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

25.1 Case Characteristics

PC cases are available in a bewildering array of sizes, shapes, and
prices. Form factor is the most important thing
about a case because it determines which motherboards and which power
supplies fit that case. Cases are available in the following form
factors:

The 1984 IBM
PC-AT introduced the AT form factor. AT cases
accept full-size AT motherboards and reduced-size Baby AT
motherboards. All AT-variant cases have a circular hole in the rear
panel for the motherboard keyboard connector and knockouts for
external DB connectors that mate to serial, parallel, and other ports
present as header pins on AT motherboards. AT cases have been
produced in two variants, which differ only in the power supply they
accept. Desktop/AT cases use the original AT
form factor power supply, with a paddle switch built into the power
supply itself. Tower/AT cases use a modified AT
power supply that instead has four main power leads that connect to a
switch built into the case. Desktop/AT cases and power supplies are
hard to find nowadays, but Tower/AT cases and power supplies are
still readily available. AT cases of either type are a poor choice
for building a new system.

AT
motherboards require large cases. The demand for smaller systems
resulted in Baby AT (BAT)
motherboards and cases. A BAT motherboard is simply a reduced-size AT
motherboard, and uses the same connectors and mounting hole
positions. Like AT cases, BAT cases have been produced in
Desktop/BAT and Tower/BAT
form factors. Desktop/BAT cases accept only Desktop/BAT power
supplies, which are smaller versions of the AT power supply, complete
with paddle switch. Tower/BAT cases accept only Tower/BAT power
supplies, which are smaller versions of Tower/AT power supplies.
Confusingly, many recent BAT desktop cases and systems were designed
to use Tower/BAT power supplies. Adding to the confusion, BAT has
become such a catchall term that some so-called BAT cases can in fact
accept full-AT motherboards and power supplies. Desktop/BAT cases and
power supplies are difficult to find new now. New Tower/BAT systems
were still being sold as recently as early 2000, so Tower/BAT cases
(in both desktop and tower styles!) are still widely available to
upgraders, although they are a poor choice for building new systems.

Corporate
demand for low-profile systems and the desire to reduce manufacturing
costs led to the creation of LPX, a variant of
BAT. LPX cases accept only LPX power supplies and LPX motherboards,
which use a riser card to arrange expansion cards horizontally rather
than vertically. Although it was for a while popular for mass-market
consumer systems and low-profile corporate systems, LPX failed as a
standard for several reasons: many manufacturers implemented
proprietary variations of LPX; expected cost savings did not
materialize, largely because most LPX implementations required
expensive supplementary cooling fans; and the riser card made it
difficult to work on the motherboard, increasing support costs. LPX
cases are still available, although their distribution is very
limited. The only reason to purchase an LPX case would be to salvage
components from an existing LPX system, but such systems are now so
old that there's nothing worth salvaging.

The preceding three form factors are obsolete, although manufacturers
continue to make them for the repair and upgrade market. The form
factors of modern cases all derive from the Intel ATX
specification (http://www.formfactors.org/). ATX includes
the following variants, whose dimensions and motherboard/case
compatibilities are detailed in Table 25-1.

Both the lack
of a formal BAT standard and some increasingly troublesome problems
with BAT component layouts led Intel to develop the ATX
form factor, which it introduced with the Advanced/ML
"Marl" motherboard in 1996. ATX
redesigned component layouts for easier access, improved cooling, and
other factors, but the important aspect of ATX for cases is that it
dispenses with the AT keyboard hole and port knockouts on the rear
panel, replacing them with a consolidated I/O panel of standard size
and positioning. Also, ATX motherboards control the power supply
directly, which means that the
"power" switch on an ATX system
actually just notifies the motherboard to turn power on or off rather
than doing so itself. The original ATX specification is often loosely
termed "full ATX" to differentiate
it from smaller ATX variants, described next. A full-ATX case is
usually the best choice for building a new PC.

As a part of the Intel ATX Specification,
Intel also defines Mini-ATX, which is simply a reduced-size ATX
motherboard. The primary motivation for Mini-ATX is manufacturing
cost reductions because four Mini-ATX boards can be produced from a
standard blank, which yields only two ATX boards. Mini-ATX cases
accept Mini-ATX (or smaller) motherboards, but are too small for ATX
motherboards. It costs nearly as much to make Mini-ATX cases as
full-ATX cases, which accept both ATX and Mini-ATX motherboards, so
Mini-ATX cases are very uncommon.

NLX,
introduced in 1997, is the ATX version of LPX, and is intended for
inexpensive, low-profile corporate and mass-market systems. NLX cases
accept only NLX motherboards, which use riser cards similar to those
used by LPX cases, and a full-length I/O panel rather than the
standard ATX I/O panel. About the only reason to buy an NLX case is
to salvage an NLX system whose case is damaged. NLX cases are readily
available from numerous sources, but are a poor choice for building
new systems.

