<s>
The	O
Standard	B-Device
Disk	I-Device
Interconnect	I-Device
(	O
SDI	O
,	O
sometimes	O
Standard	O
Disk	O
Interface	O
)	O
was	O
used	O
by	O
Digital	O
Equipment	O
Corporation	O
for	O
its	O
line	O
of	O
RAxx	O
disks	O
,	O
for	O
example	O
the	O
RA90	B-Device
.	O
</s>
<s>
There	O
were	O
two	O
bi-directional	O
serial	B-Protocol
communications	I-Protocol
paths	O
between	O
the	O
controller	B-Architecture
and	O
the	O
drive	B-Device
.	O
</s>
<s>
One	O
path	O
was	O
for	O
messages	O
(	O
from	O
the	O
controller	B-Architecture
to	O
the	O
drive	B-Device
)	O
and	O
responses	O
(	O
from	O
the	O
drive	B-Device
to	O
the	O
controller	B-Architecture
)	O
;	O
messages	O
controlled	O
seeking	O
,	O
setting	O
drive	B-Device
parameters	O
,	O
reporting	O
errors	O
,	O
etc	O
.	O
</s>
<s>
This	O
path	O
was	O
also	O
used	O
to	O
transmit	O
data	O
to	O
and	O
from	O
the	O
drive	B-Device
.	O
</s>
<s>
The	O
other	O
path	O
consisted	O
of	O
a	O
continuously	O
transmitted	O
set	O
of	O
control	O
(	O
controller-to-drive	O
)	O
or	O
status	O
(	O
drive-to-controller	O
)	O
bits	O
.	O
</s>
<s>
By	O
encoding	O
control	O
signals	O
serially	O
they	O
could	O
be	O
AC-coupled	O
(	O
with	O
a	O
transformer	O
)	O
to	O
prevent	O
ground	O
loop	O
problems	O
and	O
EMI	B-General_Concept
issues	O
between	O
the	O
controller	B-Architecture
and	O
the	O
drives	O
,	O
which	O
could	O
be	O
connected	O
by	O
long	O
cables	O
,	O
even	O
to	O
different	O
floors	O
in	O
a	O
building	O
.	O
</s>
<s>
The	O
same	O
serial	O
interconnect	O
was	O
used	O
for	O
tape	O
and	O
solid	B-Device
state	I-Device
storage	I-Device
devices	I-Device
to	O
the	O
storage	O
subsystem	O
controllers	O
.	O
</s>
<s>
Large	O
VAX	B-Device
systems	O
primarily	O
used	O
the	O
Hierarchical	O
Storage	O
Controller	B-Architecture
(	O
HSC	O
)	O
series	O
but	O
Unibus	B-Device
and	O
BI	O
Bus	O
controllers	O
to	O
SDI	O
were	O
built	O
as	O
well	O
.	O
</s>
<s>
The	O
UDA50	B-Device
for	O
the	O
Unibus	B-Device
was	O
the	O
first	O
controller	B-Architecture
shipped	O
with	O
the	O
SDI	O
interface	O
.	O
</s>
<s>
Among	O
the	O
signals	O
the	O
drive	B-Device
sent	O
to	O
the	O
controller	B-Architecture
were	O
:	O
</s>
<s>
Index	O
Pulse	O
-	O
the	O
drive	B-Device
head	I-Device
is	O
over	O
the	O
beginning	O
of	O
sector	B-Application
0	O
of	O
the	O
track	O
.	O
</s>
<s>
Sector	B-Application
Pulse	O
-	O
the	O
drive	B-Device
head	I-Device
is	O
over	O
the	O
beginning	O
of	O
some	O
other	O
sector	B-Application
.	O
</s>
<s>
The	O
controller	B-Architecture
counted	O
these	O
pulses	O
to	O
know	O
where	O
the	O
drive	B-Device
was	O
,	O
rotationally	O
.	O
</s>
<s>
Read/Write	O
Ready	O
-	O
the	O
drive	B-Device
head	I-Device
has	O
settled	O
over	O
some	O
track	O
,	O
and	O
it	O
is	O
safe	O
to	O
read	O
or	O
write	O
.	O
</s>
<s>
The	O
controller	B-Architecture
signals	O
to	O
the	O
drive	B-Device
included	O
signals	O
such	O
as	O
Read	O
Gate	O
(	O
turn	O
the	O
Read	O
Head	O
on	O
)	O
and	O
Write	O
Gate	O
(	O
turn	O
the	O
Write	O
Head	O
on	O
)	O
.	O
</s>
<s>
The	O
SDI	O
was	O
an	O
element	O
of	O
the	O
Digital	O
Storage	O
Architecture	O
(	O
DSA	O
)	O
which	O
included	O
the	O
Mass	B-Device
Storage	I-Device
Control	I-Device
Protocol	I-Device
(	O
MSCP	O
)	O
,	O
which	O
specified	O
communications	O
means	O
between	O
OS	B-General_Concept
software	I-General_Concept
and	O
all	O
flavors	O
of	O
storage	O
devices	O
.	O
</s>
