<s>
The	O
ST-506	B-Device
and	O
ST-412	B-Device
(	O
sometimes	O
written	O
ST506	B-Device
and	O
ST412	B-Device
)	O
were	O
early	O
hard	B-Device
disk	I-Device
drive	I-Device
products	O
introduced	O
by	O
Seagate	O
in	O
1980	O
and	O
1981	O
respectively	O
,	O
that	O
later	O
became	O
construed	O
as	O
hard	B-Device
disk	I-Device
drive	I-Device
interfaces	I-Device
:	O
the	O
ST-506	B-Device
disk	O
interface	B-Application
and	O
the	O
ST-412	B-Device
disk	O
interface	B-Application
.	O
</s>
<s>
Compared	O
to	O
the	O
ST-506	B-Device
precursor	O
,	O
the	O
ST-412	B-Device
implemented	O
a	O
refinement	O
to	O
the	O
seek	O
speed	O
,	O
and	O
increased	O
the	O
drive	O
capacity	O
from	O
5	O
MB	O
to	O
10	O
MB	O
,	O
but	O
was	O
otherwise	O
highly	O
similar	O
.	O
</s>
<s>
Beginning	O
with	O
its	O
selection	O
as	O
the	O
hard	B-Device
drive	I-Device
subsystem	O
for	O
the	O
original	O
IBM	B-Device
XT	I-Device
disk	B-General_Concept
drive	I-General_Concept
controllers	I-General_Concept
supporting	O
the	O
ST-412	B-Device
interface	B-Application
grew	O
to	O
become	O
ubiquitous	O
in	O
the	O
personal	B-Device
computer	I-Device
industry	O
,	O
The	O
ST-412	B-Device
interface	B-Application
and	O
its	O
variants	O
were	O
the	O
de	O
facto	O
industry	O
standard	O
for	O
personal	B-Device
computer	I-Device
hard	B-Device
disks	I-Device
until	O
the	O
advent	O
and	O
wider	O
adoption	O
of	O
the	O
IDE	B-Protocol
or	I-Protocol
ATA	I-Protocol
interface	I-Protocol
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
Both	O
interfaces	B-Application
used	O
MFM	B-Protocol
encoding	I-Protocol
;	O
the	O
subsequent	O
extension	O
of	O
the	O
ST-412	B-Device
interface	B-Application
,	O
the	O
ST-412HP	O
interface	B-Application
,	O
used	O
RLL	B-Protocol
encoding	I-Protocol
for	O
a	O
50%	O
increase	O
in	O
capacity	O
and	O
bit	O
rate	O
.	O
</s>
<s>
The	O
ST506	B-Device
HDD	O
was	O
the	O
first	O
5.25inch	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
introduced	O
in	O
1980	O
by	O
Shugart	O
Technology	O
(	O
now	O
Seagate	O
Technology	O
)	O
.	O
</s>
<s>
The	O
similar	O
,	O
10-megabyte	O
ST412	B-Device
HDD	O
was	O
introduced	O
in	O
late	O
1981	O
(	O
with	O
306	O
cylinders	O
)	O
.	O
</s>
<s>
All	O
three	O
used	O
MFM	B-Protocol
encoding	I-Protocol
,	O
a	O
widely	O
used	O
coding	O
scheme	O
.	O
</s>
<s>
A	O
subsequent	O
extension	O
of	O
the	O
ST412	B-Device
interface	B-Application
,	O
the	O
ST412HP	B-Device
interface	B-Application
,	O
used	O
RLL	B-Protocol
encoding	I-Protocol
for	O
a	O
50%	O
increase	O
in	O
capacity	O
and	O
bit	O
rate	O
.	O
</s>
<s>
The	O
ST506	B-Device
drive	O
connected	O
to	O
a	O
computer	O
system	O
through	O
a	O
disk	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
