CDs are 120mm in diameter, 1.2mm thick, and can store up to 800 MB of information. They are comprised of a layer of polycarbonate plastic, a layer of reflective metal, and a coat of lacquer. CDs are a type of optical media as opposed to magnetic media such as floppy disks, hard disks, and Zip disks.

A CD-ROM drive is comprised of the drive motor, laser assembly, tracking mechanism, and the communication circuits. The drive motor spins the CD-ROM up to the proper speed so the laser assembly can read the information. The laser assembly consists of a laser and a lens. This assembly reads the CD-ROM as it spins. The tracking mechanism is a motor and drive system that moves the lens into the correct position to access a specific area of the CD-ROM. Finally, the communication circuits are used to send the information read from the CD-ROM to the computer using the configured bus.

CD-ROM drives are categorized by their case type, bus type, and read speed.

• CD-ROM drives can be mounted internally in the computer or as an external drive. Internally mounted drives are now common with computers. These drives draw their power from the power supply and are usually connected to the computer bus through a ribbon cable. External drives are enclosed in a case and have their own power supplies. They connect either directly to an external port on the computer (such as USB, FireWire, or parallel) or to a controller installed in one of the computer's expansion slots (usually SCSI).
• A bus is the communications pipeline between a computer and the peripherals that are installed. Common internal connections include IDE and SCSI. Most motherboards have built-in IDE controllers and will automatically recognize that a CD-ROM drive has been installed. IDE communication cables are 40-pin ribbon cables that connect to the drive and the motherboard. While some motherboards offer built-in SCSI solutions, most applications require that a SCSI adapter be purchased and installed separately. There are many different varieties of SCSI and assorted cabling types.

Speed Ratings
A CD-ROM drive read speed determines the rate at which information can be pulled from the CD-ROM and sent to the communications bus. In general, the higher the read speed rating on a CD-ROM drive, the faster the drive. The read speed of a CD-ROM drive is measured in multiples of 150kb and denoted by a numeral followed by an "x". A drive rated at 1x has a read speed of 150kb per second (or 1 times 150kb). A drive rated at 10x has a read speed of 1500kb per second. Data that streams from a CD (such as full motion video) can require read speeds of 12x or more. Also, a CD-ROM drive read speed is not always uniform. A drive rated at 100x may only be able to achieve that rate near the center of the CD-ROM while only achieving a 40x rate at the outer tracks.

Installing and Configuring an IDE CD-ROM Drive
Installing and configuring an internal IDE CD-ROM drive is similar to the installation of an internal IDE hard drive. The CD-ROM drive needs to be connected to a 40-pin IDE cable that is then connected to the IDE controller. The drive is usually keyed so that the cable will only connect one way but it is important to ensure that the 1 pin on the cable matches with the 1 pin on the drive. An internal CD-ROM drive uses the standard Molex power connector that supplies 12 volts to other devices inside a PC. Next, the CD-ROM drive must be set to its proper operating status: master, slave, and single drive. A CD-ROM drive should be set as a master drive only if it is the fastest drive on that particular IDE cable. Most CD-ROM drives have a digital-audio cable that connects directly to the sound card. A CD-ROM containing a Digital-to-Analog Converter (DAC) will allow music to be played through the computer's speakers without requiring any overhead processing. Finally, certain CD-ROM drives require the installation of a software driver before they will work properly.

For further information on the step-by-step installation of IDE drives, review Chapter 3.