CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [147]
The biggest problem with ATX12V was its lack of teeth—it made a lot of recommendations but few requirements, giving PSU makers too much choice (such as choosing or not choosing to add AUX and P4 connectors) that weren’t fixed until later versions.
Figure 10-21 Molex power on motherboard
EPS12V Server motherboards are thirsty for power, and sometimes ATX12V 1.3 just didn’t cut it. An industry group called the Server System Infrastructure (SSI) developed a non-ATX standard motherboard and power supply called EPS12V. An EPS12V power supply came with a 24-pin main motherboard power connector that resembled a 20-pin ATX connector, but it offered more current and thus more stability for motherboards. It also came with an AUX connector, an ATX12V P4 connector, and a unique 8-pin connector. That’s a lot of connectors! EPS12V power supplies were not interchangeable with ATX12V power supplies.
EPS12V may not have seen much life beyond servers, but it introduced a number of power features, some of which eventually became part of the ATX12V standard. The most important issue was something called rails.
Rails Generally, all of the PC’s power comes from a single transformer that takes the AC current from a wall socket and converts it into DC current that is split into three primary DC voltage rails: 12.0 volts, 5.0 volts, and 3.3 volts. Individual lines run from each of these voltage rails to the various connectors. That means the 12-volt connector on a P4 draws from the same rail as the main 12-volt connector feeding power to the motherboard. This works fine as long as the collective needs of the connectors sharing a rail don’t exceed its capacity to feed them power. To avoid this, EPS12V divided the 12-volt supply into two or three separate 12-volt rails, each one providing a separate source of power.
ATX12V 2.0 The ATX12V 2.0 standard incorporated many of the good ideas of EPS12V into the ATX world, starting with the 24-pin connector. This 24-pin motherboard power connector is backward compatible with the older 20-pin connector so users don’t have to buy a new motherboard if they use an ATX12V 2.0 power supply. ATX12V 2.0 requires two 12-volt rails for any power supply rated higher than 230 watts. ATX12V 2.0 dropped the AUX connector and required SATA hard drive connectors.
In theory, a 20-pin motherboard power supply connector will work on a motherboard with a 24-pin socket, but doing this is risky in that the 20-pin connector may not provide enough power to your system. Try to use the right power supply for your motherboard to avoid problems. Many ATX12V 2.0 power supplies have a convertible 24-to-20-pin converter. These are handy if you want to make a nice “clean” connection, because many 20-pin connectors have capacitors that prevent plugging in a 24-pin connector. You’ll also see the occasional 24-pin connector constructed in such a way that you can slide off the extra four pins. Figure 10-22 shows 20-pin and 24-pin connectors; Figure 10-23 shows a convertible connector. Although they look similar, those extra four pins won’t replace the P4 connector. They are incompatible!
Figure 10-22 20- and 24-pin connectors
Figure 10-23 Convertible motherboard power connector
Many modern ATX motherboards feature an 8-pin CPU power connector like the one found in the EPS12V standard to help support high-end CPUs that demand a lot of power. This connector is referred to by several names, including EPS12V, EATX12V, and ATX12V 2x4. One half of this connector will be pin compatible with the P4 power connector and the other half may be under a protective