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I have PCI-E 75w limit question...

post #1 of 29
Thread Starter 
Some one has Access to PCISIG ?

I just was boring...

So.... 75w per slot in 2016 sound pretty low =P, after 650w+ monsters like 295x with just 2 x8 Pins.

Documents that were available for public.

Errata for the PCI Express Base Specification, Revision 3.0/1

Errata for the PCI Express Base Specification, Revision 2.0/1 

Errata for the PCI Express Base Specification, Revision 1.0/1 

Document about 300 w GPUs has no public access but this document is only for PCI-E 1.0/1 solts.





PCI-E 2.0/3.0 - Document









F0h = x1

F1h = x2

F2h = x4

F3h up to FFh  x8 ,x16,x32.

Means slot size X(bla) /(not sure about the slot size.) =P

So as you can see "POWER SUPPLIED BY THE SLOT OR BY"

So, what do you think guys ... ?

All documents, you can find here.

https://pcisig.com/specifications
Edited by Eroticus - 7/3/16 at 2:36pm
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post #2 of 29
75 watts is a lot of power to get through the motherboard traces, for one. Then also the card edge slot.

That's why graphics cards have the wired connections for their power. There's no real point in pulling any power from the motherboard other than looking at reference voltages for signals.

Also consider the VRM section of a motherboard up by the ATX12v connector. There ain't any components up there because all of the copper has to be used to get the 12v to the VRMs and then get the much lower voltage HUGE amperage power to the CPU. Those traces are massive and short as heck. The CPU probably uses A LOT of pins for power too.
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post #3 of 29
Doesn't really matter. I have modded 750 Tis with no PCIe power connector and they grab as much as 85-90W from the socket without breaking a sweat for hours on end, and this is on a cheap H81 motherboard. Long term, I can't say for sure but I doubt there is a risk of damage.
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post #4 of 29
I think the issue is hugely overblown, pulling more than 75W from the PCIe slot is hardly uncommon.
The only motherboards I realistically think are at risk are old, cheap, bottom of the barrel ones.

Its unfortunate, but the only downside I see to this "problem" is that this won't be a card you'll be using on sub-par motherboards and OEM systems, at least not without restricting the power limit.
post #5 of 29
Thread Starter 
@Dieselbird ? 250w CPU is fine trough motherboard, but over 75w GPU going to kill it, + most motherboards has anti surge protection.
I think 75w limit that was ony in the first PCI-E gen, later they upgraded it with PCI 2.1.

+
Edited by Eroticus - 7/3/16 at 5:00pm
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post #6 of 29
Quote:
Originally Posted by Gilles3000 View Post

I think the issue is hugely overblown, pulling more than 75W from the PCIe slot is hardly uncommon.
The only motherboards I realistically think are at risk are old, cheap, bottom of the barrel ones.

Its unfortunate, but the only downside I see to this "problem" is that this won't be a card you'll be using on sub-par motherboards and OEM systems, at least not without restricting the power limit.

It actually seems to be very uncommon, no other video card so far pulls more than 66W from the motherboard's 12V continuously. Maybe a heavily overclocked 750Ti (no auxiliary power) but nothing at stock. Everything pulls any extra power from the 6/8-pin connector.

Any examples of any other card that does this?
Quote:
Originally Posted by Eroticus View Post

@Dieselbird ? 250w CPU is fine trough motherboard, but over 75w GPU going to kill it, + most motherboards has anti surge protection.
I think 75w limit that was ony in the first PCI-E gen, later they upgraded it with PCI 2.1.

+

It was not upgraded, why would you think it was? You are supposed to use the 6/8-pin connections for any extra power you need.
Edited by Asmodian - 7/3/16 at 5:02pm
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post #7 of 29
Thread Starter 
Quote:
Originally Posted by Asmodian View Post

It actually seems to be very uncommon, no other video card so far pulls more than 66W from the motherboard's 12V continuously. Maybe a heavily overclocked 750Ti (no auxiliary power) but nothing at stock. Everything pulls any extra power from the 6/8-pin connector.

Any examples of any other card that does this?

ASUS GTX 950 2 GB (no power connector) - Average in gaming 74w / Peak 76w

I don't own this one, but i pretty sure it's could reach ~80w in FurMark.


----

What does make you think it didn't ?

as you can see in the picture...


3.0
F3H to FFh (x16 > x32 ?) = Power limit values above 300w.


2.1
Power supplied by the slot or by other means to the adapter(Cables?).


+The random image that people gave from Google, is from the old document, and doesn't belong to PCI-E 2.0 / 2.1 / 3.0 / 3.1
Edited by Eroticus - 7/3/16 at 5:12pm
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post #8 of 29
Quote:
Originally Posted by Cyclops View Post

Doesn't really matter. I have modded 750 Tis with no PCIe power connector and they grab as much as 85-90W from the socket without breaking a sweat for hours on end, and this is on a cheap H81 motherboard. Long term, I can't say for sure but I doubt there is a risk of damage.