The	O
ST506	B-Device
interface	B-Application
between	O
the	O
controller	O
and	O
drive	O
was	O
derived	O
from	O
the	O
Shugart	O
Associates	O
SA1000	O
interface	B-Application
,	O
which	O
was	O
in	O
turn	O
based	O
upon	O
the	O
floppy	B-Device
disk	I-Device
drive	I-Device
interface	B-Application
,	O
thereby	O
making	O
disk	B-General_Concept
controller	I-General_Concept
design	O
relatively	O
easy	O
.	O
</s>
<s>
The	O
ST412	B-Device
interface	B-Application
was	O
adopted	O
by	O
numerous	O
HDD	O
manufacturers	O
such	O
that	O
the	O
interface	B-Application
became	O
a	O
de	O
facto	O
industry	O
standard	O
for	O
disk	O
drives	O
well	O
into	O
the	O
1990s	O
.	O
</s>
<s>
The	O
limitations	O
of	O
the	O
ST412	B-Device
interface	B-Application
are	O
5	O
million	O
transitions	O
per	O
second	O
maximum	O
on	O
data	O
lines	O
,	O
16	O
heads	O
,	O
4	O
drive	O
units	O
and	O
a	O
cable	O
length	O
.	O
</s>
<s>
The	O
standard	O
channel	O
code	O
for	O
the	O
ST412	B-Device
(	O
and	O
ST506	B-Device
)	O
is	O
MFM	B-Protocol
with	O
one	O
data	O
bit	O
per	O
transition	O
for	O
a	O
data	O
rate	O
of	O
5Mbit/s	O
.	O
</s>
<s>
The	O
ST412HP	B-Device
RLL	B-Protocol
variant	O
averages	O
1.5	O
data	O
bits	O
per	O
transition	O
for	O
a	O
data	O
rate	O
of	O
7.5Mbit/s	O
.	O
</s>
<s>
In	O
the	O
ST506	B-Device
interface	B-Application
,	O
the	O
drive	O
connects	O
to	O
a	O
controller	O
card	O
with	O
two	O
ribbon	O
cables	O
carrying	O
signals	O
,	O
while	O
a	O
third	O
cable	O
provides	O
power	O
.	O
</s>
<s>
The	O
control	O
cable	O
interface	B-Application
is	O
very	O
similar	O
to	O
the	O
standard	O
Shugart	O
floppy	B-Device
disk	I-Device
interface	B-Application
;	O
like	O
that	O
floppy	B-Device
disk	I-Device
interface	B-Application
,	O
it	O
can	O
support	O
four	O
drives	O
.	O
</s>
<s>
The	O
data	O
represented	O
by	O
these	O
signals	O
is	O
MFM	B-Protocol
,	O
so	O
the	O
absolute	O
signal	O
states	O
are	O
not	O
significant	O
:	O
the	O
data	O
is	O
represented	O
in	O
the	O
timing	O
of	O
the	O
state	O
transitions	O
,	O
like	O
in	O
floppy	B-Device
disk	I-Device
systems	O
.	O
</s>
<s>
While	O
up	O
to	O
four	O
drives	O
can	O
share	O
a	O
control	O
cable	O
,	O
each	O
drive	O
has	O
its	O
own	O
dedicated	O
data	O
cable	O
connecting	O
it	O
to	O
the	O
hard	B-General_Concept
disk	I-General_Concept
controller	I-General_Concept
(	O
HDC	O
)	O
.	O
</s>
<s>
Four	O
of	O
the	O
control	O
cable	O
pins	O
,	O
"	O
HD	O
SLCT	O
0	O
"	O
through	O
"	O
HD	O
SLCT	O
3	O
"	O
,	O
allow	O
the	O
selection	O
among	O
up	O
to	O
16	O
heads	O
,	O
although	O
only	O
four	O
are	O
available	O
on	O
the	O
two-platter	O
ST506	B-Device
.	O
</s>
<s>
The	O
original	O
ST506/ST412	B-Device
