PCI express risers to use multiple GPUs on one motherboard - not detecting card?

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Peter Hucker
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Message 94970 - Posted: 14 Jan 2020, 12:11:13 UTC

I'm trying to plug multiple GPUs into one motherboard. The problem is the motherboard only has two PCI express x16 slots, and they're only two apart, which means the standard double width graphics cards can't both go in at once, or there's no airflow. Also I'd like to in the future put more than 2 GPUs in there, using the motherboard's x1 slots, or even multiplexing slots. So I got some PCI express risers (1x to 16x), but when I plug a card through one of those, it doesn't work properly. I get no monitor output at all. If I connect to the machine remotely, windows has booted, but calls the card something like a standard VGA, definitely no OpenCL abilities. In fact I'm not convinced the card is doing anything at all, that might just be monitor emulation over the network. I had a pack of several of these risers left over from when I was going to do bitcoin mining with them years ago, but never used them, so I've tried a few to make sure it's not just a faulty one. Does anyone have experience with these? Compatibility problems? I've read a few articles on using them for bitcoin mining and I can find nothing to suggest that any card AMD or Nvidia would object to only having 1 lane. But then mining might not need Windows to detect the card and Boinc does?
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Message 94972 - Posted: 14 Jan 2020, 12:51:47 UTC

There are an incredible number of very poor quality risers on the market.
Price and brand name are not guides to quality, only very careful inspection and testing will "prove" that you have good or bad ones. Some risers will work for a short time (hours or a couple of days), but will then fail, at least one user I know has bought dozens and only found one or two in a batch actually work.
Another thing is that some are really only designed to work with mining GPUs and will not work with "proper" GPUs.
But before you despair make sure the GPUs work when plugged into the motherboard.
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Peter Hucker
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Message 94973 - Posted: 14 Jan 2020, 13:14:22 UTC - in response to Message 94972.  
Last modified: 14 Jan 2020, 13:16:26 UTC

There are an incredible number of very poor quality risers on the market.
Price and brand name are not guides to quality, only very careful inspection and testing will "prove" that you have good or bad ones. Some risers will work for a short time (hours or a couple of days), but will then fail, at least one user I know has bought dozens and only found one or two in a batch actually work.
Another thing is that some are really only designed to work with mining GPUs and will not work with "proper" GPUs.
But before you despair make sure the GPUs work when plugged into the motherboard.


The GPUs are perfectly happy in the motherboard, even both at once (although cooling problems mean I can't run them both for more than a few minutes).

So are you saying that no GPU or motherboard or Windows itself would ever object to a single lane communication with a GPU?

I have ordered a few other risers to try different types and makes to see what works where, waiting for them to arrive in the post....

What did you mean by "Another thing is that some are really only designed to work with mining GPUs and will not work with "proper" GPUs."? Aren't mining and Boinc both using Cuda/OpenCL? Is there something missing from the riser so Windows won't detect the card?
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Message 94986 - Posted: 14 Jan 2020, 14:52:38 UTC - in response to Message 94973.  

A mining GPU doesn't necessarily have a monitor output. It's just a coprocessor on a card.
Please do not private message me for tech support, these will be ignored!
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Message 94990 - Posted: 14 Jan 2020, 15:34:56 UTC - in response to Message 94986.  
Last modified: 14 Jan 2020, 15:37:36 UTC

A mining GPU doesn't necessarily have a monitor output. It's just a coprocessor on a card.


Oh. I thought all the riser did was pass the PCI express lane and wouldn't even know what was plugged into it.

Can you recommend one you know that works please?

I have all this stuff on order, not arrived yet so untested (the one that failed to work looked a lot cheaper than the below):
https://www.ebay.co.uk/itm/372799912233
https://www.ebay.co.uk/itm/263820775128
https://www.ebay.co.uk/itm/132726755095
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Message 94991 - Posted: 14 Jan 2020, 15:40:11 UTC - in response to Message 94973.  

