Hardware Reference
In-Depth Information
Logic Boards
All HDDs have one or more logic boards mounted on them. The logic boards contain the electronics
that control the drive's spindle and head actuator systems and present data to the controller in some
agreed-upon form. On ATA drives, the boards include the controller itself, whereas SCSI drives
include the controller and the SCSI bus adapter circuit.
Many disk drive failures occur in the logic board, not in the mechanical assembly. (This statement
does not seem logical, but it is true.) Therefore, you sometimes can repair a failed drive by replacing
the logic board rather than the entire drive. Replacing the logic board, moreover, enables you to
regain access to the data on the drive—something that replacing the entire drive does not provide.
Unfortunately, none of the drive manufacturers sell logic boards separately. The only way to obtain a
replacement logic board for a given drive is to purchase a functioning identical drive and then
cannibalize it for parts. Of course, it doesn't make sense to purchase an entire new drive just to repair
an existing one except in cases in which data recovery from the old drive is necessary. Cannibalizing
new drives to repair old drives is a common practice among companies that offer data recovery
services. They stock a large number of popular drives they can use for parts to allow data recovery
from defective customer drives they receive.
Most of the time, the boards are fairly easy to change with nothing more than a screwdriver. Merely
removing and reinstalling a few screws as well as unplugging and reconnecting a cable or two are all
that is required to remove and replace a typical logic board.
Cables and Connectors
HDDs typically have several connectors for interfacing to the computer, receiving power, and
sometimes grounding to the system chassis. Most drives have at least these three types of connectors:
• Interface connector(s)
• Power connector
• Optional ground connector (tab)
Of these, the interface connectors are the most important because they carry the data and command
signals between the system and the drive. In most cases, the drive interface cables can be connected
in a daisy-chain or bus-type configuration. Most interfaces support at least two devices. Older
interfaces, such as ST-506/412 or ESDI (Enhanced Small Device Interface), used separate cables for
data and control signals, but both Parallel and Serial ATA (AT Attachment) drives have a single data
connector on each drive.
See the
Chapter 7
section, “
PATA I/O Connector
,
”
p.
387
.
The power is supplied via either a 15-pin SATA power connector or a 4-pin peripheral power
connector, both of which can be found on most PC power supplies. Most 3.5-inch and larger HDDs
use both 5- and 12-volt power, while most 2.5-inch and smaller drives use only 5-volt power. In
drives that use both 5V and 12V power, the 12-volt power runs the spindle motor and head actuator,
while the 5-volt power runs the circuitry. Make sure your power supply can supply adequate power
for the HDDs installed in your system.
The power consumption of a drive usually varies with the physical size and rotational speed of the
unit. The larger and faster the drive is, the more power it requires. More platters add rotational mass
that takes more power as well. 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 older 5 1/4-inch drives. Some of the small