Biggest issue is exceeding the current limits on the ATX 24-pin's +12v wires.

This is very hard to do with one card, but get two or three cards and some fans attached to the board without supplemental power and you can wind up with quite a bit of current.
Quote:
Originally Posted by Eroticus View Post

Really guys ? 250w CPU is fine trough motherboard, but over 75w GPU going to kill it, + most motherboards has anti surge protection.

CPU's typically have an EPS 12v connector feeding the VRM and hundreds of power/ground traces going to the CPU itself.

Without supplemental power all the PCI-E slots (as well as things like fan headers) are fed by two +12v wires on the ATX 24-pin connector, with each slot having five +12v fingers/traces.
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post #9 of 29
Thread Starter 
Quote:
Originally Posted by Blameless View Post

Biggest issue is exceeding the current limits on the ATX 24-pin's +12v wires.

This is very hard to do with one card, but get two or three cards and some fans attached to the board without supplemental power and you can wind up with quite a bit of current.
CPU's typically have an EPS 12v connector feeding the VRM and hundreds of power/ground traces going to the CPU itself.

Without supplemental power all the PCI-E slots (as well as things like fan headers) are fed by two +12v wires on the ATX 24-pin connector, with each slot having five +12v fingers/traces.

This power is still going through the motherboard.

There is even Two and Four way servers with Tons of slots.
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post #10 of 29
Do keep in mind that, though the limit is 75W, it's from two different voltages - 5.5A @ 12V and 3A @ 3.3V. That adds up to 66W of 12V and 9.9W of 3.3V for 75.9W total.

Realistically, it won't really matter. If you have multiple 480s and they're overclocked, yeah. But that's a risk that all motherboards take with multiple GPUs. This is the only source I can find, but they rate each wire for 6A. There's two whole 12V wires for 12A in total, meaning you can supply a grand total of 144W to all devices on the 12V rail. This includes PCIe cards and fans, for example.

But even that isn't a huge deal because these wires can take quite a bit more current safely than they're rated for. 18 AWG wire can handle, over short distances, 16A. That's pretty insane, providing 192W per wire and therefore a single 6-pin connector could take 384W. The plastic would probably melt by then though. Still, over 100W should be safe with a 6-pin connector.


So what's the problem? The motherboard itself. The traces - tiny copper wires running throughout the layers of the board - are quite small and cannot necessarily handle the reported 7A safely. It would be pretty dumb for any issue to come up with a single GPU though, since a motherboard that fried at 5.6A drawn from a slot would not survive day to day use. I must imagine that most decent motherboards are also overbuilt to such a degree that drawing like 10A from the slot would be safe-ish.

Quote:
Originally Posted by Eroticus View Post

Really guys ? 250w CPU is fine trough motherboard, but over 75w GPU going to kill it, + most motherboards has anti surge protection.
I think 75w limit that was ony with first PCI-E gen, later they upgraded it with PCI 2.1.
+

Nope, they haven't changed any specifications for power since the original PCIe version.

The overall power limit has gone up, however. I believe either 2.0 or 2.1 introduced the 8-pin connector. Additionally - though I do not know if this has changed recently - dual 8-pin isn't up to PCIe specs. The most you can do is 300W from one of each power source: the slot, a 6-pin cable, and an 8-pin cable. Technically, GPUs with dual 8-pin connections are not "officially" PCIe cards.
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Triumvirate
(20 items)
 
Osmium
(8 items)
 
For Sale: [FS] Z97 system: Xeon and RAM
$160 (USD) or best offer
CPUMotherboardGraphicsGraphics
i7-5775C ASUS Sabertooth Z97 Mark 2 Sapphire RX 480 (reference) MSI Low-Profile 750Ti 
RAMHard DriveHard DriveHard Drive
Corsair Dominator Platinum - 2x8GB Crucial M500 - 960GB Samsung 840 - 250GB WD Scorpio Blue - 1TB 
Hard DriveCoolingCoolingOS
ADATA SP900 - 64GB Scythe BIG Shuriken 2 Rev. B PNY Quadro 600 blower Windows 10 
MonitorMonitorMonitorKeyboard
ASUS VE247H - 1080p Gateway FPD1960 - 1280x1024 Samsung S20D300 - 900p, portrait Rosewill RK9000I - Cherry MX Blue 
PowerCaseMouseOther
Rosewill HIVE 650W Riotoro CR1080 SteelSeries Rival 100 CyberPower 1500PFCLCD 
CPUMotherboardRAMHard Drive
AMD A10-7870K ASRock FM2A88X-ITX+ G.Skill Ripjaws X - 2x4GB @ 2400MT/s Samsung 840 120GB 
CoolingOSPowerCase
Evercool EC-HPS-810CP Windows 7 Pro 64-bit PicoPSU-120-WI-25 Mini-Box M350 
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