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Author Topic: Q's regarding use of risers  (Read 6430 times)
jayeeyee
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May 13, 2013, 10:04:51 AM
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I've never sought the need to use risers before because my residency has free central cooling/heating and my rigs rarely rise above the 80c mark even during the summer.  However, I've decided to make use of my motherboard real-estate and will be adapting to risers in the near future.  I've read a few post and DIY'er regarding risers, both have conflicting advice's on the uses/application for them.  One thing for certain is the luxury of being able to re-position the card in a manner in which it will benefit cooling, effectively lowering temps and thus ultimately reducing electrical cost (unfortunately, is not free for me / I'd trade free HVAC for free electricity any day!).

Some say 16x-16x risers do not require a molex pigtail connector cable (powered type) since the PCIE 2.0 16x/8x/4x slots on the motherboard can provide enough "juice" to run the cards in addition to the 6/8pin power connector from the PSU.  However, I've seen/read mining rigs with multiple card (minimum 4 or more) on each mobo using them and I'm wondering why is that if the above former mentioned is true?  Does powered risers add an additional level of "stability" of some sort when it comes to mining?

As for 1x-16x risers, once again, conflicting results from various sources.  Some say it's not required while other's say it is.  In my opinion, I believe it makes more sense to use powered 1x-16x risers compared to powered 16x-16x risers since PCIE 1x slots are rated at a lower voltage (10w iirc, correct me if I'm wrong) and common logic would suggest a powered 1x-16x riser fits the equation if you don't want to run into stability issues during mining. However, some say it's not required since all cards running on risers (both powered and un-powered) are still being powered by the PCIE connectors from the PSU.

So what is the verdict on this conundrum?  Is there a specific application when powered or non-powered risers are required?  Thanks for your time reading this silly but yet equally important post.  Wink
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HellDiverUK
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May 13, 2013, 11:36:44 AM
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So what is the verdict on this conundrum?  

Sometimes it works, sometimes it doesn't.  I've run a HD7750 (which draws all it's power from the slot) off a 1x-16x unpowered riser.  Yet the same riser won't run a HD7770 which has it's own 6-pin power supply.

It really is "try it and see".
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May 13, 2013, 01:57:05 PM
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Yes.

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May 13, 2013, 04:46:54 PM
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It has nothing to do with 1x or 16x... all have the same power. The "extra" x's are just data-connections, not power.

It has to do with "your available power on your 12v rails" which connects to the 6-8pin connections. If you do not supply power there, it will be drawn from the PCIe rails.

If the total of your cards exceeds 75watts (the standard "must supply" for PCIe), using the (standard 25w draw max for cards on a PCIe slot), then you NEED supplemental power.

Or, if you use cheap china risers which can barely handle 15watts of power on those thin wires... then you need powered risers.

Or, if you simply have more than 3 cards, which would possibly pull more than the PCIe supply of 75w, from the multiple 25w-pull from each card. 3 = 75w (ok)... 4 = 100w (bad)... If your 6-8pin connections are not supplying the required "load".

Although the "standard" is 75watts, many OC boards are designed for more than that. Crossfire-ready and NVidia-SLi boards and PSU's are usually designed for the "extra PCIe loads". But the china-wires (old hard drive data-cables used to make cheap risers), are not.
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May 13, 2013, 04:52:50 PM
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It has nothing to do with 1x or 16x... all have the same power. The "extra" x's are just data-connections, not power.

It has to do with "your available power on your 12v rails" which connects to the 6-8pin connections. If you do not supply power there, it will be drawn from the PCIe rails.

If the total of your cards exceeds 75watts (the standard "must supply" for PCIe), using the (standard 25w draw max for cards on a PCIe slot), then you NEED supplemental power.

Or, if you use cheap china risers which can barely handle 15watts of power on those thin wires... then you need powered risers.

Or, if you simply have more than 3 cards, which would possibly pull more than the PCIe supply of 75w, from the multiple 25w-pull from each card. 3 = 75w (ok)... 4 = 100w (bad)... If your 6-8pin connections are not supplying the required "load".

