Posted by Scott Mueller on Apr 14, 2005 (13591 views, 2 comments) Rating: 2.96
New Form Factors
Unfortunately recent changes in motherboard and power supply form factors may mean that compatible power supply connections can't be taken for granted any longer. The recent changes in power supply form factors mostly relate to newer motherboards, processors and even video cards requiring ever increasing amounts of power. Let's examine the new form factors and how they differ from the traditional ATX and ATX12V 1.x standards.
ATX12V 2.x
Starting in June 2004 the new PCI Express bus first appeared on motherboards. PCI Express is a type of serial bus, with standard slots having a single channel or lane of communications. These single lane slots are called x1 slots, and are designed for normal peripheral cards such as network cards, sound cards and the like. PCI Express also includes a special higher bandwidth slot with 16 lanes (called an x16 slot), which is especially designed for use by video cards. During development it was realized that PCI Express x16 video cards could draw more power than what was allowed by the existing 20-pin Main and 6-pin Auxiliary power supply connectors, especially when it came to +12V power.
The problem was that the 20-pin Main connector had only a single +12V pin, and the new video cards would require more +12V power than a single pin could safely deliver. The +12V connector that had already been added was specifically for the CPU, and not available to other devices. Rather than add another supplemental or auxiliary connector as they had done before, Intel decided that it was finally time to upgrade the Main power connector to supply more power.
The result was officially called ATX12V 2.0, released in February 2003. ATX12V 2.0 included two major changes from the previous ATX12V 1.x specifications. This included a new 24-pin Main power connector, and the elimination of the 6-pin Auxiliary power connector. The new 24-pin Main power connector included 4 more pins supplying additional +3.3V, +5V, and +12V power plus a ground. The inclusion of these extra pins not only satisfied the power requirements for PCI Express video cards drawing up to 75 watts, but also made the older 6-pin Auxiliary connector unnecessary. The pinout of the new 24-pin Main power connector is shown in the following figure.
ATX12V 2.x 24-pin Main Power Connector:
Pin 13 might have a second orange or brown wire, used for +3.3V sense feedback - used by the power supply to monitor 3.3V regulation.
Pin 20 might be N/C (no connection) since -5V was removed from the ATX12V 2.01 and later specifications. Supplies with no connection at pin 20 should not be used with older motherboards that incorporate ISA Bus slots.
It is interesting to note that the 24-pin connector is not really that new, it actually first appeared in the SSI (Server System Infrastructure) EPS (Entry Power Supply) specification released in 1998. SSI (www.ssiforum.org) is an initiative designed to create standard interfaces for server components, including power supplies. The 24-pin Main power connector was first created for servers since at the time only servers needed the higher levels of power. Since today's PCs draw the same power levels as servers did years ago, rather than reinvent an incompatible connector the ATX12V 2.0 standard merely incorporated the 24-pin connector already specified in the SSI EPS standard.
As compared to the previous 20-pin design, the 24-pin Main power connector includes additional +3.3V, +5V and +12V terminals allowing a substantially greater amount of power to be delivered to the motherboard. Each terminal in the Main power connector is rated to handle up to 6 amps of current. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.
24-pin Main Connector Power Rating:
Volts
Terminals
Amps
Watts
+3.3V
4
24
79.2
+5V
5
30
150
+12V
2
12
144
Total Watts:
373.2
This means that the total power handling capacity of this connector is 373 watts, substantially higher than the 251 watts available in the previous 20-pin connector. Combining the 24-pin Main plus the 4-pin +12V power connector results in up to 565 watts total power available to the motherboard and processor! This is more than enough to support the highest output power supplies on the market today.
PCI Express x16 Graphics Power Specification
Although the ATX12V 2.x specifications includes a new 24-pin Main power connector with more power for devices like video cards, the design was intended to power a video card drawing up to 75 watts maximum. There are already some video cards on the market drawing more power than that, for example the NVIDIA GeForce 6800 Ultra draws 110 watts and future cards may draw even more power. Since greater than 75 watts cannot be provided via the motherboard directly, the PCI-SIG (Special Interest Group) developed a standard for supplying power directly to the video card from the power supply via an additional graphics power connector. This will likely be included in future ATX12V power supply standards as well.
The PCI Express x16 Graphics Power spec. consists of a 6-pin Molex Mini-Fit, Jr. connector housing with female terminals, used to provide power directly to the video card. For reference, the connector is Molex part number 39-01-2060 and the terminals are part number 5556. This is the same style of connector as the Main and +12V power connectors. The pinout of the connector is shown in the following figure.
