<s>
The	O
IBM	B-Device
2821	I-Device
Control	I-Device
Unit	I-Device
attaches	O
card	B-Device
readers	I-Device
and	I-Device
card	I-Device
punches	I-Device
,	O
and	O
line	O
printers	O
to	O
the	O
IBM	B-Application
System/360	I-Application
and	O
IBM	B-Device
System/370	I-Device
families	O
of	O
computers	O
.	O
</s>
<s>
The	O
IBM	B-Device
2540	I-Device
card	B-Device
reader	I-Device
and	O
card	B-Device
punch	I-Device
;	O
</s>
<s>
The	O
2821	O
was	O
originally	O
advertised	O
—	O
in	O
1964	O
,	O
before	O
System/360	B-Application
shipped	O
—	O
as	O
a	O
controller	O
for	O
the	O
IBM	B-Device
1402	I-Device
card	O
reader/punch	O
and	O
the	O
IBM	B-Device
1403	I-Device
and	O
IBM	O
2201	O
printers	O
.	O
</s>
<s>
Six	O
models	O
of	O
the	O
IBM	B-Device
2821	I-Device
Control	I-Device
Unit	I-Device
were	O
available	O
,	O
as	O
follows	O
:	O
</s>
<s>
Model	O
1	O
attaches	O
one	O
IBM	B-Device
2540	I-Device
and	O
one	O
IBM	B-Device
1403	I-Device
;	O
</s>
<s>
Model	O
2	O
attaches	O
one	O
IBM	B-Device
1403	I-Device
;	O
</s>
<s>
Model	O
3	O
attaches	O
two	O
or	O
three	O
IBM	B-Device
1403s	I-Device
;	O
</s>
<s>
Model	O
4	O
attaches	O
one	O
IBM	B-Device
2540	I-Device
and	O
one	O
IBM	O
1404	O
,	O
but	O
only	O
on	O
IBM	B-Application
System/360	I-Application
models	O
25	O
,	O
30	O
,	O
40	O
and	O
50	O
;	O
</s>
<s>
Model	O
6	O
attaches	O
one	O
IBM	B-Device
2540	I-Device
.	O
</s>
<s>
The	O
2821	O
is	O
fully	O
buffered	B-General_Concept
,	O
that	O
is	O
it	O
has	O
buffers	B-General_Concept
large	O
enough	O
to	O
hold	O
an	O
entire	O
card	O
to	O
punch	B-Device
,	O
an	O
entire	O
card	O
read	O
,	O
or	O
an	O
entire	O
print	O
line	O
for	O
each	O
attached	O
device	O
.	O
</s>
<s>
The	O
control	O
unit	O
can	O
be	O
connected	O
to	O
either	O
a	O
byte	O
multiplexer	O
channel	B-Device
or	O
a	O
selector	O
channel	B-Device
.	O
</s>
<s>
It	O
is	O
made	O
up	O
of	O
logically	O
independent	O
control	O
units	O
,	O
but	O
takes	O
up	O
only	O
one	O
of	O
the	O
eight	O
control	O
unit	O
slots	O
which	O
can	O
be	O
connected	O
to	O
a	O
channel	B-Device
.	O
</s>
<s>
A	O
two-channel	O
switch	O
is	O
available	O
,	O
allowing	O
the	O
control	O
unit	O
to	O
be	O
connected	O
to	O
two	O
channels	O
,	O
on	O
one	O
or	O
two	O
computer	O
systems	O
,	O
controlled	O
by	O
manual	O
switches	O
or	O
by	O
programming	O
.	O
</s>
<s>
The	O
circuitry	O
in	O
the	O
2821	O
is	O
largely	O
older-technology	O
IBM	B-Device
SMS	I-Device
,	O
unlike	O
much	O
of	O
the	O
System/360	B-Application
line	O
,	O
but	O
later	O
revisions	O
were	O
partially	O
implemented	O
in	O
SLT	B-Algorithm
or	O
MST	O
.	O
</s>
