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:
- AT
-
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.
- Baby AT
-
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.
- LPX
-
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/developer/specs/atx/atxspecs.htm). ATX includes the following
variants, whose dimensions and motherboard/case compatibilities are
detailed in Table 25-1:
- ATX
-
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 they 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 below. A full-ATX case is
usually the best choice for building a new PC.
- Mini-ATX
-
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
-
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.
- microATX
-
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 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.
- FlexATX
-
Just as microATX is a smaller version of ATX,
FlexATX is a
smaller version of microATX, picoATX 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 like 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.teleport.com/~ffsupprt/spec/FlexATXaddn1_0.pdf).
- WTX
-
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.wtx.org.
Table 25-1. Maximum motherboard dimensions and chassis compatibility for ATX form factor variants
|
ATX
|
12.0"/305mm
|
9.6"/244mm
|
|
|
|
|
|
|
|
Mini-ATX
|
11.2"/284mm
|
8.2"/208mm
|
|
|
|
|
|
|
|
microATX
|
9.6"/244mm
|
9.6"/244mm
|
|
|
|
|
|
|
|
FlexATX
|
9.0"/229mm
|
7.5"/191mm
|
|
|
|
|
|
|
|
NLX
|
13.6"/346mm
|
9.0"/229mm
|
|
|
|
|
|
|
|
WTX
|
16.75"/425mm
|
14.0"/356mm
|
|
|
|
|
|
|
 |
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 come equipped with a
"universal" power supply that
provides both AT and ATX motherboard main power connectors. These
cases are popular with some upgraders, as they allow using an AT
motherboard now and upgrading to an ATX model later without replacing
the case. We don't much like universal cases,
because they often focus on flexibility and low cost at the expense
of quality.
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Although form factor is the most important
characteristic of a case, numerous other factors are worth
considering:
- Size and orientation
-
Cases are available in a variety of sizes and orientations, including
low-profile desktop, standard desktop, micro-tower (for the new
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 styles—the
dividing line between them is nebulous—are 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.
- Drive bay arrangement
-
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"
external bay for a CD-ROM, DVD-ROM, or CD-RW drive, and one 3.5"
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.
Table 25-2. Typical drive bay configuration in various case styles
|
Low profile
|
1 - 2
|
1
|
1 - 2
|
0
|
3 - 4
|
|
Desktop
|
1 - 2
|
2 - 3
|
1 - 3
|
0 - 3
|
4 - 7
|
|
Micro-tower
|
1 - 2
|
1
|
1 - 2
|
0
|
3 - 4
|
|
Mini-tower
|
1 - 2
|
2 - 3
|
1 - 4
|
0 - 2
|
4 - 7
|
|
Mid-tower
|
1 - 2
|
3 - 4
|
1 - 5
|
0 - 3
|
5 - 8
|
|
Full-tower
|
0 - 2
|
3 - 12
|
0 - 8
|
0 - 8
|
8 - 14
|
- Drive mounting method
-
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" drives. Some cases address this problem by using
removable motherboard trays or removable drive bays. Others simply
have access holes punched into 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.
- Accessibility
-
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.
- Provisions for supplemental cooling
-
For basic systems, the power supply fan
and CPU fan normally suffice. More heavily
loaded systems—those with dual processors, high-performance
SCSI hard drives, lots of expansion cards, and so on—require
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.
- Construction quality
-
Cases run the gamut in construction quality. Cheap cases have flimsy
frames, thin sheetmetal, holes that don't line up,
and razor-sharp burrs and edges that make them dangerous to work on.
High-quality cases—like those from PC Power & Cooling and
Antec—have rigid frames, heavy sheetmetal, 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.
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