PCI Express x16 Graphics Power Connector:
Using standard terminals each pin in the PCI Express graphics power connector is rated to handle up to 8 amps of current. By counting the number of terminals for each voltage level, the power handling capability of the connector can be calculated as shown in the following table.
6-pin Graphics Connector Power Rating:
Volts
Terminals
Amps
Watts
+3.3V
0
0
0
+5V
0
0
0
+12V
3
24
288
Total Watts:
288
This means that the total power handling capacity of this connector is 288 watts, which should be more than enough for even the most power hungry video cards in the future.
A new power supply incorporating a 24-pin Main, 4-pin +12V, and 6-pin PCI Express graphics power connector would have a maximum rated power delivery capability to the motherboard of 853 watts! It is important to note that just because a power supply has those connectors doesn't mean that it could truly deliver that much power, only that if it could, the connectors could handle that much power without overheating.
Backwards and Forwards Compatibility
If you have reached this point, I'm sure you have some questions. For example, what happens if you purchase a new power supply that has a 24-pin Main power connector but your motherboard has only a 20-pin Main power socket? Likewise, what if you purchase a new motherboard that has a 24-pin Main power socket, but your power supply has only a 20-pin Main power connector? The answers to these questions are surprising to say the least.
First let me say that there are adapters that can convert a 24-pin connector to a 20-pin type, and the other way around, but surprisingly these adapters are NOT actually necessary, or even desirable. The plain truth is that compatibility has been engineered into the connectors, power supplies and motherboards from the start.
If you look at the 24-pin Main power connector diagram and compare it to the previous 20-pin design (both are shown earlier in this article), you'll see that the extra 4-pins are all placed on one end of the connector, and all of the other pins are defined the same as they were previously. The design of these connectors is such that it allows an interesting bit of backwards compatibility. The result is that you can plug a 24-pin Main connector directly into a motherboard that has a 20-pin socket (and vice versa), WITHOUT using an adapter! The trick is to position the connector such that the 4 extra pins are empty. Depending on the latch design, the latch on the side may not engage, but the connector will otherwise plug in and operate properly.
The following figure shows how you would connect a new power supply with a 24-pin connector to a motherboard that has only a 20-pin socket. The terminals on the 24-pin connector that are highlighted in gray would plug directly into the 20-pin socket, while the white highlighted terminals would remain free and unconnected.
Connecting a 24-pin Main Power Connector to a 20-pin Motherboard Socket:
Logically this works because the first 20-pins of the 24-pin connector that match the 20-pin motherboard socket contain the correct signals in the correct positions. The only problem that might arise is if there is some component on the motherboard directly adjacent to the end of the 20-pin power socket which would physically interfere with the 4 extra unused terminals on the 24-pin connector.
What about the opposite condition, where you have a new motherboard with a 24-pin socket, but your power supply has only a 20-pin connector? Well, this plugs in perfectly as well, as shown by the following figure. The gray highlighted terminals in the 20-pin connector plug into the gray highlighted terminals in the 24-pin socket, with the 4 white highlighted terminals in the socket remaining open and unconnected.
Connecting a 20-pin Main Power Connector to a 24-pin Motherboard Socket:
This also works because the 20-pin gray highlighted portion is the same on both the connector and the socket. But this example raises another question; will the motherboard operate properly without the extra power pins? Since the extra signals are merely additional voltage pins which are already present in the remaining part of the connector, the answer should be yes, but if the motherboard draws a lot of power, it may overload the remaining pins. After all, preventing overloads is the reason why the extra pins were added in the first place.
Fortunately even that problem has been solved. All of the motherboards I've seen which use a 24-pin Main power connector also have an additional Peripheral (i.e. disk drive) power connector on-board, which is designed to provide the extra power that would be missing if you connect a 20-pin Main power connector from your power supply. The documentation for the motherboard refers to this as an Alternate power connector. The following figure shows an Intel D925XBC motherboard, which features a 24-pin Main, 4-pin +12V, and 4-pin Alternate power connector.
Intel D925XBC Motherboard Power Connectors:
The +12V power connector is always required since that provides power to the CPU. If you also plug a 24-pin Main power connector into the 24-pin socket on the motherboard, than the Alternate power connection is not necessary. However if you plug a 20-pin Main power connector into the 24-pin Main power socket on the motherboard, then simply select a spare peripheral (disk drive) power connector from the power supply and plug it into the Alternate power connector. Most power supplies have several extra peripheral power connectors for supporting additional drives. Using a 20-pin Main and the Alternate power connector satisfies the power requirements for the motherboard and any PCI Express x16 video cards drawing up to 75 watts.
