# which feature in a x16 PCIe riser cable is more important?



## volcob (Oct 1, 2015)

hi all

I am intending to mod my case to something similar to that of LIAN LI's design where the GPU would face the towards the glass panel of the system case. So I checked on a few comments/reviews regarding riser cables online and a lot of them are negative. I summed up a couple of "expert opinions" which suggest that the cables should have a power source(molex connector) and a capacitor so it could correct minute errors along the data transfers. but I also found other "premium" cable that has shielding on it but w/o any Cap or powersource  
(I thought the shielding might compensate for the absence of the cap and molex since if no interference affects the signal then there wouldnt be a need for correction?? - I have basic knowledge in electronics but I am not an expert or anything so pardon me if im wrong) 

to be short, my question is:

- In theory which one is better?
-- should I opt for the shielded PCIe Risers or the should I go for the ones w/ power supply and capacitor(s)
--- Is there any riser cable that has both features?

I am mainly using the machine for editing and gaming
and the metal side panel has been totally replaced w/ tempered glass similar to the LIAN LI design
http://cdn5.thinkcomputers.org/wp-content/uploads/2015/01/DSC05716.jpg

Riser w/ capacitor and power source 

http://www.frozencpu.com/products/2...Capacitor_-_7.html?tl=c38s2033b77&id=C7uQPNVf

VERSUS

Riser w/o power source or capacitor BUT w/ SHIELDING!
http://www.moddiy.com/products/Prem...dE-Extension-Shielded-Cable-Riser-(19cm).html


----------



## MIRTAZAPINE (Oct 9, 2015)

I would go with regular inexpensive unpowered pcie risers. It is not much of problem if I recall right from people doing lite coin or alternative coin mining where they put up to 5 graphic cards connected to an unpowered risers. Powered risers have the risk of getting burn from drawing to much power from the experiences of the miners.


----------



## Athlonite (Oct 9, 2015)

http://www.moddiy.com/products/PCI%2dExpress-PCI%2dE-Extension-Cable-Riser-(19cm).html


----------



## R-T-B (Oct 9, 2015)

MIRTAZAPINE said:


> I would go with regular inexpensive unpowered pcie risers. It is not much of problem if I recall right from people doing lite coin or alternative coin mining where they put up to 5 graphic cards connected to an unpowered risers. Powered risers have the risk of getting burn from drawing to much power from the experiences of the miners.



This.  Powered risers are only relevant when running many GPUs on the same board, usually 4 or more.


----------



## Aquinus (Oct 9, 2015)

R-T-B said:


> This.  Powered risers are only relevant when running many GPUs on the same board, usually 4 or more.


...or if the extension is too long and the power pins on the PCI-E slot have more voltage drop than is allowable.


----------



## sneekypeet (Oct 9, 2015)

R-T-B said:


> This.  Powered risers are only relevant when running many GPUs on the same board, usually 4 or more.



Our own @AthlonX2 found that standard risers also are not enough to power dual GPU cards either. I was more than able to run my HD7950 on this cable, but his 295 draws too much power from the slot, and is forced to look into powered versions. 

I say look for the shielded ones for a normal single GPU card, only for looks as the black tended to blend into a build much better.


----------



## R-T-B (Oct 9, 2015)

Thanks for the backup opinions/facts guys.  That was conventional cryptocoin mining wisdom I was providing, which obviously is not a normal usage scenario.


----------



## MIRTAZAPINE (Oct 9, 2015)

R-T-B said:


> Thanks for the backup opinions/facts guys.  That was conventional cryptocoin mining wisdom I was providing, which obviously is not a normal usage scenario.



Oh I forget that mining you don't need to use the full PCIe lanes without much effect in mining performance unlike gaming. I learn something new from this thread.


----------



## OneMoar (Oct 9, 2015)

low resistance low capacitance
pci-e does not like noise
I would not use a powered adaptor tho you run the risk of a short and frying both the board and gpu


----------



## newtekie1 (Oct 9, 2015)

Since you are going for a build that focuses more on looks, I say get the black shielded cable without power.  For a single GPU card it shouldn't be a problem.

However, if power becomes an issues, pick up one of these. That will boost the PCI-E power for all slots, and should give enough power to the PCI-E bus to use the riser without problem.


----------



## sneekypeet (Oct 9, 2015)

newtekie1 said:


> Since you are going for a build that focuses more on looks, I say get the black shielded cable without power.  For a single GPU card it shouldn't be a problem.
> 
> However, if power becomes an issues, pick up one of these. That will boost the PCI-E power for all slots, and should give enough power to the PCI-E bus to use the riser without problem.



That was brought up when we had the issue. I am told it will not work for the adapters. That device is more for conditioning the power that runs through it, not for adding additional power through it.


----------



## newtekie1 (Oct 10, 2015)

sneekypeet said:


> That was brought up when we had the issue. I am told it will not work for the adapters. That device is more for conditioning the power that runs through it, not for adding additional power through it.



Odd, that seems to be the opposite of how it is designed.  It takes +12v from a molex connector and connects it to the PCI-E bus.  I see nothing on it that would condition power in any way.  There are no caps or anything.  It is a direct connection between the molex and the PCI-E bus.  AFAIK, this works the same way an auxilary connector on the motherboard would work, it just provides more power to the PCI-E bus.


----------



## sneekypeet (Oct 10, 2015)

newtekie1 said:


> Odd, that seems to be the opposite of how it is designed.  It takes +12v from a molex connector and connects it to the PCI-E bus.  I see nothing on it that would condition power in any way.  There are no caps or anything.  It is a direct connection between the molex and the PCI-E bus.  AFAIK, this works the same way an auxilary connector on the motherboard would work, it just provides more power to the PCI-E bus.



