<s>
In	O
computing	O
,	O
a	O
serial	B-Protocol
port	I-Protocol
is	O
a	O
serial	B-Protocol
communication	I-Protocol
interface	B-Application
through	O
which	O
information	O
transfers	O
in	O
or	O
out	O
sequentially	O
one	O
bit	O
at	O
a	O
time	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
a	O
parallel	B-Device
port	I-Device
,	O
which	O
communicates	O
multiple	O
bits	O
simultaneously	O
in	O
parallel	O
.	O
</s>
<s>
Throughout	O
most	O
of	O
the	O
history	O
of	O
personal	B-Device
computers	I-Device
,	O
data	O
has	O
been	O
transferred	O
through	O
serial	B-Protocol
ports	I-Protocol
to	O
devices	O
such	O
as	O
modems	B-Application
,	O
terminals	O
,	O
various	O
peripherals	O
,	O
and	O
directly	O
between	O
computers	O
.	O
</s>
<s>
While	O
interfaces	B-Application
such	O
as	O
Ethernet	O
,	O
FireWire	B-Protocol
,	O
and	O
USB	B-Protocol
also	O
send	O
data	O
as	O
a	O
serial	O
stream	B-Architecture
,	O
the	O
term	O
serial	B-Protocol
port	I-Protocol
usually	O
denotes	O
hardware	B-Architecture
compliant	O
with	O
RS-232	O
or	O
a	O
related	O
standard	O
,	O
such	O
as	O
RS-485	O
or	O
RS-422	O
.	O
</s>
<s>
Modern	O
consumer	O
personal	B-Device
computers	I-Device
(	O
PCs	B-Device
)	O
have	O
largely	O
replaced	O
serial	B-Protocol
ports	I-Protocol
with	O
higher-speed	O
standards	O
,	O
primarily	O
USB	B-Protocol
.	O
</s>
<s>
However	O
,	O
serial	B-Protocol
ports	I-Protocol
are	O
still	O
frequently	O
used	O
in	O
applications	O
demanding	O
simple	O
,	O
low-speed	O
interfaces	B-Application
,	O
such	O
as	O
industrial	O
automation	O
systems	O
,	O
scientific	O
instruments	O
,	O
point	O
of	O
sale	O
systems	O
and	O
some	O
industrial	O
and	O
consumer	O
products	O
.	O
</s>
<s>
Server	B-Application
computers	I-Application
may	O
use	O
a	O
serial	B-Protocol
port	I-Protocol
as	O
a	O
control	O
console	O
for	O
diagnostics	O
,	O
while	O
networking	B-Device
hardware	I-Device
(	O
such	O
as	O
routers	B-Protocol
and	O
switches	B-Protocol
)	O
commonly	O
use	O
serial	O
console	B-Protocol
ports	I-Protocol
for	O
configuration	O
,	O
diagnostics	O
,	O
and	O
emergency	O
maintenance	O
access	O
.	O
</s>
<s>
To	O
interface	B-Application
with	O
these	O
and	O
other	O
devices	O
,	O
USB-to-serial	O
converters	O
can	O
quickly	O
and	O
easily	O
add	O
a	O
serial	B-Protocol
port	I-Protocol
to	O
a	O
modern	O
PC	O
.	O
</s>
<s>
Modern	O
devices	O
use	O
an	O
integrated	O
circuit	O
called	O
a	O
UART	O
to	O
implement	O
a	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
This	O
IC	O
converts	O
characters	O
to	O
and	O
from	O
asynchronous	O
serial	O
form	O
,	O
implementing	O
the	O
timing	O
and	O
framing	O
of	O
data	O
specified	O
by	O
the	O
serial	O
protocol	O
in	O
hardware	B-Architecture
.	O
</s>
<s>
