<s>
The	O
S-100	O
bus	O
or	O
Altair	O
bus	O
,	O
IEEE	O
696-1983	O
(	O
withdrawn	O
)	O
,	O
is	O
an	O
early	O
computer	B-General_Concept
bus	I-General_Concept
designed	O
in	O
1974	O
as	O
a	O
part	O
of	O
the	O
Altair	B-Architecture
8800	I-Architecture
.	O
</s>
<s>
The	O
bus	O
was	O
the	O
first	O
industry	O
standard	O
expansion	B-Device
bus	I-Device
for	O
the	O
microcomputer	O
industry	O
.	O
</s>
<s>
The	O
bus	O
formed	O
the	O
basis	O
for	O
homebrew	O
computers	O
whose	O
builders	O
(	O
e.g.	O
,	O
the	O
Homebrew	O
Computer	O
Club	O
)	O
implemented	O
drivers	O
for	O
CP/M	B-Application
and	O
MP/M	B-Operating_System
.	O
</s>
<s>
These	O
microcomputers	O
ran	O
the	O
gamut	O
from	O
hobbyist	O
toy	O
to	O
small	O
business	O
workstation	O
and	O
were	O
common	O
in	O
early	O
home	O
computers	O
until	O
the	O
advent	O
of	O
the	O
IBM	B-Device
PC	I-Device
.	O
</s>
<s>
The	O
bus	O
is	O
a	O
passive	B-Architecture
backplane	I-Architecture
of	O
100-pin	O
printed	O
circuit	O
board	O
edge	O
connectors	O
wired	O
in	O
parallel	O
.	O
</s>
<s>
Circuit	O
cards	O
measuring	O
5	O
×	O
10-inches	O
serving	O
the	O
functions	O
of	O
CPU	B-General_Concept
,	O
memory	O
,	O
or	O
I/O	O
interface	O
plugged	O
into	O
these	O
connectors	O
.	O
</s>
<s>
The	O
bus	O
signal	O
definitions	O
closely	O
follow	O
those	O
of	O
an	O
8080	B-General_Concept
microprocessor	I-General_Concept
system	O
,	O
since	O
the	O
Intel	B-General_Concept
8080	B-General_Concept
microprocessor	I-General_Concept
was	O
the	O
first	O
microprocessor	O
hosted	O
on	O
the	O
bus	O
.	O
</s>
<s>
Power	O
supplied	O
on	O
the	O
bus	O
is	O
bulk	O
unregulated	O
+8	O
Volt	O
DC	O
and	O
±16	O
Volt	O
DC	O
,	O
designed	O
to	O
be	O
regulated	O
on	O
the	O
cards	O
to	O
+5	O
V	O
(	O
used	O
by	O
TTL	B-General_Concept
ICs	O
)	O
,	O
-5	O
V	O
and	O
+12	O
V	O
for	O
Intel	B-General_Concept
8080	I-General_Concept
CPU	I-General_Concept
IC	O
,	O
±12	O
V	O
RS-232	O
line	O
driver	O
ICs	O
,	O
+12	O
V	O
for	O
disk	O
drive	O
motors	O
.	O
</s>
<s>
The	O
bi-directional	O
8-bit	O
data	B-General_Concept
bus	I-General_Concept
of	O
the	O
Intel	B-General_Concept
8080	I-General_Concept
is	O
split	O
into	O
two	O
unidirectional	O
8-bit	O
data	B-General_Concept
buses	I-General_Concept
.	O
</s>
<s>
The	O
Sol-20	B-Architecture
used	O
a	O
variation	O
that	O
had	O
only	O
a	O
single	O
8-bit	O
bus	O
and	O
used	O
the	O
now-unused	O
pins	O
as	O
signal	O
grounds	O
to	O
reduce	O
electronic	O
noise	O
.	O
</s>
<s>
A	O
bus	O
control	O
signal	O
can	O
put	O
these	O
lines	O
in	O
a	O
tri-state	B-Device
condition	O
to	O
allow	O
direct	O
memory	O
access	O
.	O
</s>
<s>
The	O
Cromemco	B-Device
Dazzler	I-Device
,	O
for	O
example	O
,	O
is	O
an	O
early	O
card	O
that	O
retrieved	O
digital	O
images	O
from	O
memory	O
using	O
direct	O
memory	O
access	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Zilog	B-General_Concept
Z-80	I-General_Concept
processor	O
has	O
a	O
non-maskable	B-General_Concept
interrupt	I-General_Concept
line	O
that	O
the	O
Intel	B-General_Concept
8080	I-General_Concept
processor	O
does	O
not	O
.	O
</s>
<s>
One	O
unassigned	O
line	O
of	O
the	O
bus	O
then	O
was	O
