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Q. What are the major motherboard type?
The computer motherboard, also called the system board, provides the basic foundation for all of the computer's hardware including the processor, RAM, BIOS and expansion cards. Several motherboard standards are available each with a different layout and associated advantages. Common motherboard types include the BTX, AT (and baby-AT), the ATX and Mini-ATX, LPX and Mini-LPX and the NLX. The most common layout (called a form factor) in newer PCs are the ATX and NLX derivatives. See also: CPU, Clock, ROM and BIOS.
 
Q. What are the major platform form factors?
A. The major platform form factors are: ATX, Micro ATX, Flex ATX, NLX, Mini NLX, AT, Baby AT, Mini AT. In the near future a new standard will be used called BTX.
 
Q. What is Serial ATA (SATA)?
A. Serial ATA is the latest standard in IDE(Atapi) devices. The maximum speed of Serial ATA is 150MB/s (the maximum speed of Parallel ATA is 133MB/s, the ATA133 standard). The cable for serial ATA has a minimum of 4 wires (extra ground wires can be applied), a 7-pin connectro and a maximum length of 1metre (about 3.1 feet). Because of the serial protocol only 1 device can be connected to each cable. The big advantages compared to parallel ATA are higher speeds (speed wil go up in future), longer cables and better airflow in your case because of the smaller footprint of the cables.
 
Q. What kind of products does Procase sell?
A. PC cases, 19" Rackmount cases, Cabinet and Racks, Power Supplies, Server Chassis, Mobile Racks,External Enclosure, Backplane Modules, Accessoires and Barebone systems.
Q. What is SCSI?
A. SCSI stands for Small Computer Systems Interface and is basically a connection standard for all kinds of devices like harddisks, tape drives and CD-ROM's. It uses components that are different from IDE. SCSI had different standards, the biggest differences are the databus width and the maximum transfer speed, the databus can be 8 or 16 bits wide, the transfer speed can go up to as far as 320MB/s. There are many different cables and connectors for different SCSI standards.
 
Q. What is SAS (Serial Attached SCSI)?
A. Serial Attached SCSI is the future version of SCSI. It will be connected via a serial interface and thus uses cables much more compact then "normal" parallel SCSI which leads to better airflow. At launch, expected to be in the first half of 2004, the speed will be 3 Gbits/sec. SAS will support up to 128 devices and a maximum cable length of 6 metres. SAS host controllers will also be able to be used for attaching SATA (Serial-ATA) devices.

More information :

Parts of the Case - A system case should normally come with a number of physical components. This will of course vary widely from system to system, depending to some extent on the form factor and overall design of the case. If you buy a case by itself for use in building your own machine, you should be able to check for all of these items. If you bought a pre-built system, many of these components were probably already put into the case when it was assembled, with left over materials kept by the company that made it.