I'm no EE by any means. In our specific instance, we brought that idea to the manufacturer (Pretty sure the rep is an EE though) and were then told it would not provide what the card needed, IE more slot power. I may have also used the wrong term.....lets say the device delivers a better power source to the slots rather than using longer traces in the motherboard to provide the same, but better power to the slots.


----------



## volcob (Oct 10, 2015)

Aquinus said:


> ...or if the extension is too long and the power pins on the PCI-E slot have more voltage drop than is allowable.


how long is "too long" I intend to use a 250 mm (9.84 inches) riser... 



MIRTAZAPINE said:


> I would go with regular inexpensive unpowered pcie risers. It is not much of problem if I recall right from people doing lite coin or alternative coin mining where they put up to 5 graphic cards connected to an unpowered risers. Powered risers have the risk of getting burn from drawing to much power from the experiences of the miners.





OneMoar said:


> low resistance low capacitance
> pci-e does not like noise
> I would not use a powered adaptor tho you run the risk of a short and frying both the board and gpu



thanks for your Opinions guys though I'm a bit confused w/ which one of you is right... which one poses the risk of frying either the mobo or the GPU, is it the powered riser or the UNpowered riser?



newtekie1 said:


> Since you are going for a build that focuses more on looks, I say get the black shielded cable without power.  For a single GPU card it shouldn't be a problem.
> 
> However, if power becomes an issues, pick up one of these. That will boost the PCI-E power for all slots, and should give enough power to the PCI-E bus to use the riser without problem.



while I intend to improve the looks of my build, I also want it to work as a workstation for photo editing (I work as a freelance photographer during weekends) 

I have an out of the topic question though... would you happen to have any idea about the Intel co processor that looks like a GPU (or maybe it is) does it require any SLI connection? and if I add it on into my rig along w/ my NVIDIA GPU do I have to use the POWERED RISER for both now?


----------



## IBMer (Oct 11, 2015)

Thanks for starting this very interesting discussion


----------



## zvonkorp (Oct 12, 2015)

volcob said:


> hi all
> 
> I am intending to mod my case to something similar to that of LIAN LI's design where the GPU would face the towards the glass panel of the system case. So I checked on a few comments/reviews regarding riser cables online and a lot of them are negative. I summed up a couple of "expert opinions" which suggest that the cables should have a power source(molex connector) and a capacitor so it could correct minute errors along the data transfers. but I also found other "premium" cable that has shielding on it but w/o any Cap or powersource
> (I thought the shielding might compensate for the absence of the cap and molex since if no interference affects the signal then there wouldnt be a need for correction?? - I have basic knowledge in electronics but I am not an expert or anything so pardon me if im wrong)
> ...



*Good question, Why is this?*

First a watery analogy: water running in a pipe and through a tap. Now turn off the tap quickly. You'll hear a loud knock/shock sound (voltage spike) in the pipe (wire). The water (current) wants to continue flowing but it can't, for a moment the water pressure (voltage) is much higher than the static water pressure (voltage). It's the same *for a current that's switched off, because of the inductance in the power wires in the flexible riser cable it wants to keep on flowing*, voltage gets higher. This is also what causes sparks (brush fire) in a _brushed_ motor.

The reason this is relevant is because once the current leaves the traces of the motherboard slot, and starts traveling through the wires in the pci-extender cable, because of the wire inductance in that cable, it has a tendency to want to keep flowing. Wire inductance increases as the cross sectional area of the conductor decreases. So what does this have to do with capacitors? Normally these pins would have their current coming in from traces on the motherboard, but when using these powered risers, all of the wires on them are very small diameter. This means they have higher inductance, and are not designed necessarily to absorb loads like would be demanded by GPUs.


Well, depending on the game, or load, this might make a difference, or it might not, but generally, when you pause a game and go to a menu, the graphics load drops sharply & GPU usage is able to decrease & use significantly less power as it only has to render a full screen menu. Not all games have these kinds of load variations. But a lot do. So you have a GPU that uses 100-200 watts of power, and you put an riser cable between it & the motherboard, and you're playing an intensive game, and you go to pause it. The GPU load drops sharply from 100% to near 0%, the power usage goes from up to 200 or more watts down to just a trickle.

Now these power pins are coursing with energy, normally this is a non issue when the card is plugged in directly. But *add a flexible cable, and the issue of inductance comes up*. When the draw from the GPU goes from a few hundred watts down to just a few, there is excess current, with "momentum" in the wires of the flexible cable. The power pins, that have been sending current through themselves, saturate the wire with voltage that wants to keep flowing. It is here now that typically powered risers will be equipped with a capacitor, downstream, right before the female-connector, with which to soak up the momentum of the current & the increased voltage (as a result of wire inductance) & prevent it from getting to the VRMs on the GPU board. Without it, the VRMs get slammed with an inrush of current that would otherwise just be soaked up by the capacitor upstream, soldered on to the cable. It is also for this reason that these power wires are much thicker & beefier than the tiny signal wires in the flexible cable itself. The inductance of a wire decreases as it's cross sectional area increases.

So while the votes overwhelmingly agree to get the powered cable, you can now rationalize why & be able to quantify in some way the reasoning behind most of these cables having these capacitors & power leads on them. This is even more applicable when GPUs are used that have no external pci-e power ports ,and that draw all their power from the PCI-E slot off the motherboard directly.

*References*
1) hyperphysics.phy-astr.gsu.edu...indeng.html#c2
Shows that energy stored is proportional to inductance, L, and proportional to current squared, I^2
2) www.ee.scu.edu/eefac/healy/indwire.html
Shows that the inductance of a straight wire is proportional to the length of the wire times the natural log of the length of the wire ...which is close to being linearly proportional.
(References thanks Panther3001, post #685)


----------

