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[Reddit] RX 480 fails PCI-E specification - Page 104  

post #1031 of 1129
In all my years building computers I've always made sure to stay within safe levels on the power supply and have never really tried going over spec on a power cable. I currently have the Corsair TX650 which isn't cheap, but is about four years old. If the fix caps power from the PCI-E slot at 75W then about how much extra could I safely pull from the PCI-E cable? I think with overclocking it might go up to about 105W
post #1032 of 1129
This guy hacked the card to put the voltage through the roof all through the pci-e and 6pin. (on a newer board of course)
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post #1033 of 1129
Its easy to feed GCN a lot of voltage under water. Doing it on air is pretty much impossbile unless you have a really great cooler. If its not the power consumption that limits you its the temperatures.
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post #1034 of 1129
Quote:
Originally Posted by Iscaria View Post

In all my years building computers I've always made sure to stay within safe levels on the power supply and have never really tried going over spec on a power cable. I currently have the Corsair TX650 which isn't cheap, but is about four years old. If the fix caps power from the PCI-E slot at 75W then about how much extra could I safely pull from the PCI-E cable? I think with overclocking it might go up to about 105W

Short answer:
You can pull 150w easy from a single 6 pin without any issues, probably much more but most tend to say 150w is safe for 24/7 draw through the cable.

Long answer:
18 gauge wire can easily handle 14 amps as a conservative rating, 16 when used in chassis wiring lengths. That is the actual conservative spec limits of the wire itself. So each wire can safely have 14 amps through it. Now, the PCI-E connector itself is rated by Molex (who makes the connectors) for 9 amps per pin, see here: http://www.molex.com/webdocs/datasheets/pdf/en-us/0455590002_CRIMP_HOUSINGS.pdf
That means that a 6-pin PCI-E connector conforming to spec and using only two +12v connections, can have 18 amps of power at 12 volts through it, which means safe rating by the people who make the stuff is 198-216 watts for a 6 pin. This is because to get wattage we multiply amps times voltage. So: 9 amps * 2 pins = 18 amps * 12 volts = 216 watts. if we account for up to a maximum of 1 volt of droop (which is excessive) then we would use 11v for the calculation, which gives us 198 watts of power.
For an 8 pin connector we have three +12v lines, which means: 264-324 watts.

if you look at one of my previous posts I also explained how the VRM section for the 3 phases tied to the PCI-E 6-pin power cable will handle the max safe wattage a 6 pin can deliver perfectly fine.
Edited by EniGma1987 - 7/7/16 at 8:30am
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post #1035 of 1129
Quote:
Originally Posted by EniGma1987 View Post

Short answer:
You can pull 150w easy from a single 6 pin without any issues, probably much more but most tend to say 150w is safe for 24/7 draw through the cable.

Long answer:
18 gauge wire can easily handle 14 amps as a conservative rating, 16 when used in chassis wiring lengths. That is the actual conservative spec limits of the wire itself. So each wire can safely have 14 amps through it. Now, the PCI-E connector itself is rated by Molex (who makes the connectors) for 9 amps per pin, see here: http://www.molex.com/webdocs/datasheets/pdf/en-us/0455590002_CRIMP_HOUSINGS.pdf
That means that a 6-pin PCI-E connector conforming to spec and using only two +12v connections, can have 18 amps of power at 12 volts through it, which means safe rating by the people who make the stuff is 198-216 watts for a 6 pin. This is because to get wattage we multiply amps times voltage. So: 9 amps * 2 pins = 18 amps * 12 volts = 216 watts. if we account for up to a maximum of 1 volt of droop (which is excessive) then we would use 11v for the calculation, which gives us 198 watts of power.
For an 8 pin connector we have three +12v lines, which means: 264-324 watts.

if you look at one of my previous posts I also explained how the VRM section for the 3 phases tied to the PCI-E 6-pin power cable will handle the max safe wattage a 6 pin can deliver perfectly fine.

Thank you, that is actually great to hear. In my overclocking tests on the 480 I did notice a sharp increase in watts, but if it can safely pull 75+150= 225W then I have nothing to worry about. Also, the blower on the card (while loud) actually does a great job of cooling the card. I was putting it under full load at 1325MHz clock speed and the temp hovered around 61C at 75% fan speed. I play with my speakers loud enough to drown out the fan noise so it doesn't bother me at all.
post #1036 of 1129
Quote:
Originally Posted by looniam View Post

Quote:
Originally Posted by Dyson Poindexter View Post

The sense pins on the power connectors shouldn't be ground though, right? Wouldn't they be on a pullup resistor to be brought low when the ground is connected? I would think the sense pins would be high-impedance by design. If they were shorted to ground, the card would think a connector is there all the time.