The	O
IBM	B-Device
PC	I-Device
implements	O
its	O
serial	B-Protocol
ports	I-Protocol
,	O
when	O
present	O
,	O
with	O
one	O
or	O
more	O
UARTs	O
.	O
</s>
<s>
Very	O
low-cost	O
systems	O
,	O
such	O
as	O
some	O
early	O
home	O
computers	O
,	O
would	O
instead	O
use	O
the	O
CPU	B-General_Concept
to	O
send	O
the	O
data	O
through	O
an	O
output	B-General_Concept
pin	O
,	O
using	O
the	O
bit	B-Algorithm
banging	I-Algorithm
technique	O
.	O
</s>
<s>
These	O
early	O
home	O
computers	O
often	O
had	O
proprietary	O
serial	B-Protocol
ports	I-Protocol
with	O
pinouts	O
and	O
voltage	O
levels	O
incompatible	O
with	O
RS-232	O
.	O
</s>
<s>
Before	O
large-scale	O
integration	O
(	O
LSI	O
)	O
made	O
UARTs	O
common	O
,	O
serial	B-Protocol
ports	I-Protocol
were	O
commonly	O
used	O
in	O
mainframes	B-Architecture
and	O
minicomputers	B-Architecture
,	O
which	O
would	O
have	O
multiple	O
small-scale	O
integrated	O
circuits	O
to	O
implement	O
shift	O
registers	O
,	O
logic	O
gates	O
,	O
counters	O
,	O
and	O
all	O
the	O
other	O
logic	O
needed	O
.	O
</s>
<s>
As	O
PCs	B-Device
evolved	O
serial	B-Protocol
ports	I-Protocol
were	O
included	O
in	O
the	O
Super	B-Device
I/O	I-Device
chip	O
and	O
then	O
in	O
the	O
chipset	B-Device
.	O
</s>
<s>
The	O
individual	O
signals	O
on	O
a	O
serial	B-Protocol
port	I-Protocol
are	O
unidirectional	O
and	O
when	O
connecting	O
two	O
devices	O
,	O
the	O
outputs	O
of	O
one	O
device	O
must	O
be	O
connected	O
to	O
the	O
inputs	O
of	O
the	O
other	O
.	O
</s>
<s>
Devices	O
are	O
divided	O
into	O
two	O
categories	O
:	O
data	B-General_Concept
terminal	I-General_Concept
equipment	O
(	O
DTE	O
)	O
and	O
data	O
circuit-terminating	O
equipment	O
(	O
DCE	O
)	O
.	O
</s>
<s>
A	O
line	O
that	O
is	O
an	O
output	B-General_Concept
on	O
a	O
DTE	O
device	O
is	O
an	O
input	O
on	O
a	O
DCE	O
device	O
and	O
vice	O
versa	O
,	O
so	O
a	O
DCE	O
device	O
can	O
be	O
connected	O
to	O
a	O
DTE	O
device	O
with	O
a	O
straight	O
wired	O
cable	O
,	O
in	O
which	O
each	O
pin	O
on	O
one	O
end	O
goes	O
to	O
the	O
same	O
numbered	O
pin	O
on	O
the	O
other	O
end	O
.	O
</s>
<s>
Conventionally	O
,	O
computers	O
and	O
terminals	O
are	O
DTE	O
,	O
while	O
peripherals	O
such	O
as	O
modems	B-Application
are	O
DCE	O
.	O
</s>
<s>
If	O
it	O
is	O
necessary	O
to	O
connect	O
two	O
DTE	O
(	O
or	O
DCE	O
)	O
devices	O
together	O
,	O
a	O
cable	O
with	O
reversed	O
TX	O
and	O
RX	O
lines	O
,	O
known	O
as	O
a	O
cross-over	B-Protocol
,	O
roll-over	O
or	O
null	O
modem	B-Application
cable	O
must	O
be	O
used	O
.	O
</s>
<s>
Generally	O
,	O
serial	B-Protocol
port	I-Protocol
connectors	O
are	O
gendered	O
,	O
only	O
allowing	O
connectors	O
to	O
mate	O
with	O
a	O
connector	O
of	O
the	O
opposite	O
gender	O
.	O
</s>
<s>
With	O