RISERS my dear boy RISERS.
For some totally incomprehensible reason some will work quite happily with mining GPUs and not with proper GPUs.
I know that folks have managed to get quite high GPU counts using risers and splitters, and even mining motherboards, but they have spent a fair bit of time, money and effort weeding out combinations of hardware that simply do not work.

What Jord has said has jolted something in my mind - Did Windows 10 decide to update the drivers for you (without your permission or knowledge) because the fact that the GPU is sort of working on a riser sounds very much like the problems people have when they have been victims of MS's cut-down drivers (which don't have the bits needed for computation, but have the same version number as the full-blown driver)
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Message 94992 - Posted: 14 Jan 2020, 15:41:01 UTC - in response to Message 94990.  
Last modified: 14 Jan 2020, 15:43:50 UTC

As far as I know, a riser allows you to move the GPU into a different orientation than the original one on the motherboard. For instance, some cases allow the GPU to stand upright with its edge connector pointing down. This can't work as the PCIe slot on the motherboard is vertically orientated, due to the motherboard being bolted to the case. A riser allows you to put the GPU in with the edge connector down. A riser can also allow you to move the GPU completely out of the case, if you want to.

I don't have any personal experience with riser cables, so can't help you there.

What I was talking about was the monitor output. A Graphics Processor Unit will have one or more monitor outputs, either HDMI, DisplayPort, DIVX or even still VGA. A unit for mining only may not have these on board as they're not needed, miners don't use the graphics output on their GPUs for anything, they just mine with their GPUs.
Please do not private message me for tech support, these will be ignored!
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Message 94994 - Posted: 14 Jan 2020, 15:54:31 UTC - in response to Message 94990.  

Sorry, I can't help you with specific make/types, and, and far as I can see what worked for someone yesterday may not work for the same person buying today. So sadly the only way to find out is to test them one at a time and see which ones actually work FOR YOU TODAY.

There are multitude of reasons for a riser not working, including poor workmanship. The main ones are probably to do with upsetting the bus timing or causing signal reflections on the bus.

You have got "powered" risers haven't you - they appear to be a little less troublesome than unpowered ones - even when using GPUs that don't need external power.

Jord - yes the function of a riser is to allow something to be moved out of its normal position out of the way. And, it was your comment about mining GPUs not having graphical outputs that prompted my comment about drivers.
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Message 94995 - Posted: 14 Jan 2020, 15:59:14 UTC

I did a quick search on PCIe electrical compliance testing. The PCI Special Interest Group seems to be the place to go. I'm not getting anything about the sort of cables which might be used to make a riser. And if there's no standard, then there's no chance any manufacturer will be building to standard. Timing, crosstalk, waveform all come to mind as imponderables at the sort of speeds we're talking.
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Message 94999 - Posted: 14 Jan 2020, 16:30:09 UTC - in response to Message 94970.  

I'm trying to plug multiple GPUs into one motherboard. The problem is the motherboard only has two PCI express x16 slots, and they're only two apart, which means the standard double width graphics cards can't both go in at once, or there's no airflow. Also I'd like to in the future put more than 2 GPUs in there, using the motherboard's x1 slots, or even multiplexing slots. So I got some PCI express risers (1x to 16x), but when I plug a card through one of those, it doesn't work properly. I get no monitor output at all. If I connect to the machine remotely, windows has booted, but calls the card something like a standard VGA, definitely no OpenCL abilities. In fact I'm not convinced the card is doing anything at all, that might just be monitor emulation over the network. I had a pack of several of these risers left over from when I was going to do bitcoin mining with them years ago, but never used them, so I've tried a few to make sure it's not just a faulty one. Does anyone have experience with these? Compatibility problems? I've read a few articles on using them for bitcoin mining and I can find nothing to suggest that any card AMD or Nvidia would object to only having 1 lane. But then mining might not need Windows to detect the card and Boinc does?