Frame and Cover - The physical frame of the case, and its cover, are usually made of sheet metal. The cover slides off the frame when the screws that hold it to the frame are removed (though some cases use no screws at all to secure the cover, and increasing numbers of designs use removable panels instead of an integrated cover.) If the case is the most overlooked part of the PC, the quality of the frame and cover is the most overlooked of the overlooked. Consider the following when assessing the quality of the frame and cover of the case: Rigidity : Many of the components in your PC have little tolerance for being flexed or bent; this is especially true of the motherboard. A very high quality case is made of solid, heavy gauge steel (16 to 18 gauge) while slightly cheaper ones use lighter (20) gauge steel. All-steel cases have very good rigidity, feel "solid", and will keep your system from flexing. The next step down from all-steel is aluminum reinforced with steel. The cheapest cases use punched aluminum that you can literally bend with your hands. Fit : Quality cases have a good fit between their components. The cover fits securely on the frame, and any plastic panels fit without rattling or leaving large gaps. Proper fit also reduces spurious RF emissions from the PC, and helps enable the case to be FCC certified. Finish : Good cases have their metal properly finished and trimmed, while cheaper cases often leave very sharp edges that can be hazardous to those who work within them.
There are so many different configurations of cases and their covers that it would be impossible to list them all. Many companies seem to pride themselves in their ability to come up with wacky new case designs with screws in the strangest of places, it seems. The following general categories seem to cover a good chunk of what is out there, however: conventional desktop, conventional tower, front-screw desktop, front-screw tower, single screw tower/desktop, screwless tower/desktop and "Flip-Top" desktop.Another relatively new innovation in case design is the removable, rotatable, or slideable motherboard panel (sometimes called a "motherboard tray".) Many newer, higher-quality cases are designed to allow the part of the frame where the motherboard is mounted to be removed from the case or shifted for easier access. If you've ever built your own PC, or tried to replace a motherboard in an existing system, you know that this is a design that is long overdue. It makes it much easier to work within the system, and can make even a small case seem much larger.
The back of the main case frame will usually contain a number of cut-outs, which are the places where I/O connectors from the motherboard are mounted. On older XT, AT and Baby AT cases, these usually are smaller, individual holes where your serial, parallel and other interface ports are mounted, with wires that run to headers on the motherboard. Newer form factors such as NLX and ATX have the I/O ports mounted directly onto the motherboard in either a single or double row; cases designed for these form factors are sometimes equipped with exchangeable plates that match different port layouts, providing flexibility for changing motherboard designs.
Finally, don't forget your feet! They attach to the bottom of the case to provide traction, especially on slippery desks. Sometimes new cases come with their plastic feet not yet attached. Remember to install them as soon as possible when assembling the case, as it may not be possible to put them in once the motherboard and other components have been installed. Larger cases occasionally come with wheels.

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Power Supply - The power supply usually comes with the case, even though it isn't technically part of it. The power supply comes with an integrated fan, secured to the back of the case in AT form factor supplies, or the front (to blow into the case) for most ATX supplies. The power supply is discussed in its own section here.
The power switch for AT form factor cases is really part of the power supply; for ATX form factor cases it is a separate component attached to the case. The switch on most modern PCs is mounted just behind a plastic button that is accessible from the exterior of the case. Most cases also come with a standard power cord, usually black, to plug the power supply into the wall.


LEDs, Speaker and Connecting Wires - Most cases have at least two LEDs, to indicate power and hard disk activity. Some have several more LEDs to indicate turbo status or other functions (though "turbo" is now outdated and not generally seen on new PCs.) A standard PC speaker is usually mounted somewhere inside the case, but may be loose in a new case. There are special wires for all the LEDs and for the speaker, to be connected to the motherboard or drives.


Cooling Vents and Auxiliary Fans - Cooling vents are usually in the front of the case. These allow air to be circulated by the power supply fan. They should not be blocked off or poor cooling may result. Some cases come with extra, auxiliary cooling fans, and/or mounting locations for the user to add fans if he or she desires to do so. These are becoming more popular, especially with homebuilders and overclockers, since they theoretically reduce the heat level within the case. The most common location of additional cooling fans is the front of the case, opposite the main power supply fan.

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Mounting Hardware - If you are buying a new case, it should come with mounting hardware. This normally comes with the case, not the motherboard. Make sure you have mounting hardware or your system assembly will stall in pretty short order. The exact hardware included varies greatly and depends on what the manufacturer decided to include in the case, but you will generally find some combination of the following (since most cases will use a combination of mounting holes): Plastic Standoffs : Also called "spacers", "sliders", and of course the highly technical "thingamajiggies", these are generally made of white plastic and are used for mounting the motherboard to system cases that have large eyelet holes. They have a collapsible point on one end and a round disk on the other. They were originally created to make motherboard installation "easier" since they do not require screws, but in my opinion they are just a pain to deal with because they make lining up the motherboard more difficult during installation. Metal Standoffs : Again also called "spacers" and a few other names (some of them unprintable), these are 3/16" hexagonal nuts with a threaded screw on the end. They are usually made of brass, sometimes steel, and they are used for mounting to threaded holes in the system case. Screws : These are used to screw the motherboard to the brass standoffs mentioned above. Washers : Generally made of plastic or paper, these go under the screws to keep the screw head away from the circuitry on the top surface of the motherboard. These are now sometimes being omitted in new system cases because they are less necessary now than they once were (since motherboards today now tend to keep the circuitry farther away from the screwholes than they once did). Some cases actually come with their mounting hardware fixed into position and not movable. In theory, a time-saver, but it reduces your flexibility in the event that you want to upgrade down the road.


