<s>
The	O
Serial	O
Input/Output	B-General_Concept
system	O
,	O
universally	O
known	O
as	O
SIO	O
,	O
was	O
a	O
proprietary	O
peripheral	B-Architecture
bus	I-Architecture
and	O
related	O
software	O
protocol	B-Protocol
stacks	I-Protocol
used	O
on	O
the	O
Atari	B-Device
8-bit	I-Device
family	I-Device
to	O
provide	O
most	O
input/output	B-General_Concept
duties	O
for	O
those	O
computers	O
.	O
</s>
<s>
Unlike	O
most	O
I/O	B-General_Concept
systems	O
of	O
the	O
era	O
,	O
such	O
as	O
RS-232	O
,	O
SIO	O
included	O
a	O
lightweight	O
protocol	O
that	O
allowed	O
multiple	O
devices	O
to	O
be	O
attached	O
to	O
a	O
single	O
daisy-chained	B-Application
port	O
that	O
supported	O
dozens	O
of	O
devices	O
.	O
</s>
<s>
It	O
also	O
supported	O
plug-and-play	B-Device
operations	O
.	O
</s>
<s>
SIO	O
's	O
designer	O
,	O
Joe	O
Decuir	O
,	O
credits	O
his	O
work	O
on	O
the	O
system	O
as	O
the	O
basis	O
of	O
USB	B-Protocol
.	O
</s>
<s>
SIO	O
was	O
developed	O
in	O
order	O
to	O
allow	O
expansion	O
without	O
using	O
internal	O
card	O
slots	O
as	O
in	O
the	O
Apple	B-Device
II	I-Device
,	O
due	O
to	O
problems	O
with	O
the	O
FCC	O
over	O
radio	O
interference	O
.	O
</s>
<s>
Devices	O
that	O
used	O
the	O
SIO	O
interface	O
included	O
printers	O
,	O
floppy	B-Device
disk	I-Device
drives	I-Device
,	O
cassette	B-Device
decks	O
,	O
modems	O
and	O
expansion	O
boxes	O
.	O
</s>
<s>
SIO	O
required	O
logic	O
in	O
the	O
peripherals	O
to	O
support	O
the	O
protocols	O
,	O
and	O
in	O
some	O
cases	O
a	O
significant	O
amount	O
of	O
processing	O
power	O
was	O
required	O
-	O
the	O
Atari	O
810	O
floppy	B-Device
disk	I-Device
drive	I-Device
included	O
a	O
MOS	B-General_Concept
Technology	I-General_Concept
6507	I-General_Concept
for	O
instance	O
.	O
</s>
<s>
The	O
name	O
"	O
SIO	O
"	O
properly	O
refers	O
only	O
to	O
the	O
sections	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
that	O
handled	O
the	O
data	O
exchange	O
,	O
in	O
Atari	O
documentation	O
the	O
bus	O
itself	O
is	O
simply	O
the	O
"	O
serial	O
bus	O
"	O
or	O
"	O
interface	O
bus	O
"	O
,	O
although	O
this	O
is	O
also	O
sometimes	O
referred	O
to	O
as	O
SIO	O
.	O
</s>
<s>
In	O
common	O
usage	O
,	O
SIO	O
refers	O
to	O
the	O
entire	O
system	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
the	O
bus	O
and	O
even	O
the	O
physical	O
connectors	O
.	O
</s>
<s>
These	O
rules	O
were	O
undergoing	O
revisions	O
during	O
the	O
period	O
when	O
Atari	O
's	O
Grass	O
Valley	O
group	O
was	O
designing	O
the	O
Colleen	O
machine	O
that	O
would	O
become	O
the	O
Atari	B-Device
800	I-Device
.	O
</s>
<s>
The	O
Apple	B-Device
II	I-Device