Smaller translates to cheaper. Intel
released the microATX Motherboard Interface Specification in 1998 in
response to the demand for low-cost consumer systems, for which
expandability is not an issue. microATX motherboards have only four
expansion slots (versus seven for ATX), which allows using smaller
and less-expensive cases. Some microATX cases accept only SFX power
supplies (described in Chapter 26), which are
smaller, cheaper, less powerful, and less functional than standard
ATX power supplies. Because such cases are too small to accept a
standard ATX power supply, upgrade options are severely limited.
Although microATX cases are available from third-party manufacturers,
their distribution is limited and they are a very poor choice for
building new systems. microATX components are sometimes mistakenly
designated mATX.

Just as
microATX is a smaller version of ATX, FlexATX is a smaller version of
microATXpicoATX as it were. FlexATX is an addendum to the
microATX specification, intended for even smaller systems such as web
devices, set-top boxes, and novelty PCs such as the Barbie PC and the
Hot Wheels PC (we are not making this up). If for some reason you
want to know more about FlexATX, download the FlexATX
Addendum Version 1.0 to microATX Specification Version 1.0
(http://www.formfactors.org/developer\specs\FlexATXaddn1_0.pdf).

In December
2001, VIA Technologies introduced the Mini-ITX reference design,
which uses a 170mm (6.7-inch) square motherboard form factor.
Mini-ITX is a follow-on to ITX, for which reference designs existed
that never went into production. Mini-ITX is essentially a smaller
version of FlexATX and is backward-compatible with FlexATX and
microATX. Micro-ITX focuses heavily on reducing PC size and power
consumption. Mini-ITX systems accordingly use very small power
supplies (less than 100 W) and slow, cool-running processors such as
the VIA C3. Although Mini-ITX is theoretically an open standard, it
is effectively proprietary because few manufacturers other than VIA
support it. For more information about Mini-ITX, see http://www.via.com.tw/en/VInternet/mini_itx_faq.jsp.

Reversing the trend toward ever-smaller
variants of ATX, the WTX Workstation System Specification Version 1.1
defines WTX as Intel's extension of the ATX standard
to the requirements of midrange workstations, with explicit support
for dual processors, large memory configurations, AGP Pro graphics,
dual-fan power supplies, and other workstation-oriented features.
WTX-compliant cases, power supplies, and motherboards began shipping
in limited quantities in Q1 2000. For additional information about
WTX, see http://www.casereview.org/wtx/WTX-Spec-11.pdf.

Some hybrid cases are available that accept either BAT or ATX motherboards and power supplies, and include both the AT-specific keyboard connector hole and port knockouts and the ATX-specific I/O panel cutout. These cases may include a "universal" power supply that provides both AT and ATX motherboard main power connectors. These cases were formerly popular with some upgraders, as they allowed using an existing AT motherboard and upgrading to an ATX model later without replacing the case. Now that the AT form factor is nearly obsolete, such cases have become harder to find, although a few are still available.

Although form factor is the most important
characteristic of a case, numerous other factors are worth
considering:

Cases are available in a variety of sizes and orientations, including
low-profile desktop, standard
desktop, micro-tower (for microATX
boards), mini-tower,
mid-tower, and full-tower.
Low-profile cases are popular for mass-market and business-oriented
PCs, but we see little purpose for them. They take up more desk space
than towers, provide poor expandability, and are difficult to work
on. Micro-tower cases take very little desk space, but otherwise
share the drawbacks of low-profile cases. Mini/mid-tower
stylesthe dividing line between them is nebulousare
most popular because they consume little desktop space while
providing good expandability. Full-tower cases are what we really
prefer. They take up no desk space at all, and are tall enough that
CD-ROM, tape, and other external drives are readily accessible. Their
cavernous interiors make it very easy to work inside them, and they
often provide better cooling than smaller cases. The drawbacks of
full-tower cases are that they are more expensive than other cases,
sometimes significantly so, and that they may require using extension
cables for keyboard, video, and/or mouse.

A proprietary PC style called Small Form Factor (SFF) is fast gaining popularity, primarily due to the efforts of Shuttle. Such systems are generally called "cubes," although they're really about the shape and size of a shoebox. Unlike Mini-ITX systems, which are low-performance "appliance" PCs, SFF systems use standard Pentium 4 or Athlon processors, and are designed to cram the power of a full-size PC into the smallest possible box. SFF PCs have two important drawbacks. First, the form factor is nonstandard, which means you can use only motherboards that are designed to fit the specific case. Leading motherboard makers such as Intel, ASUS, Gigabyte, and MSI do not make SFF motherboards, so you are limited to motherboards from second- and third-tier makers such as Iwill, Jetway, Shuttle, Soltek, and Soyo. Second, cooling is critical when a high-performance processor, a fast hard drive, and a power supply of sufficient wattage are crammed into a shoebox-size enclosure. Although we do not have sufficient data to make an absolute prediction, we expect that the higher operating temperature of SFF PCs will lead to increased instability and a shorter life relative to similar components enclosed in a standard case. We recommend avoiding SFF PCs unless system size is the absolute highest priority.