D-subminiature	B-Protocol
connectors	I-Protocol
,	O
the	O
male	O
connectors	O
have	O
protruding	O
pins	O
,	O
and	O
female	O
connectors	O
have	O
corresponding	O
round	O
sockets	O
.	O
</s>
<s>
However	O
,	O
this	O
is	O
far	O
from	O
universal	O
;	O
for	O
instance	O
,	O
most	O
serial	O
printers	O
have	O
a	O
female	O
DB25	B-Protocol
connector	O
,	O
but	O
they	O
are	O
DTEs	O
.	O
</s>
<s>
In	O
this	O
circumstance	O
,	O
the	O
appropriately	O
gendered	O
connectors	O
on	O
the	O
cable	O
or	O
a	O
gender	B-Protocol
changer	I-Protocol
can	O
be	O
used	O
to	O
correct	O
the	O
mismatch	O
.	O
</s>
<s>
The	O
only	O
connector	O
specified	O
in	O
the	O
original	O
RS-232	O
standard	O
was	O
the	O
25-pin	O
D-subminiature	B-Protocol
,	O
however	O
,	O
many	O
other	O
connectors	O
have	O
been	O
used	O
to	O
save	O
money	O
or	O
save	O
on	O
physical	O
space	O
,	O
among	O
other	O
reasons	O
.	O
</s>
<s>
The	O
9-pin	O
DE-9	B-Protocol
connector	I-Protocol
has	O
been	O
used	O
by	O
most	O
IBM-compatible	O
PCs	B-Device
since	O
the	O
Serial/Parallel	O
Adapter	O
option	O
for	O
the	O
PC-AT	B-Operating_System
,	O
where	O
the	O
9-pin	O
connector	O
allowed	O
a	O
serial	O
and	O
parallel	B-Device
port	I-Device
to	O
fit	O
on	O
the	O
same	O
card	O
.	O
</s>
<s>
Some	O
miniaturized	O
electronics	O
,	O
particularly	O
graphing	B-Application
calculators	I-Application
and	O
hand-held	O
amateur	O
and	O
two-way	O
radio	O
equipment	O
,	O
have	O
serial	B-Protocol
ports	I-Protocol
using	O
a	O
phone	B-Device
connector	I-Device
,	O
usually	O
the	O
smaller	O
2.5	O
or	O
3.5mm	O
connectors	O
and	O
the	O
most	O
basic	O
3-wire	O
interface	B-Application
—	O
transmit	O
,	O
receive	O
and	O
ground	O
.	O
</s>
<s>
The	O
EIA/TIA	O
-561	O
standard	O
defines	O
a	O
pinout	O
using	O
this	O
connector	O
,	O
while	O
the	O
rollover	O
cable	O
(	O
or	O
Yost	O
standard	O
)	O
is	O
commonly	O
used	O
on	O
Unix	B-Application
computers	O
and	O
network	B-Device
devices	I-Device
,	O
such	O
as	O
equipment	O
from	O
Cisco	O
Systems	O
.	O
</s>
<s>
Many	O
models	O
of	O
Macintosh	B-Device
favor	O
the	O
related	O
RS-422	O
standard	O
,	O
mostly	O
using	O
circular	O
mini-DIN	B-Protocol
connectors	I-Protocol
.	O
</s>
<s>
The	O
Macintosh	B-Device
included	O
a	O
standard	O
set	O
of	O
two	O
ports	O
for	O
connection	O
to	O
a	O
printer	O
and	O
a	O
modem	B-Application
,	O
but	O
some	O
PowerBook	B-Device
laptops	O
had	O
only	O
one	O
combined	O
port	O
to	O
save	O
space	O
.	O
</s>
<s>
Another	O
common	O
connector	O
is	O
a	O
pin	B-Protocol
header	I-Protocol
common	O
on	O
motherboards	O
and	O
add-in	B-Device
cards	I-Device
which	O
is	O
usually	O
converted	O
via	O
a	O
ribbon	O
cable	O
to	O
the	O
more	O
standard	O
9-pin	O
DE-9	B-Protocol
connector	I-Protocol