,	O
one	O
of	O
the	O
few	O
pre-built	O
machines	O
that	O
connected	O
to	O
a	O
television	O
in	O
that	O
era	O
,	O
had	O
avoided	O
this	O
problem	O
by	O
not	O
including	O
the	O
RF	O
modulator	O
in	O
the	O
computer	O
.	O
</s>
<s>
Colleen	O
was	O
originally	O
intended	O
to	O
be	O
a	O
game	O
console	O
,	O
the	O
successor	O
to	O
the	O
Atari	B-General_Concept
2600	I-General_Concept
.	O
</s>
<s>
The	O
success	O
of	O
the	O
Apple	B-Device
II	I-Device
led	O
to	O
the	O
system	O
being	O
repositioned	O
as	O
a	O
home	O
computer	O
,	O
and	O
this	O
market	O
required	O
peripheral	O
devices	O
.	O
</s>
<s>
On	O
machines	O
like	O
the	O
Apple	B-Device
II	I-Device
,	O
peripherals	O
were	O
supported	O
by	O
placing	O
an	O
adapter	B-Device
card	I-Device
in	O
one	O
of	O
the	O
machine	O
's	O
internal	O
card	O
slots	O
,	O
running	O
a	O
cable	O
through	O
a	O
hole	O
in	O
the	O
case	O
,	O
and	O
connecting	O
the	O
device	O
to	O
that	O
cable	O
.	O
</s>
<s>
During	O
a	O
visit	O
in	O
early	O
1978	O
,	O
a	O
Texas	O
Instruments	O
(	O
TI	O
)	O
salesman	O
demonstrated	O
a	O
system	O
consisting	O
of	O
a	O
fibre	B-Architecture
optic	I-Architecture
cable	O
with	O
transceivers	O
molded	O
into	O
both	O
ends	O
.	O
</s>
<s>
Now	O
the	O
computer	O
could	O
have	O
normal	O
slots	O
;	O
like	O
the	O
Apple	B-Device
II	I-Device
,	O
the	O
RF	O
portion	O
would	O
be	O
entirely	O
external	O
and	O
could	O
be	O
tested	O
on	O
its	O
own	O
separately	O
from	O
the	O
computer	O
.	O
</s>
<s>
Unknown	O
to	O
the	O
Grass	O
Valley	O
team	O
,	O
TI	O
was	O
at	O
that	O
time	O
in	O
the	O
midst	O
of	O
developing	O
the	O
TI-99/4	B-Device
and	O
was	O
facing	O
the	O
same	O
problem	O
with	O
RF	O
output	O
.	O
</s>
<s>
By	O
this	O
time	O
,	O
considerable	O
work	O
had	O
been	O
carried	O
out	O
on	O
using	O
the	O
Atari	O
's	O
POKEY	B-Device
chip	O
to	O
run	O
a	O
cassette	B-Device
deck	I-Device
by	O
directly	O
outputting	O
sounds	O
that	O
would	O
be	O
recorded	O
to	O
the	O
tape	O
.	O
</s>
<s>
It	O
was	O
realized	O
that	O
,	O
with	O
suitable	O
modifications	O
,	O
the	O
POKEY	B-Device
could	O
bypass	O
digital-to-analog	O
conversion	O
hardware	O
and	O
drive	O
TTL	B-General_Concept
output	O
directly	O
.	O
</s>
<s>
To	O
produce	O
a	O
TTL	B-General_Concept
digital	O
bus	O
,	O
the	O
SIO	O
system	O
used	O
two	O
of	O
the	O
POKEY	B-Device
's	O
four	O
sound	O
channels	O
to	O
produce	O
steady	O
tones	O
that	O
represented	O
clock	O
signals	O
of	O
a	O
given	O
frequency	O
.	O
</s>
<s>
When	O
all	O
eight	O
bits	O
were	O
read	O
or	O
written	O
,	O
the	O
system	O
generated	O
an	O
interrupt	O
that	O
