ImageDestroyer (Click to show)
Originally Posted by CataclysmZA
Don't you really mean to say that few reviewers were able to measure PCI-E power draw? Not everyone has that kind of equipment on hand.
As for the information from that document, it's over 800 pages long. There's so much information available that finding anything that actually tells us what's going on here is going to take a very, very long time. It might be better to ignore those numbers and work off the PCI-E Electromechanical specification.http://download.csdn.net/detail/jianjunzaixian/9229521
I've pasted the relevant pages for everyone to read.
As you can see, at no point is the PCI-E slot ever officially
given more than 75W of power to work with. Both the board and GPU need to adhere to that spec while making use of aux inputs to make up the difference.
The average draw of the GTX 750 Ti was below 75W, which is expected. The average of the RX 480, as measured in that graph, is 80W. That should not be the case, and the RX 480 is overdrawing on the PCI-E specification most of the time.
I mean, sure, there's provision for drawing 81W, at a stretch, if you're abusing the tolerances as much as you can and guessing that the board can handle it and doesn't have cheap caps, which is what AMD is doing here. I can see why they're doing it this way, but it's not the most ideal way to go about this. Slapping on an 8-pin connector would have rendered this all a non-issue.
"it's over 800 pages long"
"There's so much information available that finding anything that actually tells us what's going on here is going to take a very, very long time."
Well, only chapter 5 (power management) and 6.8 to 6.16 (power budgeting capabilities) are about power which is about 77 pages.
Plus someone has already done the digging ( see his post below)
A few quotes from the manual,
"PCI Express provides a mechanism for software controlled limiting of the maximum power per slot
that a PCI Express adapter (associated with that slot) can consume."
" Scenario 3: An Adapter Consuming 30 W
If the adapter is plugged into a form factor #1 40 W slot, the Slot Power Limit control
mechanism is followed, and the device operates normally.
If the adapter is plugged into a form factor #1 25 W slot, the Slot Power Limit control
mechanism is followed, and the device must scale down to 25 W or disable operation.
If the adapter is plugged into a form factor #2 15 W slot, the Slot Power Limit control
mechanism is followed, and the adapter must scale down to 15 W or disable operation. An
adapter that does not scale within any of the power limits for a given form factor will always be
disabled in that form factor and should not be used. "
I dont know how the motherboard 'disables' the adapter/video card in this scenario, I assume the motherboard disables it by cutting power if it doesn't want to scale down but I haven't read it completely though.spoiler (Click to show)
Edited by Yttrium - 6/30/16 at 10:25am Quote:
Originally Posted by DaaQ
Ok, since the bandwagon is speeding along so fast that pitchforks are flying everywhere. I have wasted 45 minutes to read through this thread. I am going to provide some information I am pretty sure few will actually read. I will start by getting the stuff I am not going to bother providing any backup for first.
According to the internet, the PCIE slot pulling over 75w was from 2 reviewers out of 20, of the other reviewers who were shared this information, non were able to recreate the scenario. So not being able to recreate the current overdraw by other reviewers, really means something in itself. But AMD fail is AMD fail, such is the oh too familiar mantra.
Anyone that comes at me with
or your argument is invalid. How about you actually do some reading if your so concerned
with proof. I seen the above information earlier in the day and am not going to spend the time trying to convince people that won't be convinced anyway.
Now for the good stuff.
Here's a juicy bit from this link here, which ironically is where I heard some of the above. https://www.reddit.com/r/pcmasterrace/comments/4qfy9d/i_work_at_amd_the_time_has_come_to_ama_about/
Ektobuffer 3 points 3 hours ago
Maybe i can helb you out a bit Raja.
I have just read the PCI-E 3 specifications and they are telling me something different. In my understanding the 75 watt isnt the maximum limit, its just the default value on startup of the motherboard. The motherboard it self sets the maximum allowed watt per slot in the "Slot Capabilities Register" which you can configure up to over 300 watt per slot. In the bits 7 to 14 "Slot Power Limit Value" you can set 250, 275, 300 and above 300 watt. This will be multiplied with bits 15 to 16 "Slot Power Limit Scale" in steps x1 ,x0.1, x0.01 and x0.001. So its up to the motherboard manufacturer and the power management on it how many watt the slot is capable of.
The Specifications do define the protocol and not the hardware specs of the PCI-E slot. If a manufacturer uses better parts which can handle higher amps on the contacts and the lines, they can allow the devie in the slot a higher power consumption than 75 watt via these registers.
Sadly most people doesnt even read the specifications and judge things they dont understand.http://composter.com.ua/documents/PCI_Express_Base_Specification_Revision_3.0.pdf
Now the link provided in that quote is really interesting. ( DL it Looniam if you don't have it you will love it )
Hopefully those that are so inclined actually spend some of their time and actually read some of the info in that link.
So it's late and I need to get to bed, but this is what I quickly found in that 800+ page document.
Power limits on the platform are typically controlled by the software (for example, platform
firmware) that comprehends the specifics of the platform such as:
15 Partitioning of the platform, including slots for I/O expansion using adapters
Power delivery capabilities
This software is responsible for correctly programming the Slot Power Limit Value and Scale fields
of the Slot Capabilities registers of the Downstream Ports connected to slots. After the value has
20 been written into the register within the Downstream Port, it is conveyed to the adapter using the
Set_Slot_Power_Limit Message (see Section 220.127.116.11). The recipient of the Message must use the
value in the Message data payload to limit usage of the power for the entire adapter, unless the
adapter will never exceed the lowest value specified in the corresponding form factor specification.
It is required that device driver software associated with the adapter be able (by reading the values of
25 the Captured Slot Power Limit Value and Scale fields of the Device Capabilities register) to
configure hardware of the adapter to guarantee that the adapter will not exceed the imposed limit.
In the case where the platform imposes a limit that is below the minimum needed for adequate
operation, the device driver will be able to communicate this discrepancy to higher level
configuration software. Configuration software is required to set the Slot Power Limit to one of the
30 maximum values specified for the corresponding form factor based on the capability of the
The following rules cover the Slot Power Limit control mechanism:
Until and unless a Set_Slot_Power_Limit Message is received indicating a Slot Power Limit
35 value greater than the lowest value specified in the form factor specification for the adapter's
form factor, the adapter must not consume more than the lowest value specified.
PCI EXPRESS BASE SPECIFICATION, REV. 3.0
An adapter must never consume more power than what was specified in the most recently
received Set_Slot_Power_Limit Message or the minimum value specified in the corresponding
form factor specification, whichever is higher.
Page 527 to be specific, this concerns slot power limit control.
The above sounds to me either, vBIOS or BIOS level. Would be quite interesting if the motherboard was responsible for assigning the slot power limit wouldn't it?