Faceplates and Expansion Slot Inserts - Plastic faceplates (sometimes called bezels) are provided for drive bays that are not in use. Some cases come with these pre-installed while others do not. Make sure all open bays are covered, to improve air flow and cooling, and keep foreign materials out of the computer. A good case should also come with metal expansion slot inserts to cover the space used by slots that don't have cards in them. Watch out for the newer (usually cheaper) cases that just use stamped metal to cover these slots. Once you punch one out you can't put it back in. If you change a card's slot, you will need the kind of inserts that screw in place to cover the hole you left. Some of the cases with stamped metal slot covers also come with real ones you can screw in place later if you need them, but some don't! Leaving expansion slots open adversely affects air flow in the case, and thus cooling, and also increases the chances of system contamination.

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Styles and Sizes - Cases come in several different major styles. Despite the commonly-used names, such as "tower", "mini-tower", etc., there are no standards for case size or shape, and one company's full tower case can be very different in details from another. Some of the companies that make high-quality high-end cases incorporate very interesting features into their cases that stretch their capabilities for the same amount of space taken. As a result, one company's "mini-tower" may be able to hold more components, and cool them better, than another company's "mid-tower". The two basic styles for the case are the tower case and the desktop case. You've no doubt seen each of these before; the desktop is a rectangular case that is wider than it is tall and normally sits on the desk. The tower is like a desktop flipped on its side and can sit on the floor or on the desk.
Case Location - In deciding what style case you want your system to have, the first consideration should be the case location - where you want to put it. There are generally two choices: on the desk or on the floor. Putting the case on the floor gets it off your desk, saving desk space. However, it also means that the power and reset buttons, the indicator LEDs, and the drives, are less accessible to your fingers, and more accessible to accidents involving your feet. This is in some ways, the biggest drawback of a tower case. In some cases you will need cable extenders for your keyboard, mouse or monitor, depending on your setup. Also, if you are getting a smaller-sized monitor, say 15", you may need something to put under it so that it is at a comfortable reading level (larger monitors usually work better on the desktop directly). Finally, there's the chance the box will get kicked or knocked over, or you'll hit the power switch with your knee at the worst possible moment (Murphy's Law--ignore it at your peril!). A tower case is generally recommended for a floor location, for stability. You can put a desktop on the floor, on its side, as long as you secure it properly (you don't want the box falling over on you!) If you do go with a desktop on the floor, some companies sell (used to sell?) brackets intended to support desktops put on their side; you may still be able to find one of these. A desktop case is of course better for putting the machine on the desk; a tower case can be put on the desktop but it will take twice as much room since the monitor can sit on top of a desktop case but not a tower. One final consideration is the orientation of your drives, in particular CD-ROM drives, DVD drives, and other removable media. Many of these drives will not tolerate being mounted on their side, which means you need to consider how the system will be oriented in advance. Most modern hard drives will operate just fine mounted either vertically or horizontally.

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Full Tower - A full tower case is the largest standard PC case available on the market. These units are big, heavy and at two to three feet in height, are intended to be installed on the floor. Full towers are the most flexible, expandable cases, with many drive bays to support a large number of internal devices. They normally come with larger power supplies to support these devices (but not always) and since they are so roomy, they are the easiest to work in and the best at keeping components from overheating. They are, naturally, also the most expensive type of case.