interface	B-Application
defined	O
only	O
two	O
HD	O
SLCT	O
lines	O
,	O
providing	O
supporting	O
for	O
only	O
four	O
heads	O
,	O
but	O
a	O
third	O
HD	O
SLCT	O
line	O
was	O
shortly	O
added	O
to	O
the	O
design	O
;	O
a	O
fourth	O
was	O
added	O
a	O
not	O
much	O
later	O
by	O
redefining	O
the	O
reduced	O
Write	O
Current	O
signal	O
,	O
needed	O
only	O
by	O
very	O
early	O
drives	O
,	O
as	O
HD	O
SLCT	O
3	O
.	O
</s>
<s>
The	O
ST412	B-Device
disk	O
drive	O
,	O
among	O
other	O
improvements	O
,	O
added	O
buffered	O
seek	O
capability	O
to	O
the	O
interface	B-Application
.	O
</s>
<s>
An	O
onboard	O
microprocessor	B-Architecture
in	O
the	O
drive	O
then	O
moves	O
the	O
mechanism	O
to	O
the	O
desired	O
track	O
as	O
fast	O
as	O
possible	O
.	O
</s>
<s>
The	O
ST506	B-Device
disk	O
drive	O
without	O
buffered	O
seek	O
averages	O
170ms	O
(	O
similar	O
to	O
a	O
floppy	B-Device
drive	I-Device
or	O
modern	O
optical	O
drive	O
)	O
while	O
the	O
mechanically	O
very	O
similar	O
ST412	B-Device
disk	O
drive	O
with	O
buffered	O
seek	O
averages	O
85ms	O
.	O
</s>
<s>
By	O
the	O
late	O
1980s	O
,	O
drives	O
with	O
an	O
ST412	B-Device
interface	B-Application
were	O
capable	O
of	O
average	O
seek	O
times	O
between	O
15	O
and	O
30	O
milliseconds	O
.	O
</s>
<s>
The	O
process	O
of	O
moving	O
portions	O
of	O
the	O
command	O
interpretation	O
off	O
the	O
controller	O
card	O
and	O
onto	O
the	O
drive	O
itself	O
in	O
order	O
to	O
improve	O
performance	O
is	O
a	O
common	O
feature	O
of	O
later	O
hard	B-Device
drive	I-Device
connection	O
schemes	O
,	O
notably	O
SCSI	B-Architecture
,	O
with	O
its	O
rich	O
command	O
set	O
,	O
and	O
the	O
storage-focused	O
IDE	B-Protocol
systems	O
.	O
</s>
<s>
IDE	B-Protocol
,	O
in	O
effect	O
,	O
is	O
a	O
system	O
for	O
extending	O
the	O
computer	O
bus	O
so	O
the	O
interface	B-Application
controller	O
can	O
be	O
built	O
into	O
the	O
drive	O
unit	O
rather	O
than	O
being	O
plugged	O
into	O
the	O
computer	O
's	O
backplane	O
.	O
</s>
<s>
This	O
allows	O
a	O
single	O
"	O
controller	O
"	O
cardreally	O
just	O
an	O
interface	B-Application
cardto	O
communicate	O
with	O
multiple	O
dissimilar	O
drives	O
,	O
while	O
it	O
also	O
reduces	O
latency	O
and	O
noise	O
between	O
the	O
controller	O
and	O
drive	O
hardware	O
.	O
</s>
<s>
These	O
became	O
known	O
as	O
"	O
smart	O
"	O
drives	O
,	O
while	O
ST506-like	O
devices	O
retroactively	O
became	O
known	O
as	O
"	O
dumb	O
"	O
.	O
</s>
<s>
With	O
a	O
separated	O
controller	O
and	O
disk	O
system	O
like	O
that	O
of	O
the	O
ST506	B-Device
interface	B-Application
,	O
sometimes	O
the	O
problem	O
can	O
be	O
resolved	O
by	O
connecting	O
the	O
disk	O
drive	O
,	O
containing	O
the	O
actual	O
(	O
perhaps	O
very	O