What motherboard do you have exactly? make and model please. So I can see the exact specs and speeds of the PCIe slots. some older motherboards might only run some slots at PCIe 2.0 vs 3.0 or will have the electrical layout of the slot differ from the mechanical layout. Or even some boards will disable certain slots when others are in use. I just want to double check all that, I can find it myself if I know the make and model of board.

What project are you trying to run on the GPU? Some projects like SETI will be fine with only 1x lane, but other projects you will see major performance impacts when only using 1 lane.

I run several different kinds of multi-GPU setups. One with 16x lane PCIe 3.0 rated ribbon cables, One with the 1x lane USB 3.0 cables, and another with all cards plugged directly to the motherboard.

When using risers and trying to run full PCIe 3.0 speeds, you NEED a quality cable (either USB Ribbon) as the shielding is very important to maintain the signal integrity. many people who are using the USB cables as riser for mining can get away with rather low end cables because they run the PCIe lanes at reduced speeds most of the time, either PCIe 2.0 or even 1.0. mining has incredibly low bandwidth requirements, so the reduced speeds don't matter there, but likely will matter on BOINC projects.

On my USB riser systems, I've had to replace nearly all of my USB cables with higher quality ones, and I've helped many other users over at SETI solve riser issues by just telling them to replace their USB cables with better ones. It's probably a good idea to just do that as it's not very expensive and will remove that variable from any other troubleshooting you need to do.

I've had good luck with the UGREEN branded cables from amazon: https://www.amazon.com/UGREEN-Transfer-Enclosures-Printers-Cameras/dp/B00P0E3954/

If you want to run a full lane ribbon cable, make sure you use a SHIELDED riser. the old grey colored "floppy cable" are no good. they have no shielding and will likely cause issues most of the time. get a good quality cable from 3M/Thermaltake, etc.

Thermaltake 16x riser: https://www.amazon.com/Thermaltake-Gaming-Black-Extender-AC-053-CN1OTN-C1/dp/B06Y5YNGDJ/

I personally use some EZDIY-FAB risers from Amazon: https://www.amazon.com/dp/B07K9SRKCT/
but bear in mind that I did have to return 2 or 3 of them for QC issues (poor signal quality, dropped GPUs, low GPU utilization). but the replacements worked fine and they have been running near 24/7 for over a year now.

another thing to keep in mind with the 16x ribbon cables is that they will send the power to the GPU via this cable, make sure your motherboard can handle that. usually not a problem with only 2 GPUs but can cause issues with 4+ if you don't provide enough power to the motherboard.
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Message 95008 - Posted: 14 Jan 2020, 17:33:04 UTC - in response to Message 94999.  

I'm trying to plug multiple GPUs into one motherboard. The problem is the motherboard only has two PCI express x16 slots, and they're only two apart, which means the standard double width graphics cards can't both go in at once, or there's no airflow. Also I'd like to in the future put more than 2 GPUs in there, using the motherboard's x1 slots, or even multiplexing slots. So I got some PCI express risers (1x to 16x), but when I plug a card through one of those, it doesn't work properly. I get no monitor output at all. If I connect to the machine remotely, windows has booted, but calls the card something like a standard VGA, definitely no OpenCL abilities. In fact I'm not convinced the card is doing anything at all, that might just be monitor emulation over the network. I had a pack of several of these risers left over from when I was going to do bitcoin mining with them years ago, but never used them, so I've tried a few to make sure it's not just a faulty one. Does anyone have experience with these? Compatibility problems? I've read a few articles on using them for bitcoin mining and I can find nothing to suggest that any card AMD or Nvidia would object to only having 1 lane. But then mining might not need Windows to detect the card and Boinc does?


What motherboard do you have exactly? make and model please. So I can see the exact specs and speeds of the PCIe slots. some older motherboards might only run some slots at PCIe 2.0 vs 3.0 or will have the electrical layout of the slot differ from the mechanical layout. Or even some boards will disable certain slots when others are in use. I just want to double check all that, I can find it myself if I know the make and model of board.