Mid Tower - A mid tower case is similar to a full tower case, but slightly smaller. This size seems to be all over the place; some companies make mid tower cases that are no larger than others' mini towers. A mid tower is a good compromise for those that want more space than a mini tower offers, without going to the expense of a full tower case.


"Midi" Tower - At first, there were only full-sized tower cases. Then came mini-towers, and mid-towers, which were a compromise between full-sized towers and minis. Then the marketing people got to work--Scott Adams had it right on them--and produced the "midi" case. It's hard to get a consensus on exactly what a "midi" case is. Some contend that the name is based on a cutesy combination of "mid" and "mini", and that the midi tower is therefore one that is smaller than a mid tower but larger than a mini. Others use the term "midi" interchangeably with "mid"; this seems to be more common in Europe than in North America.


Mini Tower - Currently one of the most popular styles for new PCs, and the most popular of the stand-up style case, the mini tower is the case of choice for many homebuilders. It is roughly the same size as a desktop case, but due to its design is generally easier to use and often has more capacity. It also tends to keep components cooler than a desktop case. Since it is not nearly as large as a full tower case, many people put the mini tower on the desktop, next to the monitor. This is particularly preferable if you are using a larger (17" or larger) monitor. Putting an 80 pound 20" monitor on top of your desktop case is not good for your eyes, and not good for the system either!
Desktop - The desktop has been the de facto standard for PC cases since the original IBM PC, XT and AT machines, which came in a desktop case only. Today's desktops are different from those, both in size and construction, but the idea is the same: the box sits on the desk, and the monitor sits on the box. For those who don't want to (or cannot) put the case on the floor, the desktop actually saves space compared to a tower case, since the monitor won't sit on top of a tower case. A desktop doesn't, in general, let components cool as well as a similarly-sized tower, and the monitor sitting upon it can be part of the reason.


Slimline Case - Also sometimes called "low profile" or even "pizza box", this is a smaller version of the desktop case. It was invented in part as a cost-cutting measure but at least as much in order to reduce the amount of space the box takes up on the desk. For many people, this case is aesthetically pleasing because it takes up the least space on the desktop. Unfortunately, it fares rather poorly in every other regard. These cases have little capacity for additional drives, have the poorest expandability, are hard to work in, and have the worst cooling. Despite this, more and more slimline PCs are being sold today. The reason is simple: smaller cases are cheaper to build than bigger ones, and many people underestimate the performance and longevity drawbacks of tiny cases.

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Proprietary Case Designs - In addition to the standard case styles, there are some machines that use totally unique designs. Some Compaq models for example have the monitor and case in one large box. These machines are generally marketed to the home user or first-time buyer as a "home appliance" and the idea is supposed to be that this is "simpler" than having the monitor be in a separate box. The disadvantage of this design, aside from the fact that it is totally proprietary and therefore hard to upgrade, is that if you want to upgrade your PC you lose your monitor, and similarly, if you decide your monitor is too small and want a larger one, you have a problem because your PC is in the same box. I strongly recommend against integrated units because of their inflexibility. For servers and other high-end machines, special cases much larger than standard full towers are available. These large cases often incorporate special features such as locking front covers, slide-out drive bays and wheels (since they are heavy). They can cost as much as an entire regular PC by themselves. In addition to server-oriented full-sized cases, there are special enclosures for such things as RAID (disk) arrays available from major case manufacturers.


Comparison of Case Styles - The key features in comparison of case styles are case size, number of external bays, number of internal bays, cooling efficiency, power supply and cost. The number of bays depends on the case, but it's much more likely to find more bays in a full tower than a mini tower. For external bays, the first number is how many 3.5" bays the case typically will have, and the second is how many 5.25" bays. Internal bays are usually 3.5". Power supply ratings are measured in watts (W). Getting a full tower case is no guarantee of a 300 watt power supply, but it's much more likely to find one in a full tower than a mini tower.

