Notebook PCs are very similar to desktop PCs in terms of overall architecture, but very
different in implementation. Hard disks and hard disk interfaces are certainly no
exception to this general rule of thumb. Virtually all notebooks have built-in hard disks
using the IDE/ATA interface, but the way they are actually connected the machine is not
the same as for desktop PCs.
Hard disks are installed in notebooks in two very different ways--which method is used
on a particular model depends on the decisions made by the engineers that created it. The
first is a proprietary installation, where the hard disk is just rammed "in
there somewhere"; the second is an open or removable installation.
The differences between the two are pretty obvious. If a notebook has a proprietary hard
disk installation, it probably still uses the regular IDE/ATA interface, but the drive is
attached using special cables and connectors, and is not intended to be touched by the
user. Needless to say, this is a very inflexible arrangement, because if you ever need or
want to replace or upgrade the drive, you have to refer to qualified service personnel for
service. This was the most common way of putting hard disks into notebooks during their
early years.
Most modern hard disks today use the open arrangement; unlike the proprietary
installations these are probably easier to install and remove than the hard disks
in a regular PC! A typical hard disk will use a special hard disk bay that has room for a
standard, 2.5" form factor hard disk. Bays differ between models, especially in terms
of the height of the drive, but the form factor itself is pretty much standardized at this
point; you can read more about it here. The
advantages of a standard interface are obvious: interchangeability and competition
(allowing you choices between drive makers).
The usual method of attachment of notebook hard disks is through a special 44-pin
connector that includes all of the signals needed by the drive, instead of the usual 40-pin data connector and 4-pin power connector. This provides the
basis for allowing the drives to be quickly and easily removed and replaced. The drive
itself uses a regular set of straight pin connectors, just like a desktop drive. Since
these pin connectors are not well-suited for easy insertion and removal from a notebook,
the drive is mounted into a special carrier or "caddy". This device includes as
part of its hardware an adapter which converts the regular pins into a single connector
designed for drive swapping. The technique used for this attachment is very similar in
concept to the SCA connectors used for some types of
SCSI drives (though the two are obviously very different in all but this concept.)
The first 40 signals on a notebook's connector are the same as those of the regular
40-pin connector; the additional four signals are defined as follows:
Pin # |
Signal |
Pin # |
Signal |
41 |
+5 V (logic) |
42 |
+5 V (motor) |
43 |
GROUND |
44 |
(reserved) |
You may immediately notice that there is no +12 V connection as exists for regular drives, because
2.5" form factor drives have 5-volt motors. Two separate +5 lines are provided; one
for the motor and the other for the hard disk's circuit board.
|
Underside of a 2.5" form factor notebook hard
drive.
You can see the main connector, with its two rows of
22 pins (the second row is hard to see). On the right
are two more pins which are used for jumpers.
To see what this drive looks like in a caddy, complete
with its single attachment connector, see this page |
Notebooks are of course very limited in space, so "expansion" is usually not
an option, at least not using the built-in IDE/ATA interface. The hard disk usually is
assigned as the single device on the primary IDE/ATA channel, and the drive's optical
drive (if any) assigned to the secondary. There's no way to decide to add a second device
to these channels on a typical notebook. Expanding a notebook to add a second hard disk is
usually done using one of the specialty interfaces such as USB or PCMCIA.
Next: Independent Master/Slave Device Timing