(	O
and	O
frequently	O
mounted	O
on	O
a	O
free	O
slot	O
plate	O
or	O
other	O
part	O
of	O
the	O
housing	O
)	O
.	O
</s>
<s>
Operating	O
systems	O
usually	O
create	O
symbolic	O
names	O
for	O
the	O
serial	B-Protocol
ports	I-Protocol
of	O
a	O
computer	O
,	O
rather	O
than	O
requiring	O
programs	O
to	O
refer	O
to	O
them	O
by	O
hardware	B-Architecture
address	O
.	O
</s>
<s>
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
usually	O
label	O
the	O
serial	B-Protocol
port	I-Protocol
devices	O
.	O
</s>
<s>
TTY	O
is	O
a	O
common	O
trademark-free	O
abbreviation	O
for	O
teletype	O
,	O
a	O
device	O
commonly	O
attached	O
to	O
early	O
computers	O
 '	O
serial	B-Protocol
ports	I-Protocol
,	O
and	O
represents	O
a	O
string	O
identifying	O
the	O
specific	O
port	O
;	O
the	O
syntax	O
of	O
that	O
string	O
depends	O
on	O
the	O
operating	O
system	O
and	O
the	O
device	O
.	O
</s>
<s>
On	O
Linux	B-Application
,	O
8250/16550	O
UART	O
hardware	B-Architecture
serial	B-Protocol
ports	I-Protocol
are	O
named	O
,	O
USB	B-Device
adapters	I-Device
appear	O
as	O
and	O
various	O
types	O
of	O
virtual	O
serial	B-Protocol
ports	I-Protocol
do	O
not	O
necessarily	O
have	O
names	O
starting	O
with	O
.	O
</s>
<s>
The	O
DOS	B-Device
and	O
Windows	B-Application
environments	O
refer	O
to	O
serial	B-Protocol
ports	I-Protocol
as	O
COM	B-Architecture
ports	I-Architecture
:	O
COM1	B-Architecture
,	O
COM2	O
,..	O
etc	O
.	O
</s>
<s>
This	O
list	O
includes	O
some	O
of	O
the	O
more	O
common	O
devices	O
that	O
are	O
connected	O
to	O
the	O
serial	B-Protocol
port	I-Protocol
on	O
a	O
PC	O
.	O
</s>
<s>
Some	O
of	O
these	O
such	O
as	O
modems	B-Application
and	O
serial	O
mice	O
are	O
falling	O
into	O
disuse	O
while	O
others	O
are	O
readily	O
available	O
.	O
</s>
<s>
Serial	B-Protocol
ports	I-Protocol
are	O
very	O
common	O
on	O
most	O
types	O
of	O
microcontroller	B-Architecture
,	O
where	O
they	O
can	O
be	O
used	O
to	O
communicate	O
with	O
a	O
PC	O
or	O
other	O
serial	O
devices	O
.	O
</s>
<s>
Since	O
the	O
control	O
signals	O
for	O
a	O
serial	B-Protocol
port	I-Protocol
can	O
be	O
driven	O
by	O
any	O
digital	O
signal	O
,	O
some	O
applications	O
used	O
the	O
control	O
lines	O
of	O
a	O
serial	B-Protocol
port	I-Protocol
to	O
monitor	O
external	O
devices	O
,	O
without	O
exchanging	O
serial	O
data	O
.	O
</s>
<s>
At	O
least	O
some	O
Morse	O
code	O
training	O
software	O
used	O
a	O
code	O
key	O
connected	O
to	O
the	O
serial	B-Protocol
port	I-Protocol
to	O
simulate	O
actual	O
code	O
use	O
;	O
the	O
status	O
bits	O
of	O
the	O
serial	B-Protocol
port	I-Protocol
could	O
be	O
sampled	O
very	O
rapidly	O
and	O
at	O
predictable	O
times	O
,	O
making	O
it	O
possible	O
for	O
the	O
software	O
to	O
decipher	O