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Form Factors - In addition to coming in different styles, cases also come in different form factors. The form factor describes the general layout of the case, the positioning of the slots in the back of the case, and the way that the case matches to the major components that fit into it. In particular, there are three main components that must be matched in terms of their form factor: the case, the power supply, and the motherboard. Usually, when you buy a system case it comes with a power supply, so matching the case and power supply is not a concern.
The most popular case form factors today are the "Baby AT" and ATX styles, with the smaller microATX/SFX form factor also being used increasingly. These cases are not interchangeable, since they are shaped differently, and require motherboards with a different form factor. If you are building your own system you must ensure your motherboard and case/power supply form factors match. There are some cases that can handle both baby AT and ATX motherboards.
Not all case styles are available in all form factors. Increasingly, as "Baby AT" loses in popularity to ATX and its variants, many newer cases are becoming hard to find in the Baby AT format. The older form factors (PC/XT, and AT) are not used in modern systems at all.


PC/XT Form Factor - The original IBM PC, and its hard-drive-equipped successor, the IBM PC/XT, used the same original PC form factor. These boxes were sold as desktops only, and were constructed of heavy-gauge metal throughout (they don't make 'em like that any more, for the most part). Many older PC buffs remember what these cases were like: a "U-shaped" metal cover was fastened at the back using five screws. To open the PC you had to remove all the screws and slide the cover off the front of the base of the machine, sometimes risking the loss of a floppy drive faceplate in the process. (Maybe it's a good thing that they don't make 'em like that any more!) The power supply was tucked into the right rear of the box, and the system power was controlled by a red toggle switch (that was usually hard to reach).
These original PC boxes were also very large, and their power supplies large as well, for the amount of power produced. (How many folks realize that the original IBM PC's power supply produced only 63.5W? This was doubled to 130W for the XT.) The PC/XT form factor was replaced by the AT form factor when the IBM PC/AT was released in 1984, though IBM PC/XT cases, as well as large numbers of clones, continued to be found on the market for some time.

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AT Form Factor - The "new" IBM PC/AT didn't look all that much different on the outside from the PC/XT units it replaced; but on the inside it was a different story. The AT case was functionally similar the older PC/XT style, but was changed slightly so that it represents a different form factor. The power supply was larger than in the PC/XT and the positioning and size of the motherboard and power supply different. Therefore, the PC/XT and AT formats were not compatible.
The AT form factor was very popular in the late 1980s, and was the basis of many "clone" manufacturers' units (for compatibility with IBM.) The AT system was also the first to formally introduce different desktop and tower configurations. The desktop configuration was very similar to that of the PC/XT, with the familiar red toggle switch in the rear of the machine, on the right-hand side. The tower configuration saw the introduction of the now familiar "remote" power switch, controlled by a button on the front of the case. Due to its convenience, this was very popular and became the standard for most later designs, both tower and desktop.


Baby AT Form Factor - Not long after the introduction of the IBM PC/AT and the AT form factor, a smaller version of the AT form factor was created called the "Baby AT" form factor. Baby AT is similar to AT, except that it is smaller in the width dimension. This means that Baby AT power supplies and motherboards will fit into full-sized AT cases, but not vice-versa.
As AT-style machines took the world by storm, manufacturers quickly developed a preference for the Baby AT form factor over the AT form factor, since it provided the same capabilities at reduced cost. Users also preferred the smaller Baby AT cases to the full-sized AT ones. As a result, Baby AT quickly overtook AT machines in popularity. Until the rise of ATX, Baby AT form factor PCs dominated the industry. Baby AT cases are found in both desktop and tower configurations, like AT, and in a large variety of styles, shapes and sizes. Baby AT cases are used with Baby AT style motherboards, which can be recognized based on their dimensions and placement of components.
In the last couple of years, the ATX form factor has started to rapidly push Baby AT out of the market. Led by Intel, as more and more motherboards are offered only in ATX, the demand for Baby AT cases diminishes. However, due to the large upgrade market, and the enormous installed base of Baby AT machines, you should still be able to find Baby AT cases for some time to come.