What project are you trying to run on the GPU? Some projects like SETI will be fine with only 1x lane, but other projects you will see major performance impacts when only using 1 lane.

I run several different kinds of multi-GPU setups. One with 16x lane PCIe 3.0 rated ribbon cables, One with the 1x lane USB 3.0 cables, and another with all cards plugged directly to the motherboard.

When using risers and trying to run full PCIe 3.0 speeds, you NEED a quality cable (either USB Ribbon) as the shielding is very important to maintain the signal integrity. many people who are using the USB cables as riser for mining can get away with rather low end cables because they run the PCIe lanes at reduced speeds most of the time, either PCIe 2.0 or even 1.0. mining has incredibly low bandwidth requirements, so the reduced speeds don't matter there, but likely will matter on BOINC projects.

On my USB riser systems, I've had to replace nearly all of my USB cables with higher quality ones, and I've helped many other users over at SETI solve riser issues by just telling them to replace their USB cables with better ones. It's probably a good idea to just do that as it's not very expensive and will remove that variable from any other troubleshooting you need to do.

I've had good luck with the UGREEN branded cables from amazon: https://www.amazon.com/UGREEN-Transfer-Enclosures-Printers-Cameras/dp/B00P0E3954/

If you want to run a full lane ribbon cable, make sure you use a SHIELDED riser. the old grey colored "floppy cable" are no good. they have no shielding and will likely cause issues most of the time. get a good quality cable from 3M/Thermaltake, etc.

Thermaltake 16x riser: https://www.amazon.com/Thermaltake-Gaming-Black-Extender-AC-053-CN1OTN-C1/dp/B06Y5YNGDJ/

I personally use some EZDIY-FAB risers from Amazon: https://www.amazon.com/dp/B07K9SRKCT/
but bear in mind that I did have to return 2 or 3 of them for QC issues (poor signal quality, dropped GPUs, low GPU utilization). but the replacements worked fine and they have been running near 24/7 for over a year now.

another thing to keep in mind with the 16x ribbon cables is that they will send the power to the GPU via this cable, make sure your motherboard can handle that. usually not a problem with only 2 GPUs but can cause issues with 4+ if you don't provide enough power to the motherboard.


It's a very old motherboard: Asus P5N-D
I can run two cards ok directly in the slots (they're just too close together for cooling like that).
I'm trying to run Milkyway, but I haven't got that far! The card isn't recognised in Windows, or Boinc, or even to display anything while booting, if a riser is used.
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Message 95009 - Posted: 14 Jan 2020, 17:36:26 UTC - in response to Message 94994.  

Sorry, I can't help you with specific make/types, and, and far as I can see what worked for someone yesterday may not work for the same person buying today. So sadly the only way to find out is to test them one at a time and see which ones actually work FOR YOU TODAY.

There are multitude of reasons for a riser not working, including poor workmanship. The main ones are probably to do with upsetting the bus timing or causing signal reflections on the bus.

You have got "powered" risers haven't you - they appear to be a little less troublesome than unpowered ones - even when using GPUs that don't need external power.

Jord - yes the function of a riser is to allow something to be moved out of its normal position out of the way. And, it was your comment about mining GPUs not having graphical outputs that prompted my comment about drivers.


The risers I tried were powered, but only by a molex lead being soldered directly to it with a bit of heatshrink around it! One of the new ones I've bought (the single PCI express to 16x PCI Express using a USB cable) is powered by a 6 pin PCI express socket, and has many capacitors and a transistor on the board, so that should be more reliable. But I can't see power being too much of a problem, as I hadn't got to the point of starting a project running on it, unless the card checks for power when booting?
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Message 95010 - Posted: 14 Jan 2020, 17:38:41 UTC - in response to Message 94991.  

RISERS my dear boy RISERS.