Morse	O
code	O
.	O
</s>
<s>
In	O
modern	O
serial	B-Protocol
ports	I-Protocol
using	O
a	O
UART	O
integrated	O
circuit	O
,	O
all	O
these	O
settings	O
can	O
be	O
software-controlled	O
.	O
</s>
<s>
Hardware	B-Architecture
from	O
the	O
1980s	O
and	O
earlier	O
may	O
require	O
setting	O
switches	B-Protocol
or	O
jumpers	O
on	O
a	O
circuit	O
board	O
.	O
</s>
<s>
The	O
configuration	O
for	O
serial	B-Protocol
ports	I-Protocol
designed	O
to	O
be	O
connected	O
to	O
a	O
PC	O
has	O
become	O
a	O
de	O
facto	O
standard	O
,	O
usually	O
stated	O
as	O
9600/8	O
-N-1	O
.	O
</s>
<s>
Serial	B-Protocol
ports	I-Protocol
use	O
two-level	O
(	O
binary	O
)	O
signaling	O
,	O
so	O
the	O
data	O
rate	O
in	O
bits	O
per	O
second	O
is	O
equal	O
to	O
the	O
symbol	O
rate	O
in	O
baud	O
.	O
</s>
<s>
A	O
standard	O
series	O
of	O
rates	O
is	O
based	O
on	O
multiples	O
of	O
the	O
rates	O
for	O
electromechanical	O
teleprinters	O
;	O
some	O
serial	B-Protocol
ports	I-Protocol
allow	O
many	O
arbitrary	O
rates	O
to	O
be	O
selected	O
,	O
but	O
the	O
speeds	O
on	O
both	O
sides	O
of	O
the	O
connection	O
must	O
match	O
for	O
data	O
to	O
be	O
received	O
correctly	O
.	O
</s>
<s>
Many	O
of	O
these	O
standard	O
modem	B-Application
baud	O
rates	O
are	O
multiples	O
of	O
either	O
0.9	O
kbps	O
(	O
e.g.	O
,	O
19200	O
,	O
38400	O
,	O
76800	O
)	O
or	O
1.2	O
kbps	O
(	O
e.g.	O
,	O
57600	O
,	O
115200	O
)	O
.	O
</s>
<s>
This	O
is	O
16	O
times	O
the	O
fastest	O
bit	O
rate	O
,	O
and	O
the	O
serial	B-Protocol
port	I-Protocol
circuit	O
can	O
easily	O
divide	O
this	O
down	O
to	O
lower	O
frequencies	O
as	O
required	O
.	O
</s>
<s>
Not	O
all	O
bit	O
rates	O
are	O
possible	O
with	O
all	O
serial	B-Protocol
ports	I-Protocol
.	O
</s>
<s>
Some	O
serial	B-Protocol
port	I-Protocol
implementations	O
can	O
automatically	O
choose	O
a	O
bit	O
rate	O
by	O
observing	O
what	O
a	O
connected	O
device	O
is	O
sending	O
and	O
synchronizing	O
to	O
it	O
.	O
</s>
<s>
The	O
number	O
of	O
data	O
bits	O
in	O
each	O
character	O
can	O
be	O
5	O
(	O
for	O
Baudot	O
code	O
)	O
,	O
6	O
(	O
rarely	O
used	O
)	O
,	O
7	O
(	O
for	O
true	O
ASCII	B-Protocol
)	O
,	O
8	O
(	O
for	O
most	O
kinds	O
of	O
data	O
,	O
as	O
this	O
size	O
matches	O
the	O
size	O
of	O
a	O
byte	B-Application
)	O
,	O
or	O
9	O
(	O
rarely	O
used	O
)	O
.	O
</s>
<s>
Most	O
serial	B-Protocol
communications	I-Protocol
designs	O
send	O
the	O
data	O
bits	O
within	O
each	O
byte	B-Application
least	O
significant	O
bit	O
first	O
.	O
</s>
<s>
Also	O
possible	O
,	O
but	O
rarely	O
used	O
,	O
is	O
most	O
significant	O
bit	O
first	O
;	O
this	O
was	O
used	O
,	O
for	O
example	O
,	O
by	O
the	O
IBM	O
2741	O
printing	O
terminal	B-General_Concept
.	O
</s>
<s>
The	O
order	O
of	O
bits	O
is	O
not	O
usually	O