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LPX (and Mini LPX) Form Factor
- Without knowing it, retail PC customers have made LPX one of the most popular form factors of the last decade. Most PCs sold in slimline or "low profile" cases in the late 1980s and early 1990s use the LPX form factor, or a variant of it. Originally developed by Western Digital Corporation back when they made motherboards, the goal of the LPX design was simple: to reduce the size and cost of the PC system box. The key design decision in LPX that enables the dramatic reduction of the size of the case is the creation of a riser card that plugs into the motherboard. Expansion cards then plug into the riser card, parallel to the motherboard. By doing this, the case no longer has to be tall enough to accommodate the height of an expansion card. One problem with the LPX form factor is that it is only a "pseudo-standard"; it was never formalized into a hard standard, the way for example ATX and NLX have been. Many companies make systems that use slimline cases and LPX-style motherboards and power supplies, but they often differ slightly in size, shape, or other characteristics. This means you cannot expect to move a power supply from say, a Compaq LPX system into a similar-looking Packard Bell system. LPX systems are essentially proprietary.
There is one innovation of the LPX form factor that has carried forward into the more modern ATX and NLX designs: the use of integrated I/O connectors, and holes provided for them in the system case. The lack of this design in the Baby AT form factor led to increased cost and time of assembly, a problem avoided with the newer form factors.


ATX (and Mini ATX) Form Factor
- The first significant change in case and motherboard design in many years, the ATX form factor was invented by Intel in 1995. It is continuing to gain in popularity and is now surpassing the Baby AT form factor for use by small PC shops and homebuilders (it has been the form factor of choice for brand-name systems for some time.) The ATX design has several significant advantages over the older designs, but the enormous installed base of existing Baby AT cases and motherboards has caused the change to ATX to take many years.
The ATX case is similar to a Baby AT case except that the holes in the back are altered to fit the changed design of the ATX form factor motherboard, in particular the integrated I/O ports. The ATX case also uses a different, ATX power supply. Most ATX cases have more features than AT cases because they are newer, and they are sometimes more expensive for the same reason. The advantages of the ATX design are discussed in the form factor chapter on motherboards.
Millions of older Baby AT systems are in existence, and to make it easier for their owners to upgrade gradually to the ATX form factor, many high-quality ATX cases will also accommodate Baby AT motherboards. (Having a case that can handle either form factor motherboard also makes production easier for the manufacturer.) Sometimes additional hardware or slight modifications are needed to the case to switch from Baby AT to ATX or vice-versa.
Intel has also specified a "Mini ATX" motherboard size, which is slightly smaller than the full-sized ATX specification. These boards use the same ATX form factor power supplies and cases. microATX however is a different form factor entirely.

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NLX Form Factor - NLX is Intel's proposal for the future of mass-marketed, retail PCs, replacing LPX. It is similar in overall design to LPX, with a riser card arrangement and low profile, slimline case. However, it has been updated and modernized to allow support for the latest technologies while keeping costs down.Many slimline systems that were formerly designed to fit the LPX form factor are now moving over to NLX. One extra advantage of NLX over LPX is that it is a true standard, unlike LPX, making interchangeability of components more likely than it was for the older form factor. NLX seems destined to become of the most popular form factors in the PC world, complementing the ATX "family" of form factors.


microATX/SFX/uATX Form Factor - Computer manufacturers continue to want to shrink the size of their systems, and Intel has been happy to oblige, with another variant on the basic ATX theme called "microATX". As the name implies, microATX is not only smaller still than Mini ATX, it is actually smaller than even NLX. microATX is intended for the very low end market that Intel has decided to target with force.
Technically, it could be said that microATX is not even really a case form factor, because microATX is in fact a motherboard standard. However, with the smaller microATX motherboards, manufacturers are creating smaller cases to fit. To complement microATX, Intel created a new, low-output power supply design called SFX. SFX power supplies are designed to work with not only microATX systems, but also NLX and regular ATX systems, and this is why these power supplies were not called microATX. SFX and microATX are sometimes used interchangeably, or together, in describing this general physical format. The term uATX may also be used to refer to microATX form factor.

