HD7950 won't work where 6950 does

So, I recently bought a Radeon HD7950, but when I put it into my PC, it caused all sorts of black screens and crashes etc.

I used both a Radeon HD4950 and a HD6950 in exactly the same pc, and both worked without an issue. For my birthday I got a new 650W Power supply (replacing my old 500W, I thought this was the issue.) But when I put my card in, it worked for maybe 2 hours then the same things happened again, with the pc eventually becoming un-bootable until I put the 6950 back in.

My PC setup is;
Asus P8Z77-V LX Motherboard
Fractal 650W PSU
Intel I5 Processor
Crucial M5 SSD
Western Digital 80GB HDD
Western Digital 250GB HDD

Now, I know for certain that the 7950 is not faulty, as I tried it in my Dad's PC, (the one that previously had the 6950) and it worked fine. For some reason, the 7950 just does not agree with something in my system.

Any ideas what would be causing this? Any help would be greatly appreciated.

So, I quickly reviewed the specs and here's what I found:

The HD 6950 is PCI-e 2.0
The HD 7950 is PCI-e 3.0

The Intel Z77 chipset only supports PCI-e 2.0

Many mobo manufacturer's are claiming PCI-e 3.0 support using a 2.0 chipset. How they can do that and how it would actually work is a mystery to me. Anyway, see if there is a BIOS option to run at GEN2 speed and see if that stabilizes your rig. Now, since we have not even maxed out GEN2 yet, I don't think you'll pay any performance penalty by doing so. Please understand that I am no expert in this area but I think it might be worth a try. Good luck.

It's rare occasion for PCIe 3.0 could not run on 2.0, more likely your PSU can't cope with it. Just because it is marked 650wats does not mean it is or it does not have enough power on that branch.

CountMike said:

It's rare occasion for PCIe 3.0 could not run on 2.0, more likely your PSU can't cope with it. Just because it is marked 650wats does not mean it is or it does not have enough power on that branch.

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I agree that it will run fine on 2.0; however, the issue (as I see it) is that the OPs mobo has a PCI-e 3.0 slot and if it is set to run at Gen 3 (using the Z77 chipset), it might not work properly because of the stated Intel chipset limitation. BTW, I think his power supply is quite adequate so it will be interesting to see how the OP responds.

my2cents said:

CountMike said:

It's rare occasion for PCIe 3.0 could not run on 2.0, more likely your PSU can't cope with it. Just because it is marked 650wats does not mean it is or it does not have enough power on that branch.

Click to expand...

I agree that it will run fine on 2.0; however, the issue (as I see it) is that the OPs mobo has a PCI-e 3.0 slot and if it is set to run at Gen 3 (using the Z77 chipset), it might not work properly because of the stated Intel chipset limitation. BTW, I think his power supply is quite adequate so it will be interesting to see how the OP responds.

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You really can't tell with PSU's just because of their rated wattage. For instance, my 600W PSU has 49a available on the single 12v rail but several 1000W PSUs I've looked at with multiple 12v rails can handle only a maximum of 40a on each rail, etc. It's not the overall wattage figure that is important, it's the amps-per-rail, and of course the general efficiency of the PSU that matter most.

My 2GB 7850 is a 3.0 card but runs fine on my 2.0 motherboard--the limit of my motherboard. The fact his 79xx runs fine for a couple of hours indicates a problem other than that with PCI-e timings--if there was a problem with the PCI-e bus related to the timings then his 79xx simply wouldn't work at all from the start. Sounds to me very much like PSU problems would indicate, though.

My 2GB 7850 is a 3.0 card but runs fine on my 2.0 motherboard

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Hi waltc,

The OPs situation is a bit different; in that he has an (advertised) 3.0 capable mobo with only a 2.0 capable chipset. I have seen many problems reported because of this isuue. Now, maybe the mobo manufacturers have found some way to work around this limitation but I'd sure like to know how they are doing it.



Its just not designed to run 3.0 so maybe the mobo manufacturers have tested it and feel otherwise?

my2cents said:

My 2GB 7850 is a 3.0 card but runs fine on my 2.0 motherboard

Click to expand...

Hi waltc,

The OPs situation is a bit different; in that he has an (advertised) 3.0 capable mobo with only a 2.0 capable chipset. I have seen many problems reported because of this isuue. Now, maybe the mobo manufacturers have found some way to work around this limitation but I'd sure like to know how they are doing it.

View attachment 27748

Its just not designed to run 3.0 so maybe the mobo manufacturers have tested it and feel otherwise?

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Heh... OK! That's one very weird mboard! It doesn't appear that even at 2.0 that it supports 16 lanes! (It should.) It looks like this is a bargain-basement mboard that isn't very happy with his premium retail gpu. I've only used AMD (since 1999) and I have to say this board is among the strangest configs I've ever seen! You might be completely correct. OP should definitely go for a mainstream mboard, no doubt about it.

The fact that it runs for two hours before bombing out is also strange--and normally I would have said PSU. But in this case I can't say. Gotta' change that mboard--for sure--before doing more diagnostics.

my2cents, my friend built a new PC using a Z77 chipset and his GPU is PCI-E 3.0 and has never had this issue. Besides the PSU being the probable cause, have you updated the drivers to the latest 13.10 beta for Windows 8?

nenad285 said:

my2cents, my friend built a new PC using a Z77 chipset and his GPU is PCI-E 3.0 and has never had this issue. Besides the PSU being the probable cause, have you updated the drivers to the latest 13.10 beta for Windows 8?

