PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

26.3 Power Connectors

All power supplies provide two
types of power connectors. The first powers the motherboard and
differs according to form factor. The second powers drives and other
internal peripherals, and comes in two varieties, which are described
in the following sections. Tower/AT and Tower/BAT power supplies
include a third type of power connectora high-voltage cable
that connects the power supply to an external main power switch.

26.3.1 AT Main Power Connector

Table 26-11 lists the
standard AT Main Power Connector pinouts used by all AT power supply
variants, including Desktop/AT, Tower/AT, Desktop/BAT, Tower/BAT, and
LPX. PC/XT power supplies use the same pinouts, except that Pin P8-2
is unused. In addition to supplying various standard voltages, this
connector includes a special-purpose pin. Pin P8-1,
PG, carries the Power Good signal, which the
power supply asserts once it has started and stabilized. The
motherboard will not attempt to boot until the power supply asserts
PG.

Note that P8 and P9 are separate connectors from the power supply,
but mate to a combined connector on the motherboard.

The P8 and P9 connectors are individually keyed, but it is possible to connect P8 from the power supply to the P9 connector on the motherboard and vice versa, with potentially catastrophic results.

P8 and P9 were IBM's original designation for
these connectors. Not all power supplies or motherboards label these
connectors, so you may have to determine which is P8 and which is P9
by examining wire colors. The colors shown are those most commonly
used, but some AT power supplies use the alternative (ATX) color
coding shown in parentheses. We have also seen AT power supplies that
use completely nonstandard wire colors, such as green for ground
wires.

When installing an AT power supply, the key
factor is to ensure that, regardless of wire colors or connector
labels, the connectors are aligned so as to place all four ground
pins contiguously. It's safer still to examine the
documents for the motherboard and power supply. We have seen a few
proprietary motherboards and power supplies that used standard
connectors but with completely nonstandard pinouts. This was clearly
done to force customers to purchase replacement components from the
original vendor, and is happily not something you're
likely to encounter on any recent system.

26.3.2 ATX Main Power Connector

Table 26-12 lists the
pinouts for the ATX Main Power Connector of an ATX or ATX12V power
supply. This is a 2X10 connector with Pin 1 keyed. The motherboard
connector is a Molex 39-29-9202 or equivalent. The power supply
connector is a Molex 39-01-2200 or equivalent. All wires are 18 AWG,
except Pin 11, which is specified as 22 AWG. For power supplies 300W
or larger, the specification recommends using 16 AWG wires for
+3.3VDC, +5VDC, and COM (ground). Note that wire colors are those
recommended and commonly used, but may vary.

In addition to supplying various standard voltages, this connector
includes four special-purpose pins:

PW_OK is the ATX equivalent of the AT Power Good
signal, which the power supply asserts once it has stabilized. The
motherboard will not attempt to boot until the power supply asserts
PW_OK.

5V_sb is the +5V standby circuit, which
supplies +5V at low amperage to the motherboard even when the power
supply is off. Any ATX power supply must provide at least 10 mA
5V_SB, but motherboards with the
Wake-on-LAN (WOL) feature
require 720 mA, which all ATX 2.1-compliant power supplies and most
good ATX power supplies of earlier vintage also provide.

On the critical +3.3V rail, small loads can cause large percentage
shifts in voltage. This pin provides a means for the power supply to
detect the actual voltage present on the +3.3V rail at the main power
connector and modify its output to compensate for up to 100 mV of
drop due to cable, connectors, and PCB traces, thereby maintaining
+3.3V within tolerance.

PS_ON is used by the motherboard to turn the
power supply on and off.

Dell has used nonstandard motherboards and power supplies since September 1998. Although recent Dell power supplies and motherboards use what looks like a standard ATX power connector, the pinouts are different. Replacing a Dell power supply with a standard ATX power supply may destroy the motherboard and/or power supply as soon as you apply power to the system. Similarly, upgrading the motherboard in a Dell system with a standard ATX motherboard while continuing to use the standard Dell power supply destroys the motherboard and/or power supply as soon as you apply power to the system. This situation is particularly insidious because Dell uses what appear to be standard components. For example, it buys Intel motherboards by the million, and those "Dell-version" motherboards resemble standard Intel motherboards in all respects except that the power supply connector is wired differently. For more information, see http://www.hardwareguys.com/dellwarnl.

