<s>
Hard	B-Device
disk	I-Device
drives	I-Device
are	O
accessed	O
over	O
one	O
of	O
a	O
number	O
of	O
bus	B-General_Concept
types	O
,	O
including	O
parallel	B-Protocol
ATA	I-Protocol
(	O
PATA	B-Protocol
,	O
also	O
called	O
IDE	O
or	O
EIDE	O
;	O
described	O
before	O
the	O
introduction	O
of	O
SATA	O
as	O
ATA	O
)	O
,	O
Serial	O
ATA	O
(	O
SATA	O
)	O
,	O
SCSI	B-Architecture
,	O
Serial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
,	O
and	O
Fibre	B-Architecture
Channel	I-Architecture
.	O
</s>
<s>
Bridge	O
circuitry	O
is	O
sometimes	O
used	O
to	O
connect	O
hard	B-Device
disk	I-Device
drives	I-Device
to	O
buses	O
with	O
which	O
they	O
cannot	O
communicate	O
natively	O
,	O
such	O
as	O
IEEE	O
1394	O
,	O
USB	B-Protocol
,	O
SCSI	B-Architecture
,	O
NVMe	O
and	O
Thunderbolt	B-Protocol
.	O
</s>
<s>
Disk	O
drive	O
interfaces	O
have	O
evolved	O
from	O
simple	O
interfaces	O
requiring	O
complex	O
controllers	O
to	O
attach	O
to	O
a	O
computer	O
into	O
high	O
level	O
interfaces	O
that	O
present	O
a	O
consistent	O
interface	O
to	O
a	O
computer	O
system	O
regardless	O
of	O
the	O
internal	O
technology	O
of	O
the	O
hard	B-Device
disk	I-Device
drive	I-Device
.	O
</s>
<s>
Acronym	O
or	O
abbreviation	O
Meaning	O
Description	O
SMD	B-Architecture
Storage	B-Architecture
Module	I-Architecture
Device	I-Architecture
Bit	O
serial	O
data	O
interface	O
introduced	O
by	O
CDCStandard	O
interface	O
for	O
many	O
mini-computers	O
in	O
the	O
1970s	O
and	O
1980s	O
.	O
</s>
<s>
SASI	O
Shugart	O
Associates	O
System	O
Interface	O
Word	O
serial	O
interface	O
introduced	O
by	O
Shugart	O
Associates	O
circa	O
1978	O
;	O
Evolved	O
by	O
ANSI	O
into	O
SCSI	B-Architecture
(	O
SASI	O
is	O
a	O
compatible	O
subset	O
of	O
the	O
first	O
version	O
of	O
SCSI	B-Architecture
)	O
.	O
</s>
<s>
SCSI	B-Architecture
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
Word	O
serial	O
interface	O
sponsored	O
by	O
ANSI	O
and	O
introduced	O
in	O
mid	O
1980s	O
;	O
Standard	O
interfaces	O
for	O
most	O
enterprise	O
HDDs	O
in	O
this	O
century	O
;	O
superseded	O
by	O
SAS	O
ESDI	B-Architecture
Enhanced	B-Architecture
Small	I-Architecture
Disk	I-Architecture
Interface	I-Architecture
Bit	O
serial	O
data	O
interface	O
sponsored	O
by	O
ANSI	O
and	O
first	O
introduced	O
by	O
Maxtor	O
in	O
late	O
1980s.A	O
higher	O
data	O
rate	O
follow	O
on	O
to	O
the	O
ST-506	B-Device
family	O
into	O
the	O
mid-1990s	O
,	O
superseded	O
by	O
SCSI	B-Architecture
(	O
P	O
)	O
ATAIDE	O
(	O
Parallel	O
)	O
AT	O
AttachmentIntegrated	O
Drive	O
Electronics	O
Word	O
serial	O
interface	O
introduced	O
in	O
late	O
1980s	O
by	O
Conner	O
Peripherals	O
,	O
later	O
sponsored	O
by	O
ANSI	O
;	O
successor	O
to	O
ST-412/506/ESDI	O
.	O
</s>
<s>
Standard	O
HDD	O
interface	O
on	O
all	O
but	O
enterprise	O
HDDs	O
until	O
superseded	O
by	O
SATA	O
SATA	O
Serial	O
ATA	O
Bit	O