reassigned	O
to	O
support	O
the	O
non-maskable	B-General_Concept
interrupt	I-General_Concept
request	O
.	O
</s>
<s>
The	O
designer	O
,	O
Ed	O
Roberts	O
,	O
also	O
had	O
the	O
problem	O
of	O
the	O
backplane	B-Architecture
taking	O
up	O
too	O
much	O
room	O
.	O
</s>
<s>
The	O
backplane	B-Architecture
is	O
split	O
into	O
four	O
separate	O
cards	O
,	O
with	O
the	O
CPU	B-General_Concept
on	O
a	O
fifth	O
.	O
</s>
<s>
In	O
May	O
1984	O
,	O
Microsystems	O
published	O
a	O
comprehensive	O
product	O
directory	O
listing	O
over	O
500	O
"	O
/IEEE	O
-696	O
"	O
products	O
from	O
over	O
150	O
companies	O
.	O
</s>
<s>
The	O
bus	O
signals	O
were	O
simple	O
to	O
create	O
using	O
an	O
8080	B-General_Concept
CPU	B-General_Concept
,	O
but	O
increasingly	O
less	O
so	O
when	O
using	O
other	O
processors	O
like	O
the	O
68000	O
.	O
</s>
<s>
Nonetheless	O
by	O
1984	O
,	O
eleven	O
different	O
processors	O
were	O
hosted	O
on	O
the	O
bus	O
,	O
from	O
the	O
8-bit	O
Intel	B-General_Concept
8080	I-General_Concept
to	O
the	O
16-bit	O
Zilog	B-Device
Z-8000	I-Device
.	O
</s>
<s>
In	O
1986	O
,	O
Cromemco	O
introduced	O
the	O
XXU	O
card	O
,	O
designed	O
by	O
Ed	O
Lupin	O
,	O
utilizing	O
a	O
32-bit	O
Motorola	B-Device
68020	I-Device
processor	O
.	O
</s>
<s>
There	O
was	O
also	O
a	O
need	O
to	O
extend	O
the	O
bus	O
so	O
that	O
it	O
could	O
support	O
processors	O
more	O
capable	O
than	O
the	O
Intel	B-General_Concept
8080	I-General_Concept
used	O
in	O
the	O
original	O
Altair	O
Computer	O
.	O
</s>
<s>
The	O
computer	B-General_Concept
bus	I-General_Concept
structure	O
developed	O
by	O
Ed	O
Roberts	O
for	O
the	O
Altair	B-Architecture
8800	I-Architecture
computer	O
had	O
been	O
extended	O
,	O
rigorously	O
documented	O
,	O
and	O
now	O
designated	O
as	O
the	O
American	O
National	O
Standard	O
IEEE	O
Std	O
696	O
–	O
1983	O
.	O
</s>
<s>
IBM	O
introduced	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
in	O
1981	O
and	O
followed	O
it	O
with	O
increasingly	O
capable	O
models	O
:	O
the	O
XT	O
in	O
1983	O
and	O
the	O
AT	O
in	O
1984	O
.	O
</s>
<s>
In	O
May	O
1984	O
,	O
Sol	O
Libes	O
(	O
who	O
had	O
been	O
a	O
member	O
of	O
the	O
IEEE-696	B-Architecture
Working	O
Group	O
)	O
wrote	O
in	O
Microsystems	O
:	O
"	O
there	O
is	O
no	O
doubt	O
that	O
the	O
S-100	O
market	O
can	O
now	O
be	O
considered	O
a	O
mature	O
industry	O
with	O
only	O
moderate	O
growth	O
potential	O
,	O
compared	O
to	O
the	O
IBM	O
PC-compatible	O
market	O
"	O
.	O
</s>
<s>
As	O
the	O
IBM	B-Device
PC	I-Device
products	O
captured	O
the	O
low-end	O
of	O
the	O
market	O
,	O
machines	O
moved	O
up-scale	O
to	O
more	O
powerful	O
OEM	O
and	O
multiuser	O
systems	O
.	O
</s>
<s>
In	O
1992	O
,	O
the	O
Chicago	O
Mercantile	O
Exchange	O
,	O
for	O
example	O
,	O
replaced	O
their	O
bus	O
computers	O
with	O
the	O
IBM	B-Device
model	I-Device
PS/2	I-Device
.	O
</s>
<s>
By	O
1994	O
the	O
bus	O
industry	O
had	O
contracted	O
sufficiently	O
that	O
the	O
IEEE	O
did	O
not	O
see	O
a	O
need	O
to	O
continue	O
supporting	O
the	O
IEEE-696	B-Architecture
standard	O
.	O
</s>
<s>
The	O
IEEE-696	B-Architecture
standard	O
was	O
retired	O
on	O
June	O
14	O
,	O
1994	O
.	O
</s>