Not sure what you mean by that. I did say risers.

For some totally incomprehensible reason some will work quite happily with mining GPUs and not with proper GPUs.


Indeed, that makes no sense at all, in fact they should work with anything that uses PCI express. They're just a bus extension and shouldn't be touching the data.

What Jord has said has jolted something in my mind - Did Windows 10 decide to update the drivers for you (without your permission or knowledge) because the fact that the GPU is sort of working on a riser sounds very much like the problems people have when they have been victims of MS's cut-down drivers (which don't have the bits needed for computation, but have the same version number as the full-blown driver)


Latest Radeon driver from AMD. But it's not the driver, as it's actually stopping the monitor from displaying while booting.
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Message 95017 - Posted: 14 Jan 2020, 18:13:28 UTC - in response to Message 95008.  
Last modified: 14 Jan 2020, 18:14:54 UTC

It's a very old motherboard: Asus P5N-D
I can run two cards ok directly in the slots (they're just too close together for cooling like that).
I'm trying to run Milkyway, but I haven't got that far! The card isn't recognised in Windows, or Boinc, or even to display anything while booting, if a riser is used.


OK, that board only has PCIe 2.0 16x slots (the 1x slots are only PCIe 1.0 btw!)

It's possible the USB riser you have is faulty in some way, either the cable or the board, and yes the the system wont boot if the card does not have proper power. all power connectors need to be plugged in. I had good luck with the style of riser that takes the 6-pin PCIe power connector, but as with all things electronic, they sometimes just fail.

I would try one (or two) of the 16x shielded ribbon cables I linked. they will extend the PCIe lanes up to a remote card and will not require any additional power connectors. it's a straight extension of the slot. I think since the slots are only PCIe 2.0 you'll want all the PCIe bandwidth you can get for Milkyway.
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Message 95019 - Posted: 14 Jan 2020, 18:19:45 UTC

Exactly which AMD GPU cards are you trying to use? Do they have sockets for supplementary power input?

If they don't have sockets, they'll take all the power they need - up to 75 watts - from the motherboard via the 16-bit slots. A 1-bit slot won't supply enough power, and you'll have to check that your riser can carry enough power.
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Message 95022 - Posted: 14 Jan 2020, 18:32:49 UTC - in response to Message 95017.  

It's a very old motherboard: Asus P5N-D
I can run two cards ok directly in the slots (they're just too close together for cooling like that).
I'm trying to run Milkyway, but I haven't got that far! The card isn't recognised in Windows, or Boinc, or even to display anything while booting, if a riser is used.


OK, that board only has PCIe 2.0 16x slots (the 1x slots are only PCIe 1.0 btw!)

It's possible the USB riser you have is faulty in some way, either the cable or the board, and yes the the system wont boot if the card does not have proper power. all power connectors need to be plugged in. I had good luck with the style of riser that takes the 6-pin PCIe power connector, but as with all things electronic, they sometimes just fail.

I would try one (or two) of the 16x shielded ribbon cables I linked. they will extend the PCIe lanes up to a remote card and will not require any additional power connectors. it's a straight extension of the slot. I think since the slots are only PCIe 2.0 you'll want all the PCIe bandwidth you can get for Milkyway.


Will PCI express 1.0 be ok for Boinc? I'd like to use those slots aswell, unless I can multiplex the PCI express 2.0 slots. Is there any way I can tell how much data is being transferred on the PCI express bus? Or look up the requirement by a project? I was under the impression that Boinc was similar to bitcoin in that a small amount of data was passed to the card, it processed hard for ages, then passed back a small amount of data.

I tried 2 of the 7 risers I had lying about, and neither produced even the BIOS screen on the monitor. They have been sat in my garage (which is only heated to 10C for the parrots), and I have found some DVI cables stored in there that have gone rusty! The sooner I get a heat pump with heating/AC/dehumidification all in one the better, Scotland is DAMP!