26.3.3 ATX/ATX12V Auxiliary Power Connector

The ATX/ATX12V Auxiliary Power
Connector, shown in Table 26-13, is recommended for
configurations that require more than 18A of +3.3VDC or more than 24A
of +5VDC. This is an inline 6-pin connector with Pin 6 keyed. The
motherboard connector is a Molex inline 6-pin 15-48-0412 header or
equivalent. The power supply connector is a Molex 90331-0010 or
equivalent. All wires are 16 AWG. Wire colors are standard. Few
inexpensive power supplies provide this connector, but it is present
on some better power supplies. With the shift to ATX12V and 12V VRMs,
the importance of this connector is decreasing. Few motherboards
include this connector.

26.3.4 ATX Optional Power Supply Connector

In addition to the main and auxiliary
power connectors, ATX 2.1 defines the ATX Optional Power Supply
Connector, shown in Table 26-14. This connector is
not required for ATX compliance, but provides various benefits,
including fan monitoring and control, a remote 3.3V sense line (used
to monitor and correct output on the 3.3VDC line), and a power source
for IEEE-1394 (FireWire) devices. This is a 2X3 connector with Pin 1
keyed. The motherboard connector is a Molex 39-30-1060 or equivalent.
The power supply connector is a Molex 39-01-2060 or equivalent. All
wires are 22 AWG. Wire colors are standard. The first color indicates
the base color, and the second the stripe color.

ATX Optional Power Supply Connector signals perform the following
functions:

FanM is a 2-pulse/revolution signal generated by
the power supply fan that notifies the system of current fan speed.
If this signal drops, the motherboard realizes immediately that the
power supply fan has failed, and can shut down the system in an
orderly manner.

FanC is an optional signal generated by some
motherboards to control fan speed for power supplies that are
designed to allow this. The signal can range from 0VDC to +12VDC. A
signal of +1V or less is recognized by the fan as an order to shut
down, and a signal of +10.5V or more is recognized as an order to run
at full speed. Intermediate voltage levels, which are supported by
some motherboards and some fans, allow the system to instruct the fan
to run at some intermediate speed. If this signal is left open
(0VDC), properly designed fans run at full speed.

This is a supplementary +3.3VDC remote sense
line, which allows the power supply to monitor the actual voltage at
the motherboard connector of the nominal +3.3VDC rail and adjust it
to stay within specifications.

1394R provides an isolated ground return path
for the 1394V voltage rail, described next.

1394V is a segregated voltage supply rail for
powering IEEE-1394 devices. The voltage on this rail depends on the
IEEE-1394 implementation, may range between +8VDC and +40VDC, and is
typically unregulated. If implemented, this rail should deliver
voltage only while PS_ON on the main connector is asserted low.

26.3.5 ATX12V Power Supply Connector

With the introduction of the Pentium 4
processor, Intel extended the ATX 2.03 specification to define a new
type of power supply called ATX12V, which is a
superset of ATX 2.03, and is now incorporated in the ATX
Specification 2.1 and the ATX/ATX12V Power
Supply Design Guide Version 1.2. ATX12V power supplies
include an additional 2X2 4-pin +12V power connector, shown in Table 26-15. The motherboard connector is a Molex
39-29-9042 or equivalent. The power supply connector is a Molex
39-01-2040 or equivalent. All wires are 20 AWG. Wire colors are
standard. The purpose of this new connector is to deliver more +12V
current to the motherboard than is available from the standard ATX
main power connector. The presence of the 4-pin +12V connector
indicates that a power supply is ATX12V. The absence of that
connector indicates that it is a standard ATX power supply.

According to Intel's policy, all Pentium 4
motherboards and all power supplies used with those motherboards must
include the 4-pin +12V connector. Because installing such
motherboards in existing systems may also require upgrading the power
supply, some third-party motherboard makers have produced
motherboards that lack the 4-pin +12V connector. This allows those
motherboards to be installed in existing systems without a power
supply upgrade, but we regard this as poor practice and suggest you
avoid using such motherboards. In addition to the 4-pin +12V
connector, ATX12V power supplies must also provide higher +5VDC
standby voltage, required for systems that meet
Intel's Instantly Available PC initiative.

If you already have a good high-capacity standard ATX 2.03 or higher power supply, you needn't replace it when you install a motherboard that requires ATX12V. Instead, buy an ATX12V adapter cable. These cables have a standard female drive power connector on one end and the 4-pin +12VDC connector on the other. We use the $8 PC Power & Cooling Pentium 4 12VATX Adapter, shown in Figure 26-1, for this purpose. Note, however, that many older or inexpensive ATX power supplies have inadequate amperage and/or very loose regulation on the +12V rail, and so may not be suitable for powering a Pentium 4 motherboard.