serial	O
interface	O
successor	O
to	O
PATA	B-Protocol
sponsored	O
by	O
ANSI	O
and	O
introduced	O
in	O
2003.Most	O
common	O
interface	O
for	O
all	O
but	O
enterprise	O
HDDs	O
.	O
</s>
<s>
SAS	O
Serial	O
Attached	O
SCSI	B-Architecture
Bit	O
serial	O
interface	O
successor	O
to	O
SCSI	B-Architecture
sponsored	O
by	O
ANSI	O
and	O
introduced	O
in	O
2004.Most	O
common	O
interface	O
for	O
enterprise	O
HDDs	O
.	O
</s>
<s>
The	O
earliest	O
hard	B-Device
disk	I-Device
drive	I-Device
(	O
HDD	O
)	O
interfaces	O
were	O
bit	O
serial	O
data	O
interfaces	O
that	O
connected	O
an	O
HDD	O
to	O
a	O
controller	O
with	O
two	O
cables	O
,	O
one	O
for	O
control	O
and	O
one	O
for	O
data	O
.	O
</s>
<s>
The	O
SMD	B-Architecture
interface	O
was	O
popular	O
on	O
minicomputers	O
in	O
the	O
1970s	O
.	O
</s>
<s>
ST-506	B-Device
used	O
MFM	B-Protocol
(	O
Modified	B-Protocol
Frequency	I-Protocol
Modulation	I-Protocol
)	O
for	O
the	O
data	O
encoding	O
method	O
.	O
</s>
<s>
ST412	B-Device
,	O
an	O
ST-506	B-Device
variant	O
was	O
available	O
in	O
either	O
MFM	B-Protocol
or	O
RLL	B-Protocol
(	O
Run	B-Protocol
Length	I-Protocol
Limited	I-Protocol
)	O
encoding	O
variants	O
.	O
</s>
<s>
Enhanced	B-Architecture
Small	I-Architecture
Disk	I-Architecture
Interface	I-Architecture
(	O
ESDI	B-Architecture
)	O
was	O
an	O
industry	O
standard	O
interface	O
similar	O
to	O
ST412	B-Device
supporting	O
higher	O
data	O
rates	O
between	O
the	O
processor	O
and	O
the	O
disk	O
drive	O
.	O
</s>
<s>
Encoding	O
schemes	O
used	O
included	O
Frequency	B-Application
modulation	I-Application
(	O
FM	B-Protocol
)	O
,	O
Modified	B-Protocol
Frequency	I-Protocol
Modulation	I-Protocol
(	O
MFM	B-Protocol
)	O
and	O
RLL	B-Protocol
encoding	I-Protocol
at	O
frequencies	O
for	O
example	O
ranging	O
from	O
0.156MHz	O
(	O
FM	B-Protocol
on	O
2311	O
)	O
to	O
7.5MHz	O
(	O
RLL	B-Protocol
on	O
ST412	B-Device
)	O
MHz	O
.	O
</s>
<s>
Enhanced	B-Architecture
Small	I-Architecture
Disk	I-Architecture
Interface	I-Architecture
(	O
ESDI	B-Architecture
)	O
was	O
an	O
attempt	O
to	O
minimize	O
controller	O
design	O
time	O
by	O
supporting	O
multiple	O
data	O
rates	O
with	O
a	O
standard	O
data	O
encoding	O
scheme	O
;	O
this	O
was	O
usually	O
negotiated	O
automatically	O
by	O
the	O
disk	O
drive	O
and	O
controller	O
;	O
most	O
of	O
the	O
time	O
,	O
however	O
,	O
15	O
or	O
20megabit	O
ESDI	B-Architecture
disk	O
drives	O
were	O
not	O
downward	O
compatible	O
(	O
i.e.	O
</s>
<s>
ESDI	B-Architecture
disk	O
drives	O
typically	O
also	O
had	O
jumpers	O
to	O
set	O
the	O
number	O
of	O
sectors	O
per	O
track	O
and	O
(	O
in	O
some	O
cases	O
)	O
sector	O
size	O
.	O
</s>
<s>
Historical	O
Word	O
serial	O
interfaces	O
connect	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
to	O
a	O
bus	B-General_Concept
adapter	O
with	O
one	O
cable	O
for	O
combined	O
data/control	O
.	O
</s>