triggered	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
read	O
or	O
write	O
more	O
data	O
.	O
</s>
<s>
Unlike	O
a	O
cassette	B-Device
interface	O
,	O
where	O
only	O
a	O
single	O
device	O
would	O
normally	O
be	O
used	O
,	O
an	O
external	O
expansion	B-Device
port	I-Device
would	O
need	O
to	O
be	O
able	O
to	O
support	O
more	O
than	O
one	O
device	O
.	O
</s>
<s>
To	O
support	O
this	O
,	O
a	O
simple	O
protocol	O
was	O
developed	O
and	O
several	O
new	O
pins	O
added	O
to	O
the	O
original	O
simple	O
cassette	B-Device
port	O
.	O
</s>
<s>
Likewise	O
,	O
the	O
timing	O
signals	O
generated	O
by	O
the	O
POKEY	B-Device
were	O
sent	O
on	O
the	O
CLOCKOUT	O
and	O
CLOCKIN	O
pins	O
,	O
although	O
the	O
asynchronous	O
protocol	O
did	O
not	O
use	O
these	O
.	O
</s>
<s>
The	O
SIO	O
bus	O
was	O
implemented	O
using	O
a	O
custom	O
13-pin	O
D-connector	B-Protocol
arrangement	O
(	O
although	O
not	O
D-subminiature	B-Protocol
)	O
with	O
the	O
male	O
connectors	O
on	O
the	O
devices	O
and	O
the	O
female	O
connectors	O
on	O
either	O
end	O
of	O
the	O
cables	O
.	O
</s>
<s>
The	O
connectors	O
were	O
physically	O
robust	O
to	O
allow	O
repeated	O
use	O
,	O
with	O
very	O
strong	O
pins	O
in	O
the	O
device	O
socket	O
and	O
sprung	O
connectors	O
in	O
the	O
cables	O
,	O
as	O
opposed	O
to	O
friction	O
fit	O
as	O
in	O
a	O
typical	O
D-connector	B-Protocol
.	O
</s>
<s>
SIO	O
was	O
controlled	O
by	O
the	O
Atari	O
's	O
POKEY	B-Device
chip	O
,	O
which	O
included	O
a	O
number	O
of	O
general	O
purpose	O
timers	O
.	O
</s>
<s>
These	O
were	O
"	O
shadow	O
registers	O
"	O
,	O
locations	O
in	O
the	O
RAM	B-Architecture
that	O
mirrored	O
registers	O
in	O
the	O
various	O
support	O
chips	O
like	O
POKEY	B-Device
.	O
</s>
<s>
To	O
write	O
data	O
in	O
synchronous	O
mode	O
,	O
the	O
POKEY	B-Device
's	O
main	O
timer	O
channels	O
were	O
set	O
to	O
an	O
appropriate	O
clock	O
rate	O
,	O
say	O
9600bit/s	O
.	O
</s>
<s>
When	O
all	O
10	O
bits	O
(	O
including	O
the	O
start	O
and	O
stop	O
)	O
had	O
been	O
sent	O
,	O
POKEY	B-Device
sent	O
a	O
maskable	O
interrupt	O
to	O
the	O
CPU	O
to	O
indicate	O
it	O
was	O
ready	O
for	O
another	O
byte	O
.	O
</s>
<s>
The	O
system	O
officially	O
supported	O
speeds	O
up	O
to	O
19,200	O
bit/s	O
,	O
but	O
this	O
rate	O
was	O
chosen	O
only	O
because	O
the	O
Atari	O
engineer	O
's	O
protocol	B-Protocol
analyzer	I-Protocol
topped	O
out	O
at	O
that	O
speed	O
.	O
</s>
<s>
A	O
number	O
of	O
3rd	O
party	O
devices	O
,	O
especially	O
floppy	B-Device
drives	I-Device
,	O
used	O
custom	O
hardware	O
and	O
drivers	O
to	O
greatly	O
increase	O
the	O
transmission	O
speeds	O
to	O
as	O
much	O
as	O