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Drive Bays - Drive bays are the spaces in the system case where you mount your hard disk, floppy, CD-ROM, tape and other drives. They come in two general types, external and internal, and two sizes: 5.25" and 3.5".
External Drive Bays - This is a bit of a misnomer, since external means "outside" and these drive bays are certainly inside the case. However, they are called "external" because they allow access to the device from the outside. Any drive that uses removable media or has controls that must be operated manually must go in an external drive bay. This includes floppy disk, CD-ROM, DVD, tape and removable-storage drives.
Internal Drive Bays - These bays are entirely within the case and are not accessible from the outside. If a device does not require any access from the outside it is preferable to use an internal bay, and save the case's external bays for drives that need them. In practical terms, this means that internal drive bays are usually used for hard disk drives, which do not require any access by the user. You can of course mount a hard drive into an external drive bay. So in some ways, an internal drive bay is really an "internal only" bay. Some cases in fact do not have any internal drive bays; hard drives are mounted into external drive bays and solid faceplates left to hide the drive from the outside.
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Drive Rails and Brackets - Up until a few years ago, most PC cases used drive rails--thin strips of metal that interfaced drives to the case. Each drive that you wanted to mount into the case was actually screwed into a set of two rails. The drive-on-rails then slid into rail slots in the drive bays of the case, and the end of the rail either snapped into place or was screwed into the front of the case frame.
Newer cases, introduced over the last few years, have mostly done away with drive rails. Instead of mounting the drive on rails, the drive is mounted directly into the drive bays. This simpler design became more popular than designs using drive rails for many years. Some companies have now brought back drive rails with some of their models.
Some folks do like drive rails though. The only advantage to them that I can think of, is of relevance only if you are someone who tinkers with your PC a lot, swapping drives in and out or moving them between cases. The rails make it much faster to move drives from machine to machine. In mass-manufacturing environments, the design can also save time due to division of labor, since one person can exclusively mount drives into rails or brackets, which can be integrated into the main case frame quickly. This of course doesn't apply to an individual PC builder.
Some cases use a snap-in bracket for internal hard disks. These are a nice feature, because they let you remove the bracket, mount the hard disk, and then remount the bracket with the hard disk. Alignment is slightly more difficult than direct-mounting of the hard drive, but since exact alignment is not critical for an internal drive, this is not a major concern, and the bracket is easier to work with than bending your arms to access the inner recesses of smaller cases. The only difficulty with this design is that once the entire system is installed, cables or other devices may make it difficult to remove the bracket for servicing.
Drive Bay Sizes: 3.5" and 5.25" - Not surprisingly, these sizes correspond to the two common sizes of floppy disks in use today. (Actually, the 5.25" disks aren't really that common any more, but they're still out there.) The bays are not actually this size, because this is the size of the media and the drive obviously must be larger, but they are almost always referred to by those names. A 3.5" bay is actually about 4 inches wide, and a 5.25" bay about 6 inches. While these bays were designed to fit the two sizes of floppy drive mentioned, all modern drives have all been designed to fit into the same dimensions.
Some devices require the larger size bay. Obviously, a 5.25" floppy drive needs one, and so do all CD-ROM and DVD drives (CDs and DVDs are 4.75 inches wide). Many tape drives do as well, as do many removable storage drives. A 3.5" floppy drive will of course fit in a 3.5" bay. So will most modern hard disk drives (in fact, most internal drive bays are 3.5" for that reason). The height of the different bay types is pretty much standard; a 3.5" bay is about 1" in height, and a 5.25" bay is about 1.75" in height. It is possible to buy adapters that will make a 3.5" device fit into a 5.25" bay, if you need to do this. It is easier to do with a hard disk than a floppy disk or other device with a faceplate, because in addition to the mounting rails for the side of the drive, you need an adapter for the faceplate as well in the latter case.

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