important	O
)	O
data	O
,	O
to	O
another	O
compatible	O
controller	O
.	O
</s>
<s>
Furthermore	O
,	O
an	O
ST506	B-Device
style	O
interface	B-Application
makes	O
it	O
possible	O
and	O
easy	O
not	O
only	O
to	O
replace	O
the	O
controller	O
without	O
throwing	O
away	O
the	O
data	O
,	O
but	O
to	O
get	O
access	O
to	O
the	O
analog	O
data	O
signals	O
from	O
the	O
disk	O
drive	O
and	O
process	O
them	O
through	O
a	O
special	O
data	O
recovery	O
system	O
that	O
may	O
be	O
able	O
to	O
reconstruct	O
data	O
that	O
a	O
normal	O
controller	O
cannot	O
read	O
.	O
</s>
<s>
Such	O
data	O
recovery	O
techniques	O
are	O
much	O
more	O
difficult	O
to	O
execute	O
on	O
integrated	O
drives	O
,	O
because	O
the	O
needed	O
analog	O
signals	O
from	O
the	O
disk	O
are	O
not	O
available	O
at	O
a	O
standard	O
interface	B-Application
and	O
the	O
internal	O
data	O
recording	O
method	O
,	O
sector	O
format	O
,	O
and	O
disk	O
organization	O
of	O
nearly	O
every	O
integrated	O
drive	O
model	O
is	O
different	O
and	O
secret	O
.	O
</s>
<s>
Many	O
other	O
companies	O
quickly	O
introduced	O
drives	O
using	O
the	O
same	O
connectors	O
and	O
signals	O
,	O
creating	O
a	O
hard	B-Device
drive	I-Device
standard	O
based	O
on	O
the	O
ST506	B-Device
.	O
</s>
<s>
IBM	O
chose	O
to	O
use	O
it	O
,	O
acquiring	O
adapter	O
cards	O
for	O
the	O
PC/XT	B-Device
from	O
Xebec	O
and	O
for	O
the	O
PC/AT	B-Operating_System
from	O
Western	O
Digital	O
.	O
</s>
<s>
As	O
a	O
consequence	O
of	O
IBM	O
's	O
endorsement	O
,	O
most	O
of	O
the	O
drives	O
in	O
the	O
1980s	O
were	O
based	O
on	O
the	O
ST506	B-Device
.	O
</s>
<s>
However	O
,	O
the	O
complexity	O
of	O
the	O
controller	O
and	O
cabling	O
led	O
to	O
newer	O
solutions	O
like	O
SCSI	B-Architecture
,	O
and	O
later	O
,	O
ATA	B-Protocol
(	O
IDE	B-Protocol
)	O
.	O
</s>
<s>
A	O
few	O
early	O
SCSI	B-Architecture
drives	O
were	O
actually	O
ST506	B-Device
drives	O
with	O
a	O
SCSI	B-Architecture
to	O
ST506	B-Device
controller	O
on	O
the	O
bottom	O
of	O
the	O
drive	O
.	O
</s>
<s>
Atari	O
also	O
used	O
Adaptec	O
ACB-4000A	O
SCSI	B-Architecture
to	O
ST506	B-Device
converter	O
inside	O
its	O
own	O
line	O
of	O
SH204/SH205	O
external	O
ACSI	O
drives	O
.	O
</s>
<s>
Likewise	O
a	O
few	O
early	O
IDE	B-Protocol
drives	I-Protocol
were	O
just	O
drives	O
with	O
an	O
ST412	B-Device
interface	B-Application
attached	O
to	O
a	O
controller	O
board	O
or	O
chip	O
.	O
</s>
<s>
Ultimately	O
all	O
SCSI	B-Architecture
and	O
ATA	B-Protocol
drives	I-Protocol
had	O
built	O
the	O
controller	O
into	O
the	O
drive	O
,	O
thereby	O
eliminating	O
the	O
ST506/412	O
interface	B-Application
in	O
such	O
models	O
.	O
</s>
<s>
From	O
ST506/ST412	B-Device
OEM	O
manual	O
.	O
</s>