Figure 26-1. ATX12V adapter

The requirement
for the new connector is due both to increased current draw by
Pentium 4 processors and a shift in Intel's plans
for future motherboards. Current ATX 2.03 or higher motherboards
power components with DC/DC conversion from +5VDC and +3.3VDC
sources. However, relative to +12VDC DC/DC conversion, +5VDC and
+3.3VDC DC/DC conversion yields lower power transmission and
conversion efficiencies. The change in emphasis to +12VDC DC/DC
conversion provides increased efficiency and flexibility for
forthcoming motherboards.

Most but not all ATX12V-compliant power supplies are backward-compatible with ATX 2.03 or higher motherboards because they supply the same voltages and currents within the same specifications on the same rails as an ATX 2.03 or higher compliant power supply. However, some ATX12V power supplies do not provide the 3.3V rails required in the ATX 2.03 specification. Such power supplies are intended for special purposes, and cannot be used with standard ATX motherboards. You can therefore use most ATX12V power supplies with any existing or future ATX-family motherboard, but you cannot use an old-style ATX 2.03 power supply with motherboards that have a 4-pin +12V connector, nor can you use an ATX12V power supply that lacks 3.3V rails with an ATX motherboard, regardless of whether that motherboard has the 4-pin +12VDC connector. For that reason, any new ATX power supply you buy should be an ATX 2.1 (ATX12V)-compliant unit that provides 3.3V rails, which will allow that power supply to be used with any modern motherboard.

26.3.6 NLX Power Connectors

The NLX Main Power Supply Connector
uses the same pinouts, wire colors, and physical connectors as the
ATX Main Power Supply Connector shown in Table 26-12. The NLX specification also defines the NLX
Optional Power Supply Connector, shown in Table 26-16. This connector uses the same physical
connectors and 22 AWG wire as the ATX Optional Power Supply Connector
described in the preceding section, but uses different wire colors
and a slightly different pinout. Wire colors are standard.

Pin 5 (1394) on this connector is analogous to Pin 5 (1394V) on the
ATX connector, and also supplies voltage to unpowered 1394 devices.
However, instead of using Pin 4 as a ground return path for 1394
voltage, this connector leaves Pin 4 unconnected. It appears unwise
to share the main system ground return path with the ground return
path for unregulated 1394 voltage from Pin 5, particularly with Pin 4
sitting there so obviously unused. If you understand the reason for
this difference, please let us know.

26.3.7 SFX/SFX12V Power Connectors

The SFX Baseboard Connector uses
the same pinouts, wire colors, and physical connectors as the ATX
Main Power Supply Connector shown in Table 26-12,
with one or two exceptions:

Pin 9 (+5V_SB) is an optional signal for SFX
power supplies. If present, it allows the motherboard to control the
power supply, just as with ATX and NLX. If Pin 9 is not connected,
the power supply must be controlled by a standard AC on/off switch.

Pin 18 (-5VDC) is not connected on SFX power supplies because -5VDC
is required only by ISA expansion slots, which are not supported by
SFX systems.

SFX12V power supplies (but not SFX power supplies) also include the
ATX12V connector described previously.

The
SFX specification defines the required SFX Control Connector, shown
in Table 26-17. This connector uses the same
physical connectors and 22 AWG wire as the ATX and NLX Optional Power
Supply Connectors described in the preceding sections, but has only
one connection, FanON/OFF, which corresponds to FanC. Wire color is
standard.

26.3.8 TFX12V Power Connectors

The
TFX12V Main Power Connector uses the same
pinouts, wire colors and sizes, and physical connectors as the ATX
Main Power Connector shown in Table 26-12, with one
exception. Pin 18 (-5VDC) is not connected on TFX12V power supplies
because -5VDC is required only by ISA expansion slots, which are not
supported by TFX12V systems. TFX12V power supplies also include the
ATX12V connector described previously.

26.3.9 Power-Supply-to-Device Connectors

Power
supplies provide two types of connectors to power disk drives and
other internal peripherals:

The Peripheral Connector, shown in Figure 26-2, is often called a Molex
Connector by technicians. The cable uses a Molex
PS-8981-04P or AMP 1-480424-0 connector or equivalent, and 18 AWG
wires. Pin 1, at left as you view the connector face, carries +12V
and uses a yellow wire. Pins 2 and 3 are COM (ground) and use black
wires. Pin 4 is +5V and uses a red wire.

Figure 26-2. Peripheral Connector

The Floppy Drive Connector, shown in Figure 26-3, is often called a Berg
Connector, and is also used by some other types of drives
(5.25-inch floppy drives use the larger Molex connector also used by
hard disks and other drives). The cable uses an AMP 171822-4
connector or equivalent. The wires are 20 AWG. Pin 1, at left as you
view the connector face, carries +5V and uses a red wire. Pins 2 and
3 are COM (ground) and use black wires. Pin 4 is +12V and uses a
yellow wire.