configurable	O
within	O
the	O
serial	B-Protocol
port	I-Protocol
interface	B-Application
but	O
is	O
defined	O
by	O
the	O
host	O
system	O
.	O
</s>
<s>
To	O
communicate	O
with	O
systems	O
that	O
require	O
a	O
different	O
bit	O
ordering	O
than	O
the	O
local	O
default	O
,	O
local	O
software	O
can	O
re-order	O
the	O
bits	O
within	O
each	O
byte	B-Application
just	O
before	O
sending	O
and	O
just	O
after	O
receiving	O
.	O
</s>
<s>
When	O
parity	O
is	O
used	O
with	O
a	O
serial	B-Protocol
port	I-Protocol
,	O
an	O
extra	O
data	O
bit	O
is	O
sent	O
with	O
each	O
data	O
character	O
,	O
arranged	O
so	O
that	O
the	O
number	O
of	O
1	O
bits	O
in	O
each	O
character	O
,	O
including	O
the	O
parity	O
bit	O
,	O
is	O
always	O
odd	O
or	O
always	O
even	O
.	O
</s>
<s>
If	O
a	O
byte	B-Application
is	O
received	O
with	O
the	O
wrong	O
number	O
of	O
1s	O
,	O
then	O
it	O
must	O
have	O
been	O
corrupted	O
.	O
</s>
<s>
A	O
single	O
parity	O
bit	O
does	O
not	O
allow	O
implementation	O
of	O
error	B-Error_Name
correction	I-Error_Name
on	O
each	O
character	O
,	O
and	O
communication	O
protocols	O
working	O
over	O
serial	O
data	O
links	O
will	O
typically	O
have	O
higher-level	O
mechanisms	O
to	O
ensure	O
data	O
validity	O
and	O
request	O
retransmission	O
of	O
data	O
that	O
has	O
been	O
incorrectly	O
received	O
.	O
</s>
<s>
Aside	O
from	O
uncommon	O
applications	O
that	O
use	O
the	O
last	O
bit	O
(	O
usually	O
the	O
9th	O
)	O
for	O
some	O
form	O
of	O
addressing	O
or	O
special	O
signaling	O
,	O
mark	O
or	O
space	O
parity	O
is	O
uncommon	O
,	O
as	O
it	O
adds	O
no	O
error	B-Error_Name
detection	I-Error_Name
information	O
.	O
</s>
<s>
Odd	O
parity	O
is	O
more	O
useful	O
than	O
even	O
parity	O
since	O
it	O
ensures	O
that	O
at	O
least	O
one	O
state	O
transition	O
occurs	O
in	O
each	O
character	O
,	O
which	O
makes	O
it	O
more	O
reliable	O
at	O
detecting	O
errors	O
like	O
those	O
that	O
could	O
be	O
caused	O
by	O
serial	B-Protocol
port	I-Protocol
speed	O
mismatches	O
.	O
</s>
<s>
The	O
most	O
common	O
parity	O
setting	O
,	O
however	O
,	O
is	O
none	O
,	O
with	O
error	B-Error_Name
detection	I-Error_Name
handled	O
by	O
a	O
communication	O
protocol	O
.	O
</s>
<s>
Stop	O
bits	O
sent	O
at	O
the	O
end	O
of	O
every	O
character	O
allow	O
the	O
receiving	O
signal	O
hardware	B-Architecture
to	O
detect	O
the	O
end	O
of	O
a	O
character	O
and	O
to	O
resynchronize	O
with	O
the	O
character	O
stream	B-Architecture
.	O
</s>
<s>
To	O
cope	O
with	O
this	O
,	O
serial	B-Protocol
lines	I-Protocol
often	O
incorporate	O
a	O
handshaking	B-Protocol
method	O
.	O
</s>
<s>
There	O
are	O
hardware	B-Architecture
and	O
software	O
handshaking	B-Protocol
methods	O
.	O
</s>
<s>
Hardware	B-Architecture
handshaking	B-Protocol