<s>
The	O
word	O
nature	O
of	O
data	O
transfer	O
makes	O
the	O
design	O
of	O
a	O
host	O
bus	B-General_Concept
adapter	O
significantly	O
simpler	O
than	O
that	O
of	O
the	O
precursor	O
HDD	O
controller	O
.	O
</s>
<s>
CTL-I	O
(	O
Controller	O
Interface	O
)	O
was	O
an	O
8-bit	O
word	O
serial	O
interface	O
introduced	O
by	O
IBM	O
for	O
its	O
mainframe	O
hard	B-Device
disk	I-Device
drives	I-Device
beginning	O
with	O
the	O
3333	O
in	O
1972	O
.	O
</s>
<s>
The	O
3333	O
was	O
the	O
first	O
unit	O
in	O
a	O
string	O
of	O
up	O
to	O
eight	O
3330	O
type	O
hard	B-Device
disk	I-Device
drives	I-Device
;	O
it	O
contained	O
a	O
CTL-I	O
controller	O
and	O
two	O
3330	O
type	O
disk	O
drives	O
.	O
</s>
<s>
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
(	O
SCSI	B-Architecture
)	O
,	O
originally	O
named	O
SASI	O
for	O
Shugart	O
Associates	O
System	O
Interface	O
,	O
is	O
an	O
early	O
(	O
circa	O
1978	O
)	O
industry	O
standard	O
interface	O
explicitly	O
deployed	O
to	O
minimize	O
system	O
integration	O
efforts	O
.	O
</s>
<s>
SCSI	B-Architecture
disks	O
became	O
standard	O
on	O
servers	O
and	O
workstations	O
.	O
</s>
<s>
Commodore	O
Amiga	O
,	O
and	O
Apple	B-Device
Macintosh	I-Device
deployed	O
SCSI	B-Architecture
drive	O
through	O
the	O
mid-1990s	O
,	O
by	O
which	O
time	O
most	O
models	O
had	O
been	O
transitioned	O
to	O
ATA	O
(	O
and	O
later	O
,	O
SATA	O
)	O
family	O
disks	O
.	O
</s>
<s>
Only	O
in	O
2005	O
did	O
the	O
capacity	O
of	O
SCSI	B-Architecture
disks	O
fall	O
behind	O
ATA	O
disk	O
technology	O
,	O
though	O
the	O
highest-performance	O
disks	O
are	O
still	O
available	O
in	O
SCSI	B-Architecture
,	O
SAS	O
and	O
Fibre	B-Architecture
Channel	I-Architecture
only	O
.	O
</s>
<s>
The	O
range	O
limitations	O
of	O
the	O
data	O
cable	O
allows	O
for	O
external	O
SCSI	B-Architecture
devices	O
.	O
</s>
<s>
Originally	O
SCSI	B-Architecture
data	O
cables	O
used	O
single	O
ended	O
(	O
common	O
mode	O
)	O
data	O
transmission	O
,	O
but	O
server	O
class	O
SCSI	B-Architecture
could	O
use	O
differential	O
transmission	O
,	O
either	O
low	B-Architecture
voltage	I-Architecture
differential	I-Architecture
(	O
LVD	O
)	O
or	O
high	B-General_Concept
voltage	I-General_Concept
differential	I-General_Concept
(	O
HVD	O
)	O
.	O
</s>
<s>
(	O
"	O
Low	O
"	O
and	O
"	O
High	O
"	O
voltages	O
for	O
differential	O
SCSI	B-Architecture
are	O
relative	O
to	O
SCSI	B-Architecture
standards	O
and	O
do	O
not	O
meet	O
the	O
meaning	O
of	O
low	O
voltage	O
and	O
high	O
voltage	O
as	O
used	O
in	O
general	O
electrical	O
engineering	O
contexts	O
,	O
as	O
apply	O
e.g.	O
</s>
<s>
Parallel	B-Protocol
ATA	I-Protocol
,	O
originally	O
IDE	O
and	O
then	O
standardized	O
under	O
the	O
name	O
AT	B-Protocol
Attachment	I-Protocol
(	O
ATA	O
)	O
,	O
with	O
the	O
alias	O
P-ATA	B-Protocol
or	O
PATA	B-Protocol
retroactively	O
added	O
upon	O