A graphics card once scared the hell out of me late in the evening when I forgot to plug in the extra power connectors. As soon as I turned on the PC, it made a very loud alarm sound, and flashed in red block capitals on the screen something along the lines of me being a stupid idiot and hadn't plugged things in correctly. I thought I had a horrid boot virus or something was about to catch fire.

One of the things I've ordered is a straight ribbon cable, 16x to 16x, looks similar to yours, hopefully that will work. Although I need the 1x to 16x to work if I want a 3rd card or more.
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Message 95023 - Posted: 14 Jan 2020, 18:37:58 UTC - in response to Message 95019.  

Exactly which AMD GPU cards are you trying to use? Do they have sockets for supplementary power input?

If they don't have sockets, they'll take all the power they need - up to 75 watts - from the motherboard via the 16-bit slots. A 1-bit slot won't supply enough power, and you'll have to check that your riser can carry enough power.


AMD Sapphire R9 280 X (two of). They both have twin 8 pin PCI Express power connectors. How does the power allocation work? Are they 3 seperate inputs and the voltage regulator on the card takes what it needs in preference? Or are they just all shorted together? The riser I tried had a rather flimsy attachment of a molex cable soldered straight onto it. Can you take 75 watts from a 1 lane socket or is that only for the 16x ones?
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Message 95025 - Posted: 14 Jan 2020, 18:42:53 UTC - in response to Message 95019.  

Exactly which AMD GPU cards are you trying to use? Do they have sockets for supplementary power input?

If they don't have sockets, they'll take all the power they need - up to 75 watts - from the motherboard via the 16-bit slots. A 1-bit slot won't supply enough power, and you'll have to check that your riser can carry enough power.


USB style risers do not take any power from the slot, they will get power from the external connection on the PCIe 16x to USB PCB.

it sounds like he has one USB style riser like this:


and one 1x ribbon with external molex like this:


He should only be plugging these into the 16x slots.

but for what he's doing I still recommend just buying nice shielded ribbon risers like what I linked before. he can find the same thing from a UK based seller.
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Message 95027 - Posted: 14 Jan 2020, 18:46:04 UTC - in response to Message 95023.  

Exactly which AMD GPU cards are you trying to use? Do they have sockets for supplementary power input?

If they don't have sockets, they'll take all the power they need - up to 75 watts - from the motherboard via the 16-bit slots. A 1-bit slot won't supply enough power, and you'll have to check that your riser can carry enough power.


AMD Sapphire R9 280 X (two of). They both have twin 8 pin PCI Express power connectors. How does the power allocation work? Are they 3 seperate inputs and the voltage regulator on the card takes what it needs in preference? Or are they just all shorted together? The riser I tried had a rather flimsy attachment of a molex cable soldered straight onto it. Can you take 75 watts from a 1 lane socket or is that only for the 16x ones?
Try https://en.wikipedia.org/wiki/PCI_Express#Power
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Message 95028 - Posted: 14 Jan 2020, 18:46:25 UTC - in response to Message 95023.  

Exactly which AMD GPU cards are you trying to use? Do they have sockets for supplementary power input?

If they don't have sockets, they'll take all the power they need - up to 75 watts - from the motherboard via the 16-bit slots. A 1-bit slot won't supply enough power, and you'll have to check that your riser can carry enough power.


AMD Sapphire R9 280 X (two of). They both have twin 8 pin PCI Express power connectors. How does the power allocation work? Are they 3 seperate inputs and the voltage regulator on the card takes what it needs in preference? Or are they just all shorted together? The riser I tried had a rather flimsy attachment of a molex cable soldered straight onto it. Can you take 75 watts from a 1 lane socket or is that only for the 16x ones?


the PCIe power connectors on the GPUs have sense pins, they know when it's not plugged in and will not start unless you do so, and will usually give that warning message on the screen when you have forgotten.

you need all power connectors plugged in. If you are not doing that, then you should try that first.
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