<s>
The	O
HP-28C	B-Device
and	O
HP-28S	B-Device
were	O
two	O
graphing	B-Application
calculators	I-Application
produced	O
by	O
Hewlett-Packard	O
from	O
1986	O
to	O
1992	O
.	O
</s>
<s>
The	O
HP-28C	B-Device
was	O
the	O
first	O
handheld	O
calculator	O
capable	O
of	O
solving	O
equations	O
symbolically	O
.	O
</s>
<s>
They	O
were	O
replaced	O
by	O
the	O
HP	O
48	O
series	O
of	O
calculators	O
,	O
which	O
grew	O
from	O
the	O
menu-driven	O
RPL	B-Device
programming	I-Device
language	I-Device
interface	O
first	O
introduced	O
in	O
these	O
HP-28	B-Device
series	I-Device
.	O
</s>
<s>
Two	O
models	O
were	O
produced	O
,	O
the	O
HP-28C	B-Device
came	O
first	O
in	O
1987	O
with	O
2	O
kilobytes	O
of	O
usable	O
RAM	B-Architecture
,	O
and	O
was	O
the	O
first	O
handheld	O
calculator	O
with	O
a	O
Computer	B-General_Concept
Algebra	I-General_Concept
System	I-General_Concept
.	O
</s>
<s>
A	O
year	O
later	O
,	O
the	O
more	O
common	O
HP-28S	B-Device
was	O
released	O
with	O
32KB	O
of	O
RAM	B-Architecture
and	O
a	O
directory	O
system	O
for	O
filing	O
variables	O
,	O
functions	O
,	O
and	O
programs	O
.	O
</s>
<s>
The	O
HP-28C	B-Device
used	O
a	O
Saturn	B-Application
processor	I-Application
running	O
at	O
640kHz	O
whereas	O
the	O
HP-28S	B-Device
used	O
a	O
custom	O
chip	O
containing	O
an	O
improved	O
Saturn	B-Application
processor	I-Application
core	O
codenamed	O
Lewis	O
and	O
running	O
at	O
1MHz	O
.	O
</s>
<s>
The	O
HP-28C	B-Device
was	O
the	O
last	O
HP	O
model	O
introduced	O
with	O
the	O
suffix	O
"	O
C	O
"	O
in	O
its	O
model	O
designation	O
–	O
a	O
practice	O
which	O
HP	O
had	O
started	O
with	O
the	O
HP-25C	B-Device
back	O
in	O
1976	O
.	O
</s>
<s>
The	O
"	O
C	O
"	O
had	O
distinguished	O
those	O
models	O
as	O
having	O
continuous	B-Device
memory	I-Device
.	O
</s>
<s>
So	O
beginning	O
with	O
the	O
HP-28S	B-Device
,	O
HP-17B	O
,	O
HP-19B	B-Device
,	O
and	O
HP-27S	O
,	O
the	O
feature	O
suffix	O
"	O
C	O
"	O
was	O
replaced	O
with	O
a	O
class	O
suffix	O
which	O
was	O
more	O
meaningful	O
in	O
the	O
market	O
:	O
"	O
S	O
"	O
for	O
Scientific	O
,	O
"	O
B	O
"	O
for	O
Business	O
,	O
and	O
later	O
(	O
in	O
1993	O
)	O
"	O
G	O
"	O
for	O
Graphic	O
.	O
</s>
<s>
The	O
HP-28	B-Device
calculators	O
shared	O
a	O
flip-open	O
(	O
"	O
clamshell	O
"	O
)	O
case	O
.	O
</s>
<s>
The	O
display	O
was	O
a	O
137×32	O
LCD	B-Device
dot	B-General_Concept
matrix	I-General_Concept
,	O
usually	O
displaying	O
four	O
lines	O
of	O
information	O
(	O
3	O
stack/command	O
lines	O
,	O
plus	O
one	O
softkey	O
label	O
line	O
)	O
.	O
</s>
<s>
Among	O
the	O
drawbacks	O
of	O
the	O
HP-28	B-Device
was	O
the	O
lack	O
of	O
a	O
computer	O
interface	O
.	O
</s>
<s>
This	O
model	O
(	O
and	O
the	O
HP-19	B-Device
series	I-Device
that	O
used	O
the	O
same	O
clamshell	O
design	O
and	O
internals	O
)	O
had	O
one	O
design	O
problem	O
that	O
affected	O
many	O
units	O
:	O
the	O
springs	O
inside	O
the	O
unit	O
that	O
made	O
contact	O
with	O
its	O
three	O
N-sized	B-Algorithm
batteries	O
made	O
a	O
considerably	O
strong	O
force	O
,	O
and	O
the	O
notches	O
on	O
the	O
case	O
in	O
which	O
the	O
lid	O
edges	O
slid	O
were	O
too	O
thin	O
and	O
shallow	O
.	O
</s>
<s>
This	O
defect	O
was	O
later	O
remedied	O
on	O
the	O
HP-19BII	B-Device
,	O
by	O
putting	O
the	O
battery	O
cover	O
underneath	O
the	O
chassis	O
.	O
</s>