introduction	O
of	O
the	O
new	O
variant	O
Serial	O
ATA	O
.	O
</s>
<s>
The	O
original	O
name	O
(	O
circa	O
1986	O
)	O
reflected	O
the	O
integration	O
of	O
the	O
controller	O
with	O
the	O
hard	B-Device
drive	I-Device
itself	O
.	O
</s>
<s>
(	O
That	O
integration	O
was	O
not	O
new	O
with	O
IDE	O
,	O
having	O
been	O
done	O
a	O
few	O
years	O
earlier	O
with	O
SCSI	B-Architecture
drives	O
.	O
)	O
</s>
<s>
The	O
data	O
cable	O
was	O
originally	O
40-conductor	O
,	O
but	O
later	O
higher	O
speed	O
requirements	O
for	O
data	O
transfer	O
to	O
and	O
from	O
the	O
hard	B-Device
drive	I-Device
led	O
to	O
an	O
"	O
ultra	O
DMA	O
"	O
mode	O
,	O
known	O
as	O
UDMA	O
.	O
</s>
<s>
Modern	O
bit	O
serial	O
interfaces	O
connect	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
to	O
a	O
host	O
bus	B-General_Concept
interface	O
adapter	O
(	O
today	O
in	O
a	O
PC	O
typically	O
integrated	O
into	O
the	O
"	O
south	B-Device
bridge	I-Device
"	O
)	O
with	O
one	O
data/control	O
cable	O
.	O
</s>
<s>
DECs	O
Standard	B-Device
Disk	I-Device
Interconnect	I-Device
(	O
SDI	O
)	O
was	O
an	O
early	O
example	O
of	O
a	O
modern	O
bit	O
serial	O
interface	O
.	O
</s>
<s>
Fibre	B-Architecture
Channel	I-Architecture
(	O
FC	O
)	O
is	O
a	O
successor	O
to	O
parallel	O
SCSI	B-Architecture
interface	O
on	O
enterprise	O
market	O
.	O
</s>
<s>
In	O
disk	O
drives	O
usually	O
the	O
Fibre	B-Architecture
Channel	I-Architecture
Arbitrated	I-Architecture
Loop	I-Architecture
(	O
FC-AL	B-Architecture
)	O
connection	O
topology	O
is	O
used	O
.	O
</s>
<s>
Recently	O
other	O
protocols	O
for	O
this	O
field	O
,	O
like	O
iSCSI	B-Protocol
and	O
ATA	B-Protocol
over	I-Protocol
Ethernet	I-Protocol
have	O
been	O
developed	O
as	O
well	O
.	O
</s>
<s>
Confusingly	O
,	O
drives	O
usually	O
use	O
copper	O
twisted-pair	O
cables	O
for	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
not	O
fibre	O
optics	O
.	O
</s>
<s>
The	O
latter	O
are	O
traditionally	O
reserved	O
for	O
larger	O
devices	O
,	O
such	O
as	O
servers	O
or	O
disk	B-Device
array	I-Device
controllers	I-Device
.	O
</s>
<s>
A	O
similar	O
differential	O
signaling	O
system	O
is	O
used	O
in	O
RS485	O
,	O
LocalTalk	B-Device
,	O
USB	B-Protocol
,	O
FireWire	B-Protocol
,	O
and	O
differential	O
SCSI	B-Architecture
.	O
</s>
<s>
Serial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
.	O
</s>
<s>
SAS	O
uses	O
a	O
mechanically	O
identical	O
data	O
and	O
power	O
connector	O
to	O
standard	O
3.5-inch	O
SATA1/SATA2	O
HDDs	O
,	O
and	O
many	O
server-oriented	O
SAS	O
RAID	B-Device
controllers	I-Device
are	O
also	O
capable	O
of	O
addressing	O
SATA	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
SAS	O
uses	O
serial	O
communication	O
instead	O
of	O
the	O
parallel	O
method	O
found	O
in	O
traditional	O
SCSI	B-Architecture
devices	O
but	O
still	O
uses	O
SCSI	B-Architecture
commands	O
.	O
</s>
