<s>
A	O
USB-to-serial	B-Device
adapter	I-Device
or	O
simply	O
USB	B-Device
adapter	I-Device
is	O
a	O
type	O
of	O
protocol	O
converter	O
that	O
is	O
used	O
for	O
converting	O
USB	B-Protocol
data	O
signals	O
to	O
and	O
from	O
serial	B-Protocol
communications	I-Protocol
standards	O
(	O
serial	B-Protocol
ports	I-Protocol
)	O
.	O
</s>
<s>
Most	O
commonly	O
the	O
USB	B-Protocol
data	O
signals	O
are	O
converted	O
to	O
either	O
RS-232	O
,	O
RS-485	O
,	O
RS-422	O
,	O
or	O
TTL-level	O
UART	O
serial	O
data	O
.	O
</s>
<s>
The	O
older	O
serial	O
RS-423	O
protocol	O
is	O
rarely	O
used	O
any	O
more	O
,	O
so	O
USB	B-Protocol
to	O
RS423	O
adapters	O
are	O
less	O
common	O
.	O
</s>
<s>
USB	B-Protocol
to	O
serial	O
RS-232	O
adapters	O
are	O
often	O
used	O
with	O
consumer	O
,	O
commercial	O
and	O
industrial	O
applications	O
and	O
USB-to-serial	O
RS-485/422	O
adapters	O
are	O
usually	O
mainly	O
used	O
only	O
with	O
industrial	O
applications	O
.	O
</s>
<s>
Currently	O
,	O
USB	B-Protocol
to	O
TTL-level	O
UART	O
converters	O
are	O
used	O
extensively	O
by	O
students	O
and	O
hobbyist	O
as	O
they	O
can	O
be	O
directly	O
interfaced	O
to	O
microcontrollers	O
.	O
</s>
<s>
Adapters	O
for	O
converting	O
USB	B-Protocol
to	O
other	O
standard	O
or	O
proprietary	O
protocols	O
also	O
exist	O
;	O
however	O
,	O
these	O
are	O
usually	O
not	O
referred	O
to	O
as	O
a	O
serial	O
adapter	O
.	O
</s>
<s>
The	O
primary	O
application	O
scenario	O
is	O
to	O
enable	O
USB-based	O
computers	O
to	O
access	O
and	O
communicate	O
with	O
serial	O
devices	O
featuring	O
D-sub	B-Protocol
(	O
usually	O
DB9	O
or	O
DB25	B-Protocol
)	O
connectors	O
or	O
screw	O
terminals	O
,	O
where	O
security	O
of	O
the	O
data	O
transmission	O
is	O
not	O
generally	O
an	O
issue	O
.	O
</s>
<s>
USB	B-Protocol
serial	O
adapters	O
can	O
be	O
isolated	O
or	O
non-isolated	O
.	O
</s>
<s>
Historically	O
,	O
most	O
personal	O
computers	O
had	O
a	O
built-in	O
D-sub	B-Protocol
serial	O
RS-232	O
port	O
,	O
also	O
referred	O
to	O
as	O
a	O
COM	B-Protocol
port	I-Protocol
,	O
which	O
could	O
be	O
used	O
for	O
connecting	O
the	O
computer	O
to	O
most	O
types	O
of	O
serial	O
RS-232	O
devices	O
.	O
</s>
<s>
By	O
the	O
late	O
'	O
90s	O
,	O
many	O
computer	O
manufacturers	O
started	O
to	O
phase	O
out	O
the	O
serial	O
COM	B-Protocol
port	I-Protocol
in	O
favor	O
of	O
the	O
USB	B-Protocol
port	I-Protocol
(	O
in	O
part	O
dictated	O
from	O
the	O
PC97	O
architecture	O
,	O
from	O
Microsoft	O
)	O
.	O
</s>
<s>
By	O
the	O
mid-2000s	O
,	O
some	O
computers	O
had	O
both	O
a	O
serial	O
COM	B-Protocol
port	I-Protocol
and	O
a	O
USB	B-Protocol
port	I-Protocol
;	O
however	O
,	O
many	O
no	O
longer	O
had	O
a	O
serial	O
COM	B-Protocol
port	I-Protocol
by	O
that	O
time	O
,	O
and	O
today	O
most	O
modern	O
computers	O
have	O
no	O
serial	O
COM	B-Protocol
port	I-Protocol
but	O
instead	O
only	O
USB	B-Protocol
ports	I-Protocol
.	O
</s>
<s>
Since	O
many	O
serial	O
devices	O
with	O
a	O
RS-232	O
,	O
RS-485	O
or	O
RS-422	O
port	O
are	O
still	O
in	O
use	O
and	O
even	O
still	O
produced	O
today	O
,	O
the	O
disappearance	O
of	O
the	O
serial	O
COM	B-Protocol
port	I-Protocol
from	O
personal	O
computers	O
has	O
created	O
a	O
need	O
for	O
the	O
USB	B-Device
to	I-Device
serial	I-Device
adapter	I-Device
.	O
</s>
<s>
As	O
a	O
simplified	O
example	O
a	O
typical	O
standard	O
USB	B-Device
to	I-Device
serial	I-Device
adapter	I-Device
consists	O
of	O
a	O
USB	B-Protocol
processor	O
chip	O
which	O
processes	O
the	O
USB	B-Protocol
signals	O
.	O
</s>
<s>
The	O
USB	B-Protocol
processor	O
sends	O
the	O
processed	O
USB	B-Protocol
signals	O
to	O
a	O
serial	O
driver	O
chip	O
which	O
applies	O
the	O
correct	O
voltages	O
and	O
sends	O
the	O
processed	O
data	O
signals	O
to	O
the	O
serial	O
output	O
.	O
</s>
<s>
When	O
the	O
USB	B-Device
to	I-Device
serial	I-Device
adapter	I-Device
is	O
connected	O
to	O
the	O
computer	O
via	O
the	O
USB	B-Protocol
port	I-Protocol
the	O
driver	O
on	O
the	O
computer	O
creates	O
a	O
virtual	O
COM	B-Protocol
port	I-Protocol
which	O
shows	O
up	O
in	O
Device	O
Manager	O
on	O
Windows	O
,	O
and	O
under	O
/dev	O
on	O
Linux	O
and	O
MacOS	O
.	O
</s>
<s>
This	O
virtual	O
COM	B-Protocol
port	I-Protocol
can	O
be	O
accessed	O
and	O
used	O
as	O
if	O
it	O
was	O
a	O
built-in	O
serial	O
COM	B-Protocol
port	I-Protocol
.	O
</s>
<s>
However	O
,	O
the	O
characteristics	O
of	O
the	O
virtual	O
COM	B-Protocol
port	I-Protocol
are	O
not	O
exactly	O
the	O
same	O
as	O
a	O
real	O
internal	O
COM	B-Protocol
port	I-Protocol
,	O
mainly	O
due	O
to	O
data	O
latency	O
;	O
which	O
means	O
that	O
if	O
very	O
sensitive	O
and	O
precise	O
data	O
transfer	O
is	O
required	O
,	O
the	O
USB	B-Device
to	I-Device
serial	I-Device
adapter	I-Device
might	O
be	O
unreliable	O
and	O
not	O
a	O
desired	O
solution	O
(	O
the	O
USB	B-Protocol
bus	O
is	O
mainly	O
a	O
polled	O
data	O
bus	O
)	O
.	O
</s>