72,000	O
bit/s	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
a	O
base	O
speed	O
was	O
set	O
in	O
as	O
above	O
in	O
the	O
POKEY	B-Device
,	O
which	O
would	O
follow	O
changes	O
of	O
up	O
to	O
5%	O
from	O
this	O
base	O
rate	O
.	O
</s>
<s>
One	O
example	O
was	O
the	O
cassette	B-Device
deck	I-Device
,	O
where	O
tape	O
stretch	O
could	O
alter	O
the	O
speed	O
,	O
another	O
is	O
a	O
modem	O
,	O
there	O
the	O
remote	O
system	O
may	O
not	O
be	O
exactly	O
clocked	O
to	O
a	O
given	O
speed	O
.	O
</s>
<s>
Design	O
of	O
what	O
became	O
the	O
SIO	O
had	O
started	O
as	O
a	O
system	O
for	O
interfacing	O
to	O
cassette	B-Device
recorders	O
using	O
the	O
sound	O
hardware	O
to	O
generate	O
the	O
appropriate	O
tones	O
.	O
</s>
<s>
When	O
set	O
to	O
operate	O
the	O
cassette	B-Device
,	O
the	O
outputs	O
from	O
channel	O
1	O
and	O
2	O
of	O
the	O
POKEY	B-Device
were	O
sent	O
to	O
the	O
DATAOUT	O
rather	O
than	O
the	O
clock	O
pins	O
.	O
</s>
<s>
The	O
two	O
channels	O
were	O
set	O
to	O
produce	O
tones	O
that	O
were	O
safe	O
to	O
record	O
on	O
the	O
tape	O
,	O
3995Hz	O
for	O
a	O
zero	O
was	O
in	O
POKEY	B-Device
channel	O
2	O
and	O
5326Hz	O
for	O
a	O
one	O
was	O
in	O
channel	O
1	O
.	O
</s>
<s>
In	O
this	O
mode	O
,	O
when	O
the	O
POKEY	B-Device
read	O
bits	O
from	O
the	O
SERIN	O
,	O
any	O
1	O
's	O
resulted	O
in	O
channel	O
1	O
playing	O
into	O
the	O
data	O
pin	O
,	O
and	O
0	O
's	O
played	O
channel	O
2	O
.	O
</s>
<s>
Instead	O
,	O
the	O
cassette	B-Device
decks	O
included	O
two	O
narrow-band	O
filters	O
tuned	O
to	O
the	O
two	O
frequencies	O
.	O
</s>
<s>
An	O
inter-record	O
gap	O
between	O
the	O
blocks	O
with	O
no	O
tones	O
allowed	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
know	O
when	O
a	O
new	O
record	O
was	O
starting	O
by	O
looking	O
for	O
the	O
leading	O
zero	O
.	O
</s>
<s>
When	O
the	O
appropriate	O
command	O
was	O
entered	O
on	O
the	O
computer	O
,	O
MOTOR	O
would	O
be	O
asserted	O
and	O
the	O
cassette	B-Device
would	O
begin	O
to	O
turn	O
.	O
</s>
<s>
Another	O
dedicated	O
pin	O
was	O
the	O
AUDIOIN	O
,	O
which	O
was	O
connected	O
directly	O
to	O
the	O
sound	O
output	O
circuits	O
between	O
the	O
POKEY	B-Device
's	O
D-to-A	O
converters	O
and	O
the	O
final	O
output	O
so	O
that	O
any	O
signal	O
on	O
the	O
pin	O
mixed	O
with	O
the	O
sound	O
from	O
the	O
POKEY	B-Device
(	O
if	O
any	O
)	O
and	O
was	O
then	O
sent	O
to	O
the	O
television	O
speaker	O
.	O
</s>
<s>
This	O
was	O
connected	O
to	O
the	O
left	O
sound	O
channel	O
in	O
the	O
cassette	B-Device
,	O
while	O
the	O
right	O
channel	O
was	O
connected	O
to	O
the	O
data	O
pins	O
.	O
</s>
