CD-ROM Drive Construction

CD drives come in three forms here. Some drives use caddies to hold the disks. You put the CD in a caddie then stick the caddie into the drive itself. The caddie helps to stabilize the disk, as well as protect the disk. Another kind of drive is one with the slide out drawer. When you press eject, the drawer pops open and you lay the disk in there. This is much like an audio CD player. This is the most common type of drive. It is much easier to use, but the mechanism that opens the drive is a point of possible failure later on. The last kind is the CD-changer. These kind are just like the caddie system, but you can put three or more disks in one caddie and switch between them without ejecting. These drives are nice, but more expensive.

The innards of a CD-ROM drive are mounted inside of a solid frame. The frame is designed typically from aluminum or some type of stainless steal. The front bezel, along with its tray opening latch, volume control, eject button, etc., are mounted onto the front of the frame. The rest of the drive lies behind the bezel. It is divided into the electronic package and the drive engine. The electronics constists of the logic board as well as the PCB that handles audio amplification for the headphones jack on the front of the drive. The drive engine is the component that handles the actual CD while it is inserted. The drive engine is mounted onto a BC-7C sub-frame which is equipped with four rubber shock absorbers to stabilize it within the overall drive frame. A motor/gear assembly manages the loading/unloading of the disc mechanism. When the CD is inserted, the tray closes on a slider assembly, and the cover shield (visible from the outside) closes. The CD is clamped so that it is palced securely over the drive spindle. A spindle motor mounted to the sub-frame spins the CD, using a thrust retainer to ensure smooth operating.

The optical device is the most important component, and is responsible for reading the CD. The optical hardware slides along two guide rails, and is mounted on a device called a “sled” that moves along those rails. The sled is moved around using linear motors which operate much the same way as a voice coil. This assembly is what is responsible for moving the optical device across the radius of the CD, thereby allowing the data to be read anywhere on the CD.

One last thing to consider is whether you want an internal drive or an external drive. An internal drive fits into one of the case’s 5.25″ drive bays and gets its power through the computer’s power supply. They are also cheaper because they have the IDE interface instead of the more expensive SCSI interface. The main drawback is that it cannot be moved from one computer to another, but if you have no need for this, I would definitely buy an internal drive. An external drive, on the other hand, sits on the desk next to the computer. They can be easily moved to another computer. They also use the SCSI interface, allowing the drive to work quickly and be placed up to 3 meters from the PC. The main disadvantage here is that you have another part laying on your desk. They are also more expensive, and more rare. Some external drives now operate off of USB or Firewire, as well.

CD-ROM Software

Even though internal CD-ROM drives operate on the IDE interface, they do not share the native support of hard drives. Due to some of the differences between manufacturers, native BIOS support for CD-ROM drives is difficult. So, manufacturers provide device drivers for their drives that allow their drives to communicate properly with the PC. These drivrs usually come with a .SYS extention and are enabled by placing its command line to the CONFIG.SYS file. An MS-DOS extention called MSCDEX provides file handling as well as driver lettering support. Under Windows operating systems, MSCDEX is not used as those OS’s provide their own protective-mode drivers for CD drives. MSCDEX is enabled using the AUTOEXEC.BAT file. Most CD-ROM install routines for DOS will automate the setup of these things. Here is a rundown, though, of the command-line switches for MSCDEX:

Conclusion

CD-ROMs are becoming more and more advanced as time goes on. They are getting faster and cheaper. Also, recordable CD-ROM drives have come out and are going down in price. These drives use a high intensity laser beam to burn data into the disk. This CD is a special blank version. This drive is sometimes called a WORM drive, short for Write Once, Read Many. Such drives are good for backup, but the disks are not erasable once you have used them.