is	O
done	O
with	O
extra	O
signals	O
,	O
often	O
the	O
RS-232	O
RTS/CTS	O
or	O
DTR/DSR	O
signal	O
circuits	O
.	O
</s>
<s>
RTS	O
and	O
CTS	O
are	O
used	O
to	O
control	O
data	O
flow	O
,	O
signaling	O
,	O
for	O
instance	O
,	O
when	O
a	O
buffer	B-General_Concept
is	O
almost	O
full	O
.	O
</s>
<s>
Software	O
handshaking	B-Protocol
is	O
done	O
for	O
example	O
with	O
ASCII	B-Protocol
control	O
characters	O
XON/XOFF	O
to	O
control	O
the	O
flow	O
of	O
data	O
.	O
</s>
<s>
When	O
the	O
receiver	O
's	O
buffers	B-General_Concept
approach	O
capacity	O
,	O
the	O
receiver	O
sends	O
the	O
XOFF	O
character	O
to	O
tell	O
the	O
sender	O
to	O
stop	O
sending	O
data	O
.	O
</s>
<s>
Later	O
,	O
after	O
the	O
receiver	O
has	O
emptied	O
its	O
buffers	B-General_Concept
,	O
it	O
sends	O
an	O
XON	O
character	O
to	O
tell	O
the	O
sender	O
to	O
resume	O
transmission	O
.	O
</s>
<s>
The	O
advantage	O
of	O
hardware	B-Architecture
handshaking	B-Protocol
is	O
that	O
it	O
can	O
be	O
extremely	O
fast	O
,	O
it	O
works	O
independently	O
of	O
imposed	O
meaning	O
such	O
as	O
ASCII	B-Protocol
on	O
the	O
transferred	O
data	O
and	O
it	O
is	O
stateless	B-Application
.	O
</s>
<s>
Its	O
disadvantage	O
is	O
that	O
it	O
requires	O
more	O
hardware	B-Architecture
and	O
cabling	O
,	O
and	O
both	O
ends	O
of	O
the	O
connection	O
must	O
support	O
the	O
hardware	B-Architecture
handshaking	B-Protocol
protocol	O
used	O
.	O
</s>
<s>
The	O
advantage	O
of	O
software	O
handshaking	B-Protocol
is	O
that	O
it	O
can	O
be	O
done	O
with	O
absent	O
or	O
incompatible	O
hardware	B-Architecture
handshaking	B-Protocol
circuits	O
and	O
cabling	O
.	O
</s>
<s>
The	O
former	O
is	O
normally	O
dealt	O
with	O
by	O
the	O
operating	O
system	O
or	O
device	O
driver	O
;	O
the	O
latter	O
normally	O
by	O
ensuring	O
that	O
control	O
codes	O
are	O
escaped	O
(	O
such	O
as	O
in	O
the	O
Kermit	B-Protocol
protocol	I-Protocol
)	O
or	O
omitted	O
by	O
design	O
(	O
such	O
as	O
in	O
ANSI	O
terminal	B-General_Concept
control	O
)	O
.	O
</s>
<s>
If	O
no	O
handshaking	B-Protocol
is	O
employed	O
,	O
an	O
overrun	O
receiver	O
might	O
simply	O
fail	O
to	O
receive	O
data	O
from	O
the	O
transmitter	O
.	O
</s>
<s>
Approaches	O
for	O
preventing	O
this	O
include	O
reducing	O
the	O
speed	O
of	O
the	O
connection	O
so	O
that	O
the	O
receiver	O
can	O
always	O
keep	O
up	O
,	O
increasing	O
the	O
size	O
of	O
buffers	B-General_Concept
so	O
it	O
can	O
keep	O
up	O
averaged	O
over	O
a	O
longer	O
time	O
,	O
using	O
delays	O
after	O
time-consuming	O
operations	O
(	O
e.g.	O
</s>
<s>
in	O
termcap	B-Application
)	O
or	O
employing	O
a	O
mechanism	O
to	O
resend	O
data	O
which	O
has	O
not	O
been	O
received	O
correctly	O
(	O
e.g.	O
</s>
<s>
TCP	B-Protocol
)	O
.	O
</s>
