Section 25.1. Case Characteristics

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

			Fits ATX case style(s)
Motherboard	Widthmax	Depthmax	Full	Mini	micro	Flex	NLX	WTX
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.

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

	External bays		Internal bays		Total bays
Case Style	3.5"	5.25"	3.5"	5.25"
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.