Figure 26-3. Floppy Drive Connector

Note that wire colors map to the same voltages on the Peripheral Connector and the Floppy Drive Connector, but the pinouts are exactly reversed. While building a home-made cable, we once toasted a drive by assuming the pinouts were identical, thereby putting +12V on a +5V device.

The number and type of device connectors provided are
loosely linked to the form factor and power rating of the power
supply. Generally, power supplies with low power ratings and those
designed for smaller cases provide fewer device connectors, sometimes
as few as three. Power supplies with higher power ratings and those
intended to fit large tower cases provide more device connectors,
sometimes as many as a dozen. So long as you do not exceed the power
supply capacity, you can freely clone device connectors by adding
Y-splitters, which are available for a couple of dollars at any
computer store.

Serial ATA hard drives use a different power connector. As of July 2003, few power supplies provide that connector, so the only option is to use an adapter cable to power Serial ATA drives from a standard Molex peripheral connector. We expect Serial ATA power connectors to become increasingly common on power supplies shipped after July 2003, and by early 2004, nearly all power supplies are likely to include Serial ATA power connectors.

26.3.10 Main Power Switch Power Connectors

Desktop/AT and Desktop/BAT power
supplies have a built-in paddle switch to turn power on and off. ATX
and ATX-variant power supplies seldom have a physical power switch
because they are turned on and off by the motherboard. Some ATX power
supplies have a rocker switch on the back of the power supply that
disconnects the power supply entirely from mains power. This can be
useful because the alternative is disconnecting the power cable when
you need to kill all power to the system, including
+5V_SB, which is ordinarily always present.

Tower/AT and Tower/BAT power supplies have no built-in main power
switch. Instead, they have four power leads that connect to a
push-button or toggle switch on the case, as shown in Figure 26-4. These leads, which are usually white, black,
blue, and brown, carry AC mains voltage to the power supply.

Figure 26-4. Typical connections for a Tower/AT or Tower/BAT power switch

Although these four wire colors are relatively standard, different switches require connecting them differently. These wires carry full mains voltage, which can kill you, so never work on them without first disconnecting the main power cable from the power supply . Connecting them improperly can also damage the power supply and the computer, so never use trial and error or guess about which wire goes where. Contact the case and power supply manufacturers to verify it. Because these wires carry high voltage, we recommend using electrical tape to insulate the connections.

26.3.11 Real-World Power Supplies Compared

Table 26-18 lists
the output by voltage of three nominally 350-watt ATX12V power
supplies. The PC Power & Cooling Turbo Cool 350 ATX/ATX12V is a
premium unit, with a street price of about $65; the Antec SL350 is a
name-brand unit that sells for about $45; the Sparkle Power Inc.
(SPI) FSP350-60BN is another name-brand unit that sells for about
$50. We would like also to have compared a no-name Pacific Rim power
supply, but finding technical specifications for such units is
impossible, probably because they vary so much from one lot to
another that no one bothers to test them.

The three key voltage rails are +3.3V, +5V, and +12V. Note that,
although all the power supplies list individual maximum outputs for
+3.3V and +5V, they also note maximum combined output of +3.3V and
+5V215W for the PC Power & Cooling unit, 230W for the
Antec, and 220W for the SPI unit. Antec is alone, however, in
specifying maximum combined output on the +3.3V, +5V, and +12V rails.

The main differences between these power supplies are as follows:

The most obvious difference is in total nominal wattage. Simply
adding the wattages on the individual rails tells us that the PC
Power & Cooling and SPI units both total 453.5W, while the Antec
is noticeably higher at 481.5W. That measure is meaningless, however,
because the wattage deliverable on one rail may be constrained by the
amount of wattage being drawn on another voltage rail.

For all three power supplies, the total deliverable combined wattage
on the +3.3V and +5V rails is related. For example, the PC Power
& Cooling unit can deliver up to 92.4W of +3.3V and 160W of +5V,
but the combined total wattage on those two rails cannot exceed 215W.
That means if you happen to be drawing 90W on the +3.3V rail, you can
draw at most 125W on the +5V rail, even though that rail is rated for
a maximum of 160W. In addition to limiting the combined +3.3V/+5V
draw, the Antec unit also limits the combined +3.3V/+5V/+12V draw to
at most 330W. Taking these combined limits into account, the PC Power
& Cooling and SPI units can both theoretically deliver more than
400W, while the Antec unit is limited to at most 352.1W.

