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The different interfaces and cable specifications are covered in the sections on drive interfaces later in this chapter.

The power connector usually is the same type that is used in floppy drives, and the same power-supply connector plugs into it. Most hard disk drives use both 5- and 12-volt power, although some of the smaller drives designed for portable applications use only 5-volt power. In most cases, the 12-volt power runs the spindle motor and head actuator, and the 5-volt power runs the circuitry. Make sure that your power supply can supply adequate power for the hard disk drives installed in your system.

The 12-volt power consumption of a drive usually varies with the physical size of the unit. The larger the drive is and the more platters there are to spin, the more power is required. Also, the faster the drive spins, the more power will be required. For example, most of the 3 1/2-inch drives on the market today use roughly one-half to one-fourth the power (in watts) of the full-size 5 1/4-inch drives. Some of the very small (2 1/2- or 1.8-inch) hard disks barely sip electrical power and actually use 1 watt or less!

Ensuring an adequate power supply is particularly important with some systems, such as the original IBM AT. These systems have a power supply with three disk drive power connectors, labeled P10, P11, and P12. The three power connectors may seem to be equal, but the technical-reference manual for these systems indicates that 2.8 amps of 12-volt current is available on P10 and P11, and that only 1 amp of 12-volt current is available on P12. Because most full-height hard drives draw much more power than 1 amp, especially at startup, the P12 connector can be used only by floppy drives or half-height hard drives. Some 5 1/2-inch drives draw as much as 4 amps of current during the first few seconds of startup. These drives also can draw as much as 2.5 amps during normal operation. Most PC-compatible systems have a power supply with four or more disk drive power connectors that provide equal power.

A grounding tab provides a positive ground connection between the drive and the system's chassis. In most systems, the hard disk drive is mounted directly to the chassis using screws so the ground wire is unnecessary. On some systems, the drives are installed on plastic or fiberglass rails, which do not provide proper grounding. These systems must provide a grounding wire plugged into the drive at this grounding tab. Failure to ground the drive may result in improper operation, intermittent failure, or general read and write errors.

Configuration Items

To configure a hard disk drive for installation in a system, several jumpers (and, possibly, terminating resistors) usually must be set or configured properly. These items will vary from interface to interface and often from drive to drive as well.

The Faceplate or Bezel

Many hard disk drives offer as an option a front faceplate, or bezel (see Figure 1-8). A bezel usually is supplied as an option for the drive rather than as a standard item. In most cases today, the bezel is a part of the case and not the drive itself.


FIG. 1-8  A hard drive faceplate (bezel).

Older systems had the drive installed so that it was visible outside the system case. Drives could have an optional bezel or faceplate to cover the holes. Bezels often come in several sizes and colors to match various PC systems. Many faceplate configurations for 3 1/2-inch drives are available, including bezels that fit 3 1/2-inch drive bays as well as 5 1/4-inch drive bays. You even have a choice of colors (usually, black, cream, or white).

Some bezels feature a light-emitting diode (LED) that flickers when your hard disk is in use. The LED is mounted in the bezel; the wire hanging off the back of the LED plugs into the drive or perhaps the controller. In some drives, the LED is permanently mounted on the drive, and the bezel has a clear or colored window so that you can see the LED flicker while the drive is accessed.

One type of LED problem occurs with some older hard disk installations: If the drive has an LED, the LED may remain on continuously, as though it were a "power-on" light rather than an access light.

This problem happens because the controller in those systems have a direct connection for the LED, thus altering the drive LED function. Some controllers have a jumper that enables the controller to run the drive in what is called latched or unlatched mode. Latched mode means that the drive is selected continuously and that the drive LED remains lighted; in unlatched mode (to which we are more accustomed), the LED lights only when the drive is accessed. Check to see whether your controller has a jumper for changing this function; if so, you may be able to control the way that the LED operates.

In systems in which the hard disk is hidden by the unit's cover, a bezel is not needed. In fact, using a bezel may prevent the cover from resting on the chassis properly, in which case the bezel will have to be removed.

Hard Disk Features

To make the best decision in purchasing a hard disk for your system, or to understand what differentiates one brand of hard disk from another, you must consider many features. This section examines the issues that you should consider when you evaluate drives:

  Reliability
  Shock mounting
  Speed
  Cost
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