being backward compatible. They in fact are very much compatible after a motherboard update (In some cases even the update wasn't needed. Sorry for the misguided information but I tried my best of reaching to the root of this controversy and reached a partially mistaken conclusion. So once again guys, if your graphic card doesn't work on your old motherboard then an UPDATE from from motherboard manufacturer will most likely solve your problem. My sincere apologies for any inconvenience or confusion caused. Thank you for all sharing your helpful experiences, as my final aim was to help ease the confusion and frustration that is involved with building a PC for the 1st time. Cheers to PC gaming!)