CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [146]
1. Switch your multimeter to DC, somewhere around 20 V DC if you need to make that choice. Make sure your leads are plugged into the multimeter properly: red to hot, black to ground. The key to testing DC is that, which lead you touch to which wire matters. Red goes to hot wires of all colors; black always goes to ground.
2. Plug the red lead into the red wire socket of a free Molex connector and plug the black lead into one of the two black wire sockets. You should get a reading of ~5 V. What do you have?
3. Now move the red lead to the yellow socket. What voltage do you get?
4. Testing the P1 connector is a little more complicated. You push the red and black leads into the top of P1, sliding in alongside the wires until you bottom out. Leave the black lead in one of the black wire ground sockets. Move the red lead through all of the colored wire sockets. What voltages do you find?
ATX
The original ATX power supplies had two distinguishing physical features: the motherboard power connector and soft power. Motherboard power came from a single cable with a 20-pin P1 motherboard power connector. ATX power supplies also had at least two other cables, each populated with two or more Molex or mini connectors for peripheral power.
When plugged in, ATX systems have 5 volts running to the motherboard. They’re always “on” even when powered down. The power switch you press to power up the PC isn’t a true power switch like the light switch on the wall in your bedroom. The power switch on an ATX system simply tells the computer whether it has been pressed. The BIOS or operating system takes over from there and handles the chore of turning the PC on or off. This is called soft power.
Using soft power instead of a physical switch has a number of important benefits. Soft power prevents a user from turning off a system before the operating system has been shut down. It enables the PC to use power-saving modes that put the system to sleep and then wake it up when you press a key, move a mouse, or receive an e-mail. (See Chapter 21, “Portable Computing,” for more details on sleep mode.)
All of the most important settings for ATX soft power reside in CMOS setup. Boot into CMOS and look for a Power Management section. Take a look at the Power On Function option in Figure 10-19. This determines the function of the on/off switch. You may set this switch to turn off the computer, or you may set it to the more common 4-second delay.
ATX did a great job supplying power for more than a decade, but over time more powerful CPUs, multiple CPUs, video cards, and other components began to need more current than the original ATX provided. This motivated the industry to introduce a number of updates to the ATX power standards: ATX12V 1.3, EPS12V, multiple rails, ATX12V 2.0, other form factors, and active PFC.
Figure 10-19 Soft power setting in CMOS
ATX12V 1.3 The first widespread update to the ATX standard, ATX12V 1.3, came out in 2003. This introduced a 4-pin motherboard power connector, unofficially but commonly called the P4, that provided more 12-volt power to assist the 20-pin P1 motherboard power connector. Any power supply that provides a P4 connector is called an ATX12V power supply. The term “ATX” was dropped from the ATX power standard, so if you want to get really nerdy you can say—accurately—that there’s no such thing as an ATX power supply. All power supplies—assuming they have a P4 connector—are ATX12V or one of the later standards.
Figure 10-20 Auxiliary power connector
The ATX12V 1.3 standard also introduced a 6-pin auxiliary connector—commonly called an AUX connector—to supply increased 3.3- and 5.0-volt current to the motherboard (see Figure 10-20). This connector was based on the motherboard power connector from the precursor of ATX, called AT.
The introduction of these two extra power connectors