Hardware Reference
In-Depth Information
Compaq 386 systems in 1986. I remember seeing this drive for the first time in 1986 at the fall
COMDEX show. Besides the (at the time) unique 40-pin ribbon cable, I remember being surprised by
the green activity LED on the front bezel. (Most drives up until then used red LEDs.)
Compaq was the first to incorporate a special bus adapter in its system to adapt the 98-pin AT-bus
(also known as ISA) edge connector on the motherboard to a smaller 40-pin, header-style connector
into which the drive would plug. The 40-pin connector was all that was necessary because it was
known that a disk controller never would need more than 40 of the ISA bus lines. Smaller 2 1/2-inch
ATA drives found in laptop computers use a superset 44-pin or 50-pin connection, which includes
additional pins for power and configuration. The pins from the original ISA bus used in ATA are the
only signal pins required by a standard-type AT hard disk controller. For example, because a primary
AT-style disk controller uses only interrupt request (IRQ) line 14, the primary motherboard ATA
connector supplies only that IRQ line; no other IRQ lines are necessary. Even if your ATA interface
is integrated within the motherboard chipset South Bridge or I/O Controller Hub chip (as it would be
in newer systems) and runs at higher bus speeds, the pinout and functions of the pins are still the same
as the original design taken right off the ISA bus.
See the
Chapter 4
section, “
Motherboard Connectors
,
”
p.
228
.
See the
Chapter 4
section, “
The ISA Bus
,
”
p.
245
.
Note
Many people who use systems with ATA connectors on the motherboard believe that a hard
disk controller is built into their motherboards, but in a technical sense the controller is
actually in the drive. Although the integrated ATA ports on a motherboard often are referred to
as controllers, they are more accurately called
host adapters
(although you'll rarely hear this
term). You can think of a host adapter as a device that connects a controller to a bus.
Eventually, the 40-pin ATA connector and drive interface design was placed before one of the ANSI
standards committees that, in conjunction with drive manufacturers, ironed out some deficiencies, tied
up some loose ends, and then published what was known as the CAM ATA (Common Access Method
AT Attachment) interface. The CAM ATA Committee was formed in October 1988, and the first
working document of the ATA interface was introduced in March 1989. Before the CAM ATA
standard, many companies, such as Conner Peripherals (which later merged with Seagate
Technology), made proprietary changes to the original interface as designed by CDC. As a result,
many older ATA drives from the late 1980s are difficult to integrate into a dual-drive setup because
minor differences in the interfaces can cause compatibility problems among the drives. By the early
1990s, most drive manufacturers brought their drives into full compliance with the official standard,
which eliminated many of these compatibility problems.
Some areas of the ATA standard have been left open for vendor-specific commands and functions.
These vendor-specific commands and functions are the reason it is important to use the OEM-specific
programs for testing ATA drives. To work to full capability, the diagnostic program you are using
typically must know the specific vendor-unique commands for remapping defects. Unfortunately,
these and other specific drive commands differ from OEM to OEM, thus clouding the standard
somewhat. Most ATA drive manufacturers publish their drive-formatting/initialization software on
their websites.