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You answer is extremely generic! So, you think that because one configuration is known to work that all configurations should work as well? Any chance it is the same gpu and same mobo as the OP's configuration? I'll bet not. So, your answer is to just go out and buy a new PSU? My suggestion wil cost the OP nothing more than a bios change to test this issue. All the OP has to do is change the PCI-e 3.0 slot to run at Gen2 and see if that fixes his issue.

One more thing. Since the power supply keeps popping up as an issue, I decided to look up the power requirements for both the 6950 and the 7950 and the power requirements are the same for both cards:

Video Card Comparison - GPUReview.com

my2cents said:

nenad285 said:

my2cents, my friend built a new PC using a Z77 chipset and his GPU is PCI-E 3.0 and has never had this issue. Besides the PSU being the probable cause, have you updated the drivers to the latest 13.10 beta for Windows 8?

Click to expand...

You answer is extremely generic! So, you think that because one configuration is known to work that all configurations should work as well? Any chance it is the same gpu and same mobo as the OP's configuration? I'll bet not. So, your answer is to just go out and buy a new PSU? My suggestion wil cost the OP nothing more than a bios change to test this issue. All the OP has to do is change the PCI-e 3.0 slot to run at Gen2 and see if that fixes his issue.

Click to expand...

Well I apologize for such a generic answer, I thought the option to change the lanes back a Gen was removed as the lanes change dynamically based on the PCI-E ver of your card.

Well all that just goes to show that until some combination of HW is tried and tested and fat lady sings, there's no sure way to know.

nenad285 said:

my2cents said:

nenad285 said:

my2cents, my friend built a new PC using a Z77 chipset and his GPU is PCI-E 3.0 and has never had this issue. Besides the PSU being the probable cause, have you updated the drivers to the latest 13.10 beta for Windows 8?

Click to expand...

You answer is extremely generic! So, you think that because one configuration is known to work that all configurations should work as well? Any chance it is the same gpu and same mobo as the OP's configuration? I'll bet not. So, your answer is to just go out and buy a new PSU? My suggestion wil cost the OP nothing more than a bios change to test this issue. All the OP has to do is change the PCI-e 3.0 slot to run at Gen2 and see if that fixes his issue.

Click to expand...

Well I apologize for such a generic answer, I thought the option to change the lanes back a Gen was removed as the lanes change dynamically based on the PCI-E ver of your card.

Click to expand...

All I need you to do is explain "how" the Intel Z77 chipset is technically able to support the HD Radeon 7950 running at 3.0 speed? If you know, explain it, and if you don't know (technically), then just say "I don't know" because that's exactly my point so I'll say it again. "I don't know how the Intel Z77 chipset is able to technically support PCI-e 3.0." And this mobo has both.

If his Processor is a Ivy Bridge then it must not have a PCI-E 3.0 controller built in but instead a 2.0 which would explain why his GPU would run at 2.0 speeds. If he has a Sandy Bridge i5 that would also explain not running at 3.0 speeds as SB does not support PCI-E 3.0. As far as bandwidth goes, no gpu fully uses the PCI-E 3.0 bandwidth but still uses the bandwidth 2.0 has to offer. This is the info I know on PCI-E gens if it does not satisfy the answer you wanted then I'm sorry, I don't know and I hope the OP solves his issue and is able to finally use his 7950.

nenad285 said:

If his Processor is a Ivy Bridge then it must not have a PCI-E 3.0 controller built in but instead a 2.0 which would explain why his GPU would run at 2.0 speeds. If he has a Sandy Bridge i5 that would also explain not running at 3.0 speeds as SB does not support PCI-E 3.0. As far as bandwidth goes, no gpu fully uses the PCI-E 3.0 bandwidth but still uses the bandwidth 2.0 has to offer. This is the info I know on PCI-E gens if it does not satisfy the answer you wanted then I'm sorry, I don't know and I hope the OP solves his issue and is able to finally use his 7950.

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Hi nenad,

Thanks for that response! With your explanation, then it looks like there is no way he could use that PCI-e 3.0 card. Let me break it down for you:

If his Processor is a Ivy Bridge then it must not have a PCI-E 3.0 controller

Click to expand...

He does have a 3.0 controller.

If he has a Sandy Bridge i5 that would also explain not running at 3.0 speeds as SB does not support PCI-E 3.0.

Click to expand...

Unknown at this point.

As far as bandwidth goes, no gpu fully uses the PCI-E 3.0 bandwidth but still uses the bandwidth 2.0 has to offer.

Click to expand...

I already made that clear in a previous post.

Actually, here's the way the mobo manufacturer's are implementing PCI-e 3.0 using the Z77 chipset:



Actually, you almost had it. The PCI-e 3.0 slot is required and it must be an Ivy Bridge processor. However, I still say the change to GEN2 speed might have worked and/or if he has a PCI-e 2.0 slot that might have worked as well. But I guess we'll never know because the OP has never responded since his opening statement and I can guarantee you that I will not be responding to this thread again becasue I have had it with the arguing over my recommendation to try a simple BIOS change and everyone else is recommending a costly PSU replacement. OK, I'm done.

In any event, I am glad to hear someone else at least use the phrase "I don't know" in a response. Very refreshing.

OK, sorry for my lack of reply, I've just gotten back off of holiday. I will have a look at some of these issues when I have some time in the next few days, then report back with the results.