<s>
The	O
16550	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
is	O
frequently	O
used	O
to	O
implement	O
the	O
serial	B-Protocol
port	I-Protocol
for	O
IBM	O
PC	O
compatible	O
personal	O
computers	O
,	O
where	O
it	O
is	O
often	O
connected	O
to	O
an	O
RS-232	O
interface	O
for	O
modems	B-Application
,	O
serial	B-Device
mice	I-Device
,	O
printers	O
,	O
and	O
similar	O
peripherals	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
serial	O
chip	O
used	O
in	O
the	O
IBM	B-Device
PS/2	I-Device
line	O
,	O
which	O
were	O
introduced	O
in	O
1987	O
.	O
</s>
<s>
A	O
UART	O
function	O
that	O
is	O
register-compatible	O
with	O
the	O
16550	B-Device
is	O
usually	O
a	O
feature	O
of	O
multifunction	O
I/O	O
cards	O
for	O
IBM	O
PC-compatible	O
computers	O
and	O
may	O
be	O
integrated	O
on	O
the	O
motherboard	O
of	O
other	O
compatible	O
computers	O
.	O
</s>
<s>
Replacement	O
of	O
the	O
factory-installed	O
8250	B-Device
UART	I-Device
was	O
a	O
common	O
upgrade	O
for	O
owners	O
of	O
IBM	O
PC	O
,	O
XT	O
,	O
and	O
compatible	O
computers	O
when	O
high-speed	O
modems	B-Application
became	O
available	O
.	O
</s>
<s>
Above	O
9600	O
baud	O
,	O
owners	O
discovered	O
that	O
the	O
serial	B-Protocol
ports	I-Protocol
of	O
the	O
computers	O
were	O
not	O
able	O
to	O
handle	O
a	O
continuous	O
flow	O
of	O
data	O
without	O
losing	O
characters	O
.	O
</s>
<s>
Exchange	O
of	O
the	O
8250	B-Device
(	O
having	O
only	O
a	O
one-byte	O
received	O
data	B-General_Concept
buffer	I-General_Concept
)	O
with	O
a	O
16550	B-Device
,	O
and	O
occasionally	O
patching	O
or	O
setting	O
system	O
software	O
to	O
be	O
aware	O
of	O
the	O
FIFO	B-Operating_System
feature	O
of	O
the	O
new	O
chip	O
,	O
improved	O
the	O
reliability	O
and	O
stability	O
of	O
high-speed	O
connections	O
.	O
</s>
<s>
Main	O
features	O
of	O
the	O
16550	B-Device
include	O
:	O
</s>
<s>
The	O
ability	O
to	O
convert	O
data	O
from	O
serial	O
to	O
parallel	O
,	O
and	O
from	O
parallel	O
to	O
serial	O
,	O
using	O
shift	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
Handshake	O
lines	O
for	O
control	O
of	O
an	O
external	B-Application
modem	I-Application
,	O
controllable	O
by	O
software	O
.	O
</s>
<s>
An	O
interrupt	B-Application
function	O
to	O
the	O
host	O
microprocessor	B-Architecture
.	O
</s>
<s>
An	O
on-chip	O
FIFO	B-Operating_System
buffer	B-General_Concept
for	O
both	O
incoming	O
and	O
outgoing	O
data	O
;	O
this	O
gives	O
the	O
host	O
system	O
more	O
time	O
to	O
respond	O
to	O
an	O
interrupt	B-Application
generated	O
by	O
the	O
UART	O
,	O
without	O
loss	O
of	O
data	O
.	O
</s>
<s>
Both	O
the	O
computer	B-Architecture
hardware	I-Architecture
and	O
software	O
interface	O
of	O
the	O
16550	B-Device
are	O
backward	O
compatible	O
with	O
the	O
earlier	O
8250	B-Device
UART	I-Device
and	O
16450	O
UART	O
.	O
</s>
<s>
The	O
16550A	O
and	O
newer	O
is	O
pin-compatible	O
with	O
the	O
16450	O
,	O
but	O
the	O
Microsoft	B-Application
diagnostics	I-Application
program	O
(	O
MSD	B-Application
)	O
supplied	O
with	O
MS-DOS	O
6.x	O
,	O
Windows	O
9x	O
,	O
Windows	O
Me	O
,	O
and	O
Windows	O
2000	O
often	O
report	O
the	O
16450	O
chip	O
as	O
an	O
8250	B-Device
chip	O
.	O
</s>
<s>
One	O
drawback	O
of	O
the	O
earlier	O
8250	B-Device
UARTs	I-Device
and	O
16450	O
UARTs	O
was	O
that	O
interrupts	B-Application
were	O
generated	O
for	O
each	O
byte	O
received	O
.	O
</s>
<s>
This	O
generated	O
high	O
rates	O
of	O
interrupts	B-Application
as	O
transfer	O
speeds	O
increased	O
.	O
</s>
<s>
More	O
critically	O
,	O
with	O
only	O
a	O
1-byte	O
buffer	B-General_Concept
there	O
is	O
a	O
genuine	O
risk	O
that	O
a	O
received	O
byte	O
will	O
be	O
overwritten	O
if	O
interrupt	B-Application
service	O
delays	O
occur	O
.	O
</s>
<s>
To	O
overcome	O
these	O
shortcomings	O
,	O
the	O
16550	B-Device
series	O
UARTs	O
incorporated	O
a	O
16-byte	O
FIFO	B-Operating_System
buffer	B-General_Concept
with	O
a	O
programmable	O
interrupt	B-Application
trigger	O
of	O
1	O
,	O
4	O
,	O
8	O
,	O
or	O
14bytes	O
.	O
</s>
<s>
The	O
original	O
16550	B-Device
had	O
a	O
bug	O
that	O
prevented	O
this	O
FIFO	B-Operating_System
from	O
being	O
used	O
.	O
</s>
<s>
Not	O
all	O
manufacturers	O
adopted	O
this	O
nomenclature	O
,	O
however	O
,	O
continuing	O
to	O
refer	O
to	O
the	O
fixed	O
chip	O
as	O
a	O
16550	B-Device
.	O
</s>
<s>
According	O
to	O
another	O
source	O
,	O
the	O
FIFO	B-Operating_System
issue	O
was	O
corrected	O
only	O
in	O
the	O
16550AF	O
model	O
,	O
with	O
the	O
A	O
model	O
still	O
being	O
buggy	O
.	O
</s>
<s>
The	O
16550	B-Device
also	O
incorporates	O
a	O
transmit	O
FIFO	B-Operating_System
,	O
though	O
this	O
feature	O
is	O
less	O
critical	O
as	O
delays	O
in	O
interrupt	B-Application
service	O
would	O
only	O
result	O
in	O
sub-optimal	O
transmission	O
speeds	O
and	O
not	O
actual	O
data	O
loss	O
.	O
</s>
<s>
The	O
16550A(F )	O
version	O
was	O
a	O
must-have	O
to	O
use	O
modems	B-Application
with	O
a	O
data	O
transmit	O
rate	O
of	O
9600baud	O
.	O
</s>
<s>
Dropouts	O
occurred	O
with	O
14.4kbit/s	O
(	O
v.32bis	O
and	O
higher	O
)	O
units	O
and	O
as	O
compression	O
was	O
added	O
with	O
v.42	O
getting	O
more	O
data	O
per	O
interrupt	B-Application
was	O
critical	O
as	O
data	O
speed	O
continued	O
to	O
increase	O
.	O
</s>