As a side note, you should be careful when plugging in the mismatched connectors so that they are offset properly. The Main, +12V, and PCI Express graphics connectors are Molex Mini-Fit, Jr. type connectors that are keyed by virtue of a series of different shaped plastic protrusions used around the terminals, which fit similar shaped holes in the mating connectors. This keying is designed to prevent backwards or improper off-center insertion, however I have found two problems with the keying that should be noted. One is that some alternate low-quality connector brands are built to looser tolerances than the original high-quality Molex versions, and the sloppier fit of the low-quality versions may allow improper insertion. The other problem is that with sufficient force, the keying on even the high-quality versions can be overcome. Since plugging a 20-pin connector into a 24-pin socket, or likewise a 24-pin connector into a 20-pin socket is designed to work even though they don't fully match up, you need to make sure you have the offsets correct or you risk damaging the board when you power it up.
Conclusion
New motherboards with PCI Express slots incorporate a 24-pin Main power connector, and new power supplies conforming to the ATX12V 2.x form factor standards also include this connector as well. The 24-pin Main power connector found on these newer supplies will plug directly into the 20-pin Main power socket on older motherboards with no adapter required. Likewise, you can plug an older 20-pin Main power connector into the 24-pin socket on a newer motherboard, as long as you also plug a Peripheral (disk drive) power connector into the Alternate power connector on the motherboard as well.
Using a power supply with a 24-pin Main connector, or combining a 20-pin Main plus the Alternate connector, allows PCI Express motherboards to provide power for up to 75 watt PCI Express x16 video cards. If the video card needs more than 75 watts of power, then the card and power supply will incorporate a 6-pin PCI Express Graphics power connector to deliver the additional power required.
Note that adapters are not required for the Main power connector when converting from 24-pin to 20-pin and vice versa. Adapters can be useful to convert Peripheral (disk drive) power connectors into either +12V power or PCI Express Graphics power connectors if your existing power supply lacks those connectors.
With backwards compatibility insuring that the new 24-pin power connector will plug into older 20-pin motherboard sockets, when purchasing a new power supply I now recommend only those with 24-pin Main power connectors, which are usually sold as ATX12V 2.x or PCI Express models. For the most flexible and future-proof supply, also make sure the power supply includes a 6-pin PCI Express Graphics connector as well as integrated SATA (Serial ATA) drive power connectors. An example of a highly rated power supply incorporating all of these features is the Turbo-Cool 510 Express from PC Power and Cooling (www.pcpowercooling.com). Choosing a power supply with these features provides flexibility that allows it to work not only in newer systems, but in virtually all older ATX systems as well, and with no adapters required.
Current Rating for this Article: 2.96 Rate Article:
#1 Posted by Legolad (1 posts) at 22 Jun 2005 - 19:06
Wow. Wow. Wow.
I just got caught short by this very issue. I offered to help a friend of mine build a new PC because I've built them for years. I know (or thought I knew) all the things to research and all the answers to find.
Then I went to plug in the ATX 20-pin power connector and was totally stumped by the existence of those four extra pins.
Thank you Mr. Mueller for your excellent document. This is extremely clear and helpful and addressed ALL of my questions about this move to 24-pin ATX power connectors.
As a 15-year veteran tech writer, all I can say is "Thank you."
But I want to say it again and again.
Thank you. Thank you. Thank You.
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#2 Posted by gbruce (1 posts) at 19 Feb 2006 - 23:30
I am a bit less experienced than legolad wrt to building PC's as I have only done this twice for personal use only. Twice I have had problems building my PC's and both times it has been the power supply catching me out, first with the Baby AT to ATX12V change and after I thought I knew as much as I needed to know about PC's and power supplies, the v2.0 took me completely by surprise. I looked up the form factors web-site and I could see that it looked backward compatible, however I needed the reassurance of your document to fill me with confidence. Alas my Giga-Byte Titan P4 Dual BIOS motherboard c/w PCI Express does not have the 24 pin connector and the new supply I just bought which has the 24pin connector does not physically fit due to the proximity of the IDE connector. I don't want to waste what is probably a good power supply so will either seek an adaptor or get the razor blade out! (not my wrists)
Anyway, great article and thanks for your help and detail.
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