Point being, the sense lines are never meant to carry meaningful current. I know we've talked about the 2 vs 3 current pairs before on the connectors.

though i am sure you are talking about the sense pin on the card; the specs do state the sense pin on the connector (#5) from the PSU is grounded to the PSU: Warning: Spoiler! (Click to show)

but there is nothing that i see specifying the connection of the sense pin(s) on the card.

since supposedly the the sense pin is used to measure resistance if there is a connection but the voltage controller (IR 3567B), unlike other controllers(!), can determine if all the vrms are energized; it could be redundant to have both features. so instead of the sense pin being used to determine if there is an auxiliary connection, the IR 3567B knows and can operate the card as required (on/off).
Yep, on the card side. I'm just inferring it from the nature of it being a "sense" pin. To be able to detect if a grounded conductor is on pin 5, three things must be true:
  • Pin 5 on the card side must not be tied to ground
  • Pin 5 on the card side must have a voltage on it (likely whatever VCC for the controller is)
  • To be able to detect when this voltage changes from VCC to 0v, the impedance on that pin must be high (otherwise you'd blow out your controller)
These three things show that (at least at boot time) pin 5 is absolutely not grounded on the card. While it might be possible to tie pin 5 into the ground plane later in the boot cycle, I suspect it remains purely a logic signal at all times. Irregardless of the presence of a +12V conductor on pin 2 (which I admit is in the spec) your current draw is going to be limited by the remaining two ground conductors in positions 4 and 6, unless you rely on another ground on the slot or PCIe bracket.
post #1037 of 1129
Quote:
Originally Posted by Dyson Poindexter View Post

The sense pins on the power connectors shouldn't be ground though, right? Wouldn't they be on a pullup resistor to be brought low when the ground is connected? I would think the sense pins would be high-impedance by design. If they were shorted to ground, the card would think a connector is there all the time.

Point being, the sense lines are never meant to carry meaningful current. I know we've talked about the 2 vs 3 current pairs before on the connectors.

Most power supplies just supply three grounds and three +12V on the six-pin connector.

The sense checks for ground, nothing more, IIRC. If you aren't going to be detecting with the sense pin, you can just short the pins on the card - this is perfectly acceptable for any card not trying to pull 200W over a six-pin PCI-e power connector.

The eight-pin connector just adds two more grounds. The 'sense B' wire being grounded is how the card checks to verify that an eight-pin connector is plugged in - shorting the 'sense B' pin on the eight-pin connector is only acceptable if shorting to the extra ground provided on the eight-pin connector... unless you aren't exceeding the current limits for a 6-pin connector and the 8-pin is just being provided so users can optionally provide improved grounding.

In the end, you can short the two +12V with the 'blank' pin 2 on the connector as well as shorting all grounds to each other and to both sense pins without violating spec or posing any danger to the system.
post #1038 of 1129
Quote:
Originally Posted by ProclusLycaeus View Post

Thank you, that is actually great to hear. In my overclocking tests on the 480 I did notice a sharp increase in watts, but if it can safely pull 75+150= 225W then I have nothing to worry about. Also, the blower on the card (while loud) actually does a great job of cooling the card. I was putting it under full load at 1325MHz clock speed and the temp hovered around 61C at 75% fan speed. I play with my speakers loud enough to drown out the fan noise so it doesn't bother me at all.
What'd you turn it up to? I'm still seeing around 81 with target temp of 70 and 2500ish as the target fan speed. No throttling in sight on the stock clock.
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post #1039 of 1129
https://www.maximintegrated.com/en/app-notes/index.mvp/id/3605
This should help explaining things. thumb.gif

Edit: The ground for the MUN2236T1 bias-resistor transistor is the #5 sense pin (even though it does not state this, that is how the card "senses" the 6-pin is plugged in)
Edited by 777ESS - 7/7/16 at 11:49am
post #1040 of 1129
AMD's fix. Source


Edited by Derp - 7/7/16 at 11:56am
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