<s>
The	O
8250	B-Device
UART	I-Device
(	O
universal	O
asynchronous	O
receiver-transmitter	O
)	O
is	O
an	O
integrated	O
circuit	O
designed	O
for	O
implementing	O
the	O
interface	O
for	O
serial	B-Protocol
communications	I-Protocol
.	O
</s>
<s>
It	O
was	O
commonly	O
used	O
in	O
PCs	B-Device
and	O
related	O
equipment	O
such	O
as	O
printers	O
or	O
modems	B-Application
.	O
</s>
<s>
The	O
8250	B-Device
included	O
an	O
on-chip	O
programmable	O
bit	O
rate	O
generator	O
,	O
allowing	O
use	O
for	O
both	O
common	O
and	O
special-purpose	O
bit	O
rates	O
which	O
could	O
be	O
accurately	O
derived	O
from	O
an	O
arbitrary	O
crystal	O
oscillator	O
reference	O
frequency	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
original	O
8250	B-Device
was	O
soon	O
followed	O
by	O
the	O
8250A	B-Device
and	O
8250B	O
versions	O
that	O
corrected	O
some	O
bugs	O
.	O
</s>
<s>
In	O
particular	O
,	O
the	O
original	O
8250	B-Device
could	O
repeat	O
transmission	O
of	O
a	O
character	O
if	O
the	O
CTS	O
line	O
was	O
asserted	O
asynchronously	O
during	O
the	O
first	O
transmission	O
attempt	O
.	O
</s>
<s>
The	O
16450(A )	O
UART	O
,	O
commonly	O
used	O
in	O
IBM	B-Operating_System
PC/AT	I-Operating_System
-series	O
computers	O
,	O
improved	O
on	O
the	O
8250	B-Device
by	O
permitting	O
higher	O
serial	O
line	O
speeds	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
on	O
PC	B-Device
hardware	O
,	O
such	O
as	O
OS/2	B-Application
,	O
Windows	B-Device
NT	I-Device
or	O
various	O
flavours	O
of	O
UNIX	B-Application
,	O
the	O
short	O
time	O
available	O
to	O
serve	O
character-by-character	O
interrupt	B-Application
requests	O
became	O
a	O
problem	O
,	O
therefore	O
the	O
IBM	B-Device
PS/2	I-Device
serial	O
ports	O
introduced	O
the	O
16550(A )	O
UARTs	O
that	O
had	O
a	O
built-in	O
16	O
byte	O
FIFO	B-Operating_System
or	O
buffer	O
memory	O
to	O
collect	O
incoming	O
characters	O
.	O
</s>
<s>
Later	O
models	O
added	O
larger	O
memories	O
,	O
supported	O
higher	O
speeds	O
,	O
combined	O
multiple	O
ports	O
on	O
one	O
chip	O
and	O
finally	O
became	O
part	O
of	O
the	O
now-common	O
Super	B-Device
I/O	I-Device
circuits	O
combining	O
most	O
input/output	O
logic	O
on	O
a	O
PC	B-Device
motherboard	I-Device
.	O
</s>
<s>
The	O
interrupt	B-Application
line	I-Application
will	O
(	O
when	O
the	O
IER	O
bit	O
has	O
enabled	O
it	O
)	O
be	O
triggered	O
to	O
go	O
high	O
when	O
one	O
of	O
the	O
following	O
events	O
occur	O
:	O
Receiver	O
line	O
status	O
,	O
Received	O
data	O
available	O
,	O
Transmitter	O
holding	O
register	O
empty	O
,	O
and	O
MODEM	B-Application
status	O
.	O
</s>
<s>
The	O
interrupt	B-Application
signal	O
is	O
reset	O
to	O
low	O
level	O
upon	O
the	O
appropriate	O
interrupt	B-Application
service	O
or	O
a	O
reset	O
operation	O
(	O
via	O
MR	O
)	O
.	O
</s>
<s>
8250	B-Device
UART	I-Device
was	O
introduced	O
with	O
the	O
IBM	B-Device
PC	I-Device
(	O
1981	O
)	O
.	O
</s>
<s>
The	O
8250A	B-Device
and	O
8250B	O
revisions	O
were	O
later	O
released	O
,	O
and	O
the	O
16450	O
was	O
introduced	O
with	O
the	O
IBM	B-Operating_System
Personal	I-Operating_System
Computer/AT	I-Operating_System
(	O
1984	O
)	O
.	O
</s>
<s>
IBM	O
refused	O
to	O
use	O
Intel	B-Device
8251	I-Device
in	O
the	O
serial	O
port	O
adapter	O
because	O
Intel	O
chip	O
had	O
no	O
internal	O
baudrate	O
generator	O
,	O
and	O
the	O
adapter	O
had	O
to	O
be	O
more	O
complex	O
and	O
expensive	O
.	O
</s>