Although the "standard" is 75watts, many OC boards are designed for more than that. Crossfire-ready and NVidia-SLi boards and PSU's are usually designed for the "extra PCIe loads". But the china-wires (old hard drive data-cables used to make cheap risers), are not.

Not always true. Some cards have separate power planes on the card. The HD 5870 is notorious for drawing the current to feed the memory subsystem from the PCIe slot.

That being said I've run many 5870s off 1x-16x risers with no ill effect. I do run them on Rampage III Extremes with auxiliary 4-pin molex connectors for mobo's PCIe power plane. The +12v pins on my 24-pin and aux molex connectors are warm to the touch and the risers are warm to the touch but not in danger of melting the insulation.
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May 13, 2013, 05:26:48 PM
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There is no such thing as separate power plains on cards. They all share the same power supply, the same ground, and gate regulated 12v sources...

Again... as I said... if your 6-8pin does NOT supply enough POWER, it will DRAW from the PCIe rails.

Obviously, you do NOT supply enough power through your other connections.

If it was isolated, it would NEVER be able to power the card. It would have to power something unrelated to the card, like a fan or data-controller, which is still connected to the other sources.

It may have horrible designed gate regulators, but that is NOT "isolation". If that were true, then "everyone" should have that issue, and those who use correct power, do not, thus, it is your assumption that it is isolated.

I just tested one, and the traces are NOT isolated, and all connections test as connected.

Cards haven't been built with "isolation" since the voodoo cards came-out.

Check your power. Your PSU is obviously "short" on its values. Possibly you are seeing the MAX/peak values, and treating them as if they are constant values. (Read the AMPS off your 6-8pins, your card regulator is just trying to distribute the load across all three supplies.)

If you are using a 5870 still... Power is the least of your worries.
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May 13, 2013, 07:13:14 PM
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There is no such thing as separate power plains on cards. They all share the same power supply, the same ground, and gate regulated 12v sources...

Again... as I said... if your 6-8pin does NOT supply enough POWER, it will DRAW from the PCIe rails.

Obviously, you do NOT supply enough power through your other connections.

If it was isolated, it would NEVER be able to power the card. It would have to power something unrelated to the card, like a fan or data-controller, which is still connected to the other sources.

It may have horrible designed gate regulators, but that is NOT "isolation". If that were true, then "everyone" should have that issue, and those who use correct power, do not, thus, it is your assumption that it is isolated.

I just tested one, and the traces are NOT isolated, and all connections test as connected.

Cards haven't been built with "isolation" since the voodoo cards came-out.

Check your power. Your PSU is obviously "short" on its values. Possibly you are seeing the MAX/peak values, and treating them as if they are constant values. (Read the AMPS off your 6-8pins, your card regulator is just trying to distribute the load across all three supplies.)


Please read mrb's post with measured numbers showing the current draw through the PCIe connection for various cards -> https://bitcointalk.org/index.php?topic=199706.msg2082941#msg2082941. This is a known issue, these cards draw memory power through the PCIe slot. This was rectified in the HD 7xxx series cards. BTC miners weren't the only people to fry the 24-pin on boards. When I used to competitively overclock, HD 5870s on liquid nitrogen were toasting boards most effectively. I used a 1x PCIe extender wired up with a 4-pin molex to provide a safer path for additional current when running quadfire.

The quality of the PSU does not matter when it is a fact that the cards draw a significant amount of power from the PCIe slot. Also, last I checked the 100 amps of +12vdc that my Corsair AX1200 PSUs can pump out is probably sufficient to power 1, maybe 2 HD 5870s if I'm lucky. 


If you are using a 5870 still... Power is the least of your worries.

I love my 5870s and 5970s. Two years later and besides fan replacements they are still hashing away, having hit ROI multiple times. People starting to mine now will have trouble achieving the same thing with HD 7950s meanwhile I'll continue to mine at just the cost of power(which is cheap) and replacement fans.
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