In effect, that means that the Antec is not really capable of
delivering 350W unless you can load the +3.3V, +5V, and +12V rails in
combination at or very near their maximum ratings. That may not
happen with a real-world system. Instead, you may have one or two
rails loaded to their individual limits, while another voltage rail
has a great deal of reserve. Having a lot of unused +12V, for
example, does you no good if what you really need is more +3.3V.

Conversely, the PC Power & Cooling and SPI units are more
flexible because they place fewer constraints on combined wattages.
Although each of them is capable of delivering more than 400W, they
are of course still rated as only 350W power supplies. The difference
between them and the Antec unit is that they place no limit on
combined +3.3V/+5V/+12V wattage. That means that if a system has a
very high draw on +12V (such as a fast Pentium 4 system), the power
supply can still pump as much +3.3V and +5V as necessary, within its
total 350W limit.

Remember that wattage rating depends heavily on the temperature at which the rating is done. We know that the PC Power & Cooling unit was rated at a realistic 40º C. We have no idea what temperature Antec or SPI used for their testing, but 25º C seems to be the industry standard. A nominal 350W power supply tested at 25º C actually delivers about the same wattage as a 230W power supply rated at 40º C.

The PC Power & Cooling and SPI units are both within ATX/ATX12V
specifications for regulation on all voltage rails. The Antec unit is
not ATX-compliant because its 5% load regulation on +3.3VDC is less
stringent than the ATX-required 4%. In practical terms, the Antec
unit is likely to function adequately, but its poor +3.3V regulation
is cause for concern. The SPI unit is regulated within ATX
specifications. The PC Power & Cooling unit meets ATX
requirements on every rail and greatly exceeds them on the critical
+3.3V rail. Tight voltage regulation is key for system stability, so
we know which of these power supplies we'd choose.

A more subtle aspect of nominal wattage is component loading. A
premium power supply may deliver its rated wattage while driving its
components at only 50% of their rated capacity; a midrange name-brand
power supply may deliver near its rated wattage while driving its
components at 70% of their rated capacity. A no-name power supply
will likely deliver nowhere near its rated wattage, and will drive
its components at 100% (or even more) of their rated capacity to do
so.

Component loading has two important aspects. First, components driven
at a fraction of their rated capacity are likely to exceed their
design life significantly, while those driven at (or above) their
designed limits are likely to be short-lived. Second, a component
that is "loafing" is likely to
perform much better than one that is being driven at or above its
design capacity. For example, components designed to supply +3.3V and
driven at 50% of capacity are likely to supply 3.3V bang-on; those
driven at 100% of capacity may deliver 3.3V nominal, but the actual
voltage may vary significantly. Unfortunately, short of disassembling
the power supply (never a good idea) and checking the number, size,
and quality of the components, about the only thing you can do to
assure that the power supply you choose uses good components is to
buy a good name-brand power supply.

Comparing the full spec sheets for these three power supplies turns
up other differences not shown in the table, of varying significance.
For example, the hold time of the PC Power & Cooling unit is 20
ms, which is the minimum required for ATX compliance. The hold time
of the SPI unit is specified as "16.6 ms
minimum" and that of the Antec is unspecified, which
means that technically neither of these units is ATX-compliant based
on the listed specifications. Similarly, the PWR_OK delay times of
the Antec and SPI units are listed as 100 - 500 milliseconds, which
is technically within ATX requirements, but far inferior to the
specific 300 millisecond PWR_OK delay specified for the PC Power
& Cooling unit.

Do not confuse hold time with PWR_OK delay. Hold time specifies the duration for which output power remains within specifications after input power is lost, as when the power flickers. PWR_OK delay time refers to when the power supply is turned on, and specifies the lag between the time all voltage rails begin supplying stable voltage and the time the power supply asserts the PWR_OK (Power Good) signal to the motherboard. Predictable PWR_OK delay is critical to the timing of the boot sequence.

In short, there are very real differences
between power supplies, even if they have the same nominal rating.
The PC Power & Cooling unit is a superb power supply, which will
supply clean, closely regulated power even when driven at its rated
wattage, and is unlikely to fail even if used 24X7 at its rated
output power. The SPI unit is a very good power supply, likely to
work reliably in less-demanding applications. The Antec unit is
relatively poorly regulated, particularly relative to the PC Power
& Cooling unit, and is an appropriate choice only when system
cost is a very high priority. (In fairness, we should point out that
the Antec SL350 is an economy power supply and that Antec does make
better power supplies, including the superb TruePower series.) About
the best you can say for no-name units is that power is likely to
come out of them, although how much, how well-regulated, and for how
long are in doubt.