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FastEddie
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The Dual Rail Myth
In looking at the newest ATX standard, the ATX12VPower Supply Design Guide, ver 2.01, we find two major changes from the past in that 2 x 12v rails are now advocated, rather than a single 12v rail, in addition to a 24-pin main ATX power connector interface. How does this dynamically impacts us AMD users? First, let?s look at some of the changes made from the ATX 1.3 standard and see how they impact us, as power users.
The most important change is the incorporation of 2 x 12v rails, or ?Dual Rail,? technology, where the processor now has basically its very own power interface that is separated from all other power consuming devices on the motherboard. A Quote from Section 1.2.1, entitled ?Increased +12VDC Output Capability? of the Guide:
This may sound like a good thing, and I?m sure that one day it will be a major improvement over past power supply interfaces, however, as an AMD user, the major drawback is that this newest standard (being designed around, and in support of, Intel based technology), took no consideration of AMD/Sli. ?Our? problem isn?t that AUX ATX 12v Rail-1 is isolated with anywhere from 15amps to 18amps of power for the processor alone, but that the remainder of the entire system must depend upon ATX 12v Rail-2, with it?s 15amps to 18amps, to power all the other devices on the motherboard. This is a lopsided balance of power in regulating and controlling devices in the average computer system, and becomes more than simple balancing act when this ?design? becomes the limiting factor for system stability in our high powered enthusiast systems.
What?s added to ATX 12v Rail-2 that makes system stability an ?issue? is related to the plethora of new technologies that must all be powered by this single rail. Aside from SATA/SATA II, standard PCI, in addition to all ?onboard? peripheral interface devices such as multiple gig-lan network connections, multiple (up to ten) USB 2.0 connections, IEEE 1394 interfaces, standard serial/parallel port interfaces, legacy IDE interfaces and audio functions, the ?Guide? takes into consideration a new power hungry interface, being PCI-E, that consumes anywhere from 40watts to 120 watts of power in a single card. The cpu no longer has the distinction of being the highest power consumer in a computer system, when one considers the amount of power a 6800 Ultra needs to operate properly. This is a vast amount of power to take into consideration when building a new computer system, and one in which is compounded two fold with the release of nForce 4 SLI, where two such power hungry graphic adapters can coexist in the same system.
Many of you have followed the Dual Rail ?vs- Single Rail argument as it?s been played out in the implementation of NF4 Sli, where I have maintained that dual rail psu?s, as they exist today, are not an efficient use of the power resources these power supplies provide. Where AUX ATX 12v Rail-1 (for the processor) would run at approximately 35%-50% of it?s available output 60%-70% of the time, ATX 12v Rail-2 would run at or near 100% of its available output 100% of the time. Power Supplies are not designed to work under these conditions and it seems that the ?Guide? agrees and has taken this into consideration, where at Sec. 1.2.2. Minimum Efficiency, it points out:
What this tells us is that, indeed, ATX 12v Rail-2 shouldn?t be required to run at more than 70% MAX of it?s available output on a constant ?full and typical? basis. And it means that Dual Rail psu?s are not the best choice for powering nForce 4 Sli as these power supplies weren?t designed to work in such a lopsided environment. It is much more efficient to use a high output Single Rail psu with at lease 26a on that 12v rail, where 28a for the Sli dual 6800 Ultra?s is a much more stable environment to run these top of the line cards. And for those that say, ?well these boards were designed around the ATX 2.01 standard, so they should work just fine,? my answer is that they weren?t ?designed? around NF4 Sli by nature of the limitations the ?Guide? itself places on that second 12v rail. Another response regards accumulation of the two 12v rails, where 18a & 18a = 36a and plenty to power any motherboard. My response is this is a total misconception as these power supplies have no ability what-so-ever to accumulate their output current from the two totally separate rails on the fly. Sure, there are one or two psu?s that do have the ability to combine their output, via a switch, but that ?switch? turns them into essentially a more powerful single rail unit, with less amperage than the total of both 12v rails. Until a psu is designed for Sli, don?t be drawn into the marketing hype about ?Sli Ready? and ?Sli Capable? as their use goes contrary to the very guide that implements them. You would be much better served with a single rail power supply that can ?efficiently? power that Sli gamers box.
