<s>
Storage	B-General_Concept
tubes	I-General_Concept
are	O
a	O
class	O
of	O
cathode-ray	B-Device
tubes	I-Device
(	O
CRTs	O
)	O
that	O
are	O
designed	O
to	O
hold	O
an	O
image	O
for	O
a	O
long	O
period	O
of	O
time	O
,	O
typically	O
as	O
long	O
as	O
power	O
is	O
supplied	O
to	O
the	O
tube	O
.	O
</s>
<s>
A	O
specialized	O
type	O
of	O
storage	B-General_Concept
tube	I-General_Concept
,	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
,	O
was	O
used	O
as	O
a	O
main	O
memory	O
system	O
on	O
a	O
number	O
of	O
early	O
computers	O
,	O
from	O
the	O
late	O
1940s	O
into	O
the	O
early	O
1950s	O
.	O
</s>
<s>
They	O
were	O
replaced	O
with	O
other	O
technologies	O
,	O
notably	O
core	B-General_Concept
memory	I-General_Concept
,	O
starting	O
in	O
the	O
1950s	O
.	O
</s>
<s>
Storage	B-General_Concept
tubes	I-General_Concept
made	O
a	O
comeback	O
in	O
the	O
1960s	O
and	O
1970s	O
for	O
use	O
in	O
computer	O
graphics	O
,	O
most	O
notably	O
the	O
Tektronix	B-General_Concept
4010	I-General_Concept
series	O
.	O
</s>
<s>
Today	O
they	O
are	O
obsolete	O
,	O
their	O
functions	O
provided	O
by	O
low-cost	O
memory	B-General_Concept
devices	I-General_Concept
and	O
liquid	B-Device
crystal	I-Device
displays	I-Device
.	O
</s>
<s>
These	O
two	O
effects	O
were	O
both	O
utilized	O
in	O
the	O
construction	O
of	O
a	O
storage	B-General_Concept
tube	I-General_Concept
.	O
</s>
<s>
The	O
easiest	O
example	O
to	O
understand	O
are	O
the	O
early	O
computer	B-General_Concept
memory	I-General_Concept
systems	O
as	O
typified	O
by	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
.	O
</s>
<s>
In	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
,	O
these	O
values	O
were	O
read	O
by	O
measuring	O
the	O
capacitance	O
of	O
a	O
metal	O
plate	O
just	O
in	O
front	O
of	O
the	O
display	O
side	O
of	O
the	O
tube	O
.	O
</s>
<s>
The	O
earliest	O
computer	O
graphics	O
systems	O
,	O
like	O
those	O
of	O
the	O
TX-2	B-Device
and	O
DEC	B-Device
PDP-1	I-Device
,	O
required	O
the	O
entire	O
attention	O
of	O
the	O
computer	O
to	O
maintain	O
.	O
</s>
<s>
This	O
generally	O
occurred	O
frequently	O
enough	O
that	O
there	O
was	O
little	O
time	O
to	O
do	O
anything	O
else	O
,	O
and	O
interactive	O
systems	O
like	O
Spacewar	B-Application
!	I-Application
</s>
<s>
For	O
practical	O
use	O
,	O
graphical	O
displays	O
were	O
developed	O
that	O
contained	O
their	O
own	O
memory	O
and	O
an	O
associated	O
very	O
simple	O
computer	O
which	O
offloaded	O
the	O
refreshing	O
task	O
from	O
the	O
mainframe	B-Architecture
.	O
</s>
<s>
This	O
was	O
not	O
inexpensive	O
;	O
the	O
IBM	B-Device
2250	I-Device
graphics	O
terminal	O
used	O
with	O
the	O
IBM	B-Application
S/360	I-Application
cost	O
$	O
280,000	O
in	O
1970	O
.	O
</s>
<s>
A	O
storage	B-General_Concept
tube	I-General_Concept
could	O
replace	O
most	O
or	O
all	O
of	O
the	O
localized	O
hardware	O
by	O
storing	O
the	O
vectors	O
directly	O
within	O
the	O
display	O
,	O
instead	O
of	O
an	O
associated	O
local	O
computer	O
.	O
</s>
<s>
No	O
need	O
for	O
display-local	O
RAM	B-Architecture
(	O
as	O
in	O
modern	O
terminals	O
)	O
,	O
which	O
was	O
prohibitively	O
expensive	O
at	O
the	O
time	O
.	O
</s>
<s>
Generally	O
speaking	O
,	O
storage	B-General_Concept
tubes	I-General_Concept
could	O
be	O
divided	O
into	O
two	O
categories	O
.	O
</s>
<s>
The	O
Tektronix	O
Direct-View	O
Bistable	O
Storage	B-General_Concept
Tube	I-General_Concept
was	O
perhaps	O
the	O
best	O
example	O
in	O
this	O
category	O
.	O
</s>
<s>
Other	O
storage	B-General_Concept
tubes	I-General_Concept
were	O
able	O
to	O
store	O
greyscale/halftoned	O
images	O
;	O
the	O
tradeoff	O
was	O
usually	O
a	O
much-reduced	O
storage	O
time	O
.	O
</s>
<s>
Some	O
pioneering	O
storage	B-General_Concept
tube	I-General_Concept
displays	O
were	O
MIT	O
Project	O
MAC	O
's	O
ARDS	O
(	O
Advanced	O
Remote	O
Display	O
Station	O
)	O
,	O
the	O
Computek	O
400	O
Series	O
Display	O
terminals	O
(	O
a	O
commercial	O
derivative	O
)	O
,	O
which	O
both	O
used	O
a	O
Tektronix	O
type	O
611	O
storage	O
display	O
unit	O
,	O
and	O
Tektronix	O
's	O
4014	B-General_Concept
terminal	O
,	O
the	O
latter	O
becoming	O
a	O
de	O
facto	O
computer	O
terminal	O
standard	O
some	O
time	O
after	O
its	O
introduction	O
(	O
later	O
being	O
emulated	O
by	O
other	O
systems	O
due	O
to	O
this	O
status	O
)	O
.	O
</s>
<s>
The	O
first	O
generalized	O
computer	O
assisted	O
instruction	O
system	O
,	O
PLATO	B-Operating_System
I	I-Operating_System
,	O
c	O
.	O
1960	O
on	O
ILLIAC	B-Device
I	I-Device
,	O
used	O
a	O
storage	B-General_Concept
tube	I-General_Concept
as	O
its	O
computer	O
graphics	O
display	O
.	O
</s>
<s>
PLATO	B-Operating_System
II	I-Operating_System
and	O
PLATO	B-Operating_System
III	I-Operating_System
also	O
used	O
storage	B-General_Concept
tubes	I-General_Concept
as	O
displays	O
.	O
</s>