The number and arrangement of drive bays
may be unimportant if the system is unlikely to be upgraded later.
All current cases provide at least one 3.5" external
bay for a floppy drive, one 5.25-inch external bay for a CD-ROM,
DVD-ROM, or CD-RW drive, and one 3.5-inch internal bay for a hard
disk. That may change, however, as Microsoft and Intel strive to rid
the world of "legacy" devices,
including the venerable floppy drive. Table 25-2
shows typical arrangements for various case styles. Some cases can be
configured as desktops or towers, and the number of drive bays may
differ between configurations.

Drives mount in most cases via screws driven directly through the
chassis into the sides of the drives. This method is secure, provides
good electrical grounding, and allows the drives to use the chassis
as a heatsink. The drawback is that, for some chassis, it is
difficult to access the screws on the right side of 3.5-inch drives.
Some cases address this problem by using removable motherboard trays
or removable drive bays. Others simply have access holes punched in
the right side of the chassis. Some cases use mounting rails, which
screw or snap onto the drive and fit slots in the drive bays. Rails
are less likely to physically torque a drive, which can cause
read/write problems, and make it easier to remove and replace drives.
On the downside, rails provide inferior electrical and thermal
contact compared to direct mounting, may rattle if they fit loosely,
and sometimes cause vertical alignment problems where one drive that
should fit an adjacent bay will not do so because the faceplate is a
tiny bit too large. In practice, we've never much
cared whether a case required securing drives directly or used rails.
If you are building a system that you will seldom open, drive
mounting method is relatively unimportant. If you are building a
test-bed or other system in which you will frequently swap drives,
either buy a case that uses rails or simply don't
use screws to secure the drives.

Cases vary widely in how easy they are to work on. Some use thumb
screws and pop-off panels that allow complete disassembly in seconds
without tools, while disassembling others requires a screwdriver and
more work. Similarly, some cases have removable motherboard trays or
drive cages that make it easier to install and remove components. The
flip side of easy access is that, unless they are properly
engineered, easy-access cases are often less rigid than traditional
cases. Years ago we worked on a system that experienced seemingly
random disk errors. We replaced the hard disk, cables, disk
controller, power supply, and other components, but errors persisted.
As it turned out, the user kept a stack of heavy reference books on
top of the case. As she added and removed books, the case was flexing
enough to torque the hard disk in its mounting, causing disk errors.
Rigid cases prevent such problems. The other aspect of accessibility
is sheer size. It's easier to work inside a
full-tower case than a smaller case simply because
there's more room.

For basic systems, the power supply fan and CPU fan normally suffice.
More heavily loaded systemsthose with dual processors,
high-performance SCSI hard drives, lots of expansion cards, and so
onrequire adding supplemental fans. Some cases have no
provision for adding fans, while others provide mounting positions
for half a dozen or more fans. A few towers and hobbyist-oriented
cases have supplemental fans as standard features, but most cases do
not.

Cases run the gamut in construction quality. Cheap cases have flimsy
frames, thin sheet metal, holes that don't line up,
and razor-sharp burrs and edges that make them dangerous to work on.
High-quality casessuch as those from PC Power & Cooling
and Antechave rigid frames, heavy sheet metal, properly
aligned holes, and all edges rolled or deburred. One seldom-noticed
specification is weight, which is largely determined by the thickness
of the frame and panels, and can provide a good clue to case quality.
Without power supply, for example, the PC Power & Cooling
Personal Mid-Tower weighs 18 pounds (8.2 kg) and the similar Antec
KS-288 weighs 23 pounds (10.5 kg). We have seen no-name cases of
similar size that weigh as little as 12 pounds (5.5 kg). For cases,
heavier is usually better.

PC cases have traditionally been made of thin-sheet steel panels,
with a rigid steel chassis to prevent flexing. Steel is inexpensive,
durable, and strong, but it is also heavy. In the last few years, the
popularity of "LAN parties" has
increased, fueling a demand for lighter cases. A steel case light
enough to be conveniently portable is insufficiently stiff, which has
led case makers to produce aluminum cases for this specialty market.
Although aluminum cases are indeed lighter than equivalent steel
models, they are also much more expensive. For example, the steel PC
Power & Cooling Personal Mid-Tower case weighs 18 pounds and
sells for $49. The PC Power & Cooling Aluminum Mid-Tower case
weighs 14 pounds and sells for $179. Such extreme price disparities
are common for aluminum models, and
"economy" aluminum models are so
flimsy that they are not worth having. Unless saving a few pounds is
a very high priority, we recommend you avoid aluminum models. Some
specialty cases are made from such unusual materials as transparent
plastic or wood. We suggest you avoid all such gimmick cases, and
stick with a steel model from a good manufacturer such as PC Power
& Cooling or Antec.