In looking at the newest ATX standard, the ATX12VPower Supply Design Guide, ver 2.01, we find two major changes from the past in that 2 x 12v rails are now advocated, rather than a single 12v rail, in addition to a 24-pin main ATX power connector interface. How does this dynamically impacts us AMD users? First, let?s look at some of the changes made from the ATX 1.3 standard and see how they impact us, as power users.
The most important change is the incorporation of 2 x 12v rails, or ?Dual Rail,? technology, where the processor now has basically its very own power interface that is separated from all other power consuming devices on the motherboard. A Quote from Section 1.2.1, entitled ?Increased +12VDC Output Capability? of the Guide:
This may sound like a good thing, and I?m sure that one day it will be a major improvement over past power supply interfaces, however, as an AMD user, the major drawback is that this newest standard (being designed around, and in support of, Intel based technology), took no consideration of AMD/Sli. ?Our? problem isn?t that AUX ATX 12v Rail-1 is isolated with anywhere from 15amps to 18amps of power for the processor alone, but that the remainder of the entire system must depend upon ATX 12v Rail-2, with it?s 15amps to 18amps, to power all the other devices on the motherboard. This is a lopsided balance of power in regulating and controlling devices in the average computer system, and becomes more than simple balancing act when this ?design? becomes the limiting factor for system stability in our high powered enthusiast systems.
What?s added to ATX 12v Rail-2 that makes system stability an ?issue? is related to the plethora of new technologies that must all be powered by this single rail. Aside from SATA/SATA II, standard PCI, in addition to all ?onboard? peripheral interface devices such as multiple gig-lan network connections, multiple (up to ten) USB 2.0 connections, IEEE 1394 interfaces, standard serial/parallel port interfaces, legacy IDE interfaces and audio functions, the ?Guide? takes into consideration a new power hungry interface, being PCI-E, that consumes anywhere from 40watts to 120 watts of power in a single card. The cpu no longer has the distinction of being the highest power consumer in a computer system, when one considers the amount of power a 6800 Ultra needs to operate properly. This is a vast amount of power to take into consideration when building a new computer system, and one in which is compounded two fold with the release of nForce 4 SLI, where two such power hungry graphic adapters can coexist in the same system.
Many of you have followed the Dual Rail ?vs- Single Rail argument as it?s been played out in the implementation of NF4 Sli, where I have maintained that dual rail psu?s, as they exist today, are not an efficient use of the power resources these power supplies provide. Where AUX ATX 12v Rail-1 (for the processor) would run at approximately 35%-50% of it?s available output 60%-70% of the time, ATX 12v Rail-2 would run at or near 100% of its available output 100% of the time. Power Supplies are not designed to work under these conditions and it seems that the ?Guide? agrees and has taken this into consideration, where at Sec. 1.2.2. Minimum Efficiency, it points out:
What this tells us is that, indeed, ATX 12v Rail-2 shouldn?t be required to run at more than 70% MAX of it?s available output on a constant ?full and typical? basis. And it means that Dual Rail psu?s are not the best choice for powering nForce 4 Sli as these power supplies weren?t designed to work in such a lopsided environment. It is much more efficient to use a high output Single Rail psu with at lease 26a on that 12v rail, where 28a for the Sli dual 6800 Ultra?s is a much more stable environment to run these top of the line cards. And for those that say, ?well these boards were designed around the ATX 2.01 standard, so they should work just fine,? my answer is that they weren?t ?designed? around NF4 Sli by nature of the limitations the ?Guide? itself places on that second 12v rail. Another response regards accumulation of the two 12v rails, where 18a & 18a = 36a and plenty to power any motherboard. My response is this is a total misconception as these power supplies have no ability what-so-ever to accumulate their output current from the two totally separate rails on the fly. Sure, there are one or two psu?s that do have the ability to combine their output, via a switch, but that ?switch? turns them into essentially a more powerful single rail unit, with less amperage than the total of both 12v rails. Until a psu is designed for Sli, don?t be drawn into the marketing hype about ?Sli Ready? and ?Sli Capable? as their use goes contrary to the very guide that implements them. You would be much better served with a single rail power supply that can ?efficiently? power that Sli gamers box.
