<s>
Plug	B-Device
compatible	I-Device
refers	O
to	O
"	O
hardware	B-Architecture
that	O
is	O
designed	O
to	O
perform	O
exactly	O
like	O
another	O
vendor	O
's	O
product.	O
"	O
</s>
<s>
The	O
term	O
PCM	O
was	O
originally	O
applied	O
to	O
manufacturers	O
who	O
made	O
replacements	O
for	O
IBM	O
peripherals	B-Device
.	O
</s>
<s>
Before	O
the	O
rise	O
of	O
the	O
PCM	O
peripheral	O
industry	O
,	O
computing	O
systems	O
were	O
either	O
configured	O
with	O
peripherals	B-Device
designed	O
and	O
built	O
by	O
the	O
CPU	O
vendor	O
,	O
or	O
designed	O
to	O
use	O
vendor-selected	O
rebadged	O
devices	O
.	O
</s>
<s>
The	O
first	O
example	O
of	O
plug	B-Device
compatible	I-Device
IBM	O
subsystems	O
were	O
tape	O
drives	O
and	O
controls	O
offered	O
by	O
Telex	O
beginning	O
1965	O
.	O
</s>
<s>
Memorex	O
in	O
1968	O
was	O
first	O
to	O
enter	O
the	O
IBM	O
plug-compatible	B-Device
disk	O
followed	O
shortly	O
thereafter	O
by	O
a	O
number	O
of	O
suppliers	O
such	O
as	O
CDC	O
,	O
Itel	O
,	O
and	O
Storage	B-Application
Technology	I-Application
Corporation	I-Application
.	O
</s>
<s>
Ultimately	O
plug-compatible	B-Device
products	O
were	O
offered	O
for	O
most	O
peripherals	B-Device
and	O
system	O
main	O
memory	O
.	O
</s>
<s>
A	O
plug-compatible	B-Device
machine	O
is	O
one	O
that	O
has	O
been	O
designed	O
to	O
be	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
a	O
prior	O
machine	O
.	O
</s>
<s>
In	O
particular	O
,	O
a	O
new	O
computer	O
system	O
that	O
is	O
plug-compatible	B-Device
has	O
not	O
only	O
the	O
same	O
connectors	O
and	O
protocol	O
interfaces	O
to	O
peripherals	B-Device
,	O
but	O
also	O
binary-code	B-General_Concept
compatibility	I-General_Concept
—	O
it	O
runs	O
the	O
same	O
software	O
as	O
the	O
old	O
system	O
.	O
</s>
<s>
A	O
plug	B-Device
compatible	I-Device
manufacturer	I-Device
or	O
PCM	O
is	O
a	O
company	O
that	O
makes	O
such	O
products	O
.	O
</s>
<s>
One	O
recurring	O
theme	O
in	O
plug-compatible	B-Device
systems	O
is	O
the	O
ability	O
to	O
be	O
bug	B-General_Concept
compatible	I-General_Concept
as	O
well	O
.	O
</s>
<s>
Otherwise	O
,	O
the	O
new	O
system	O
may	O
generate	O
unpredictable	O
results	O
,	O
defeating	O
the	O
full	O
compatibility	B-Device
objective	O
.	O
</s>
<s>
The	O
original	O
example	O
of	O
PCM	O
mainframes	B-Architecture
was	O
the	O
Amdahl	O
470	O
mainframe	B-Architecture
computer	I-Architecture
which	O
was	O
plug-compatible	B-Device
with	O
the	O
IBM	B-Application
System	I-Application
360	I-Application
and	O
370	B-Device
,	O
costing	O
millions	O
of	O
dollars	O
to	O
develop	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
plug-compatible	B-Device
cooling	O
fan	O
may	O
need	O
to	O
have	O
not	O
only	O
the	O
same	O
physical	O
size	O
and	O
shape	O
,	O
but	O
also	O
similar	O
capability	O
,	O
run	O
from	O
the	O
same	O
voltage	O
,	O
use	O
similar	O
power	O
,	O
attach	O
with	O
a	O
standard	O
electrical	O
connector	O
,	O
and	O
have	O
similar	O
mounting	O
arrangements	O
.	O
</s>
<s>
Some	O
non-conforming	O
units	O
may	O
be	O
re-packaged	O
or	O
modified	O
to	O
meet	O
plug-compatible	B-Device
requirements	O
,	O
as	O
where	O
an	O
adapter	O
plate	O
is	O
provided	O
for	O
mounting	O
,	O
or	O
a	O
different	O
tool	O
and	O
instructions	O
are	O
supplied	O
for	O
installation	O
,	O
and	O
these	O
modifications	O
would	O
be	O
reflected	O
in	O
the	O
bill	O
of	O
materials	O
for	O
such	O
components	O
.	O
</s>
<s>
In	O
general	O
,	O
plug-compatible	B-Device
systems	O
are	O
designed	O
where	O
industry	O
or	O
de	O
facto	O
standards	O
have	O
rigorously	O
defined	O
the	O
environment	O
,	O
and	O
there	O
is	O
a	O
large	O
installed	O
population	O
of	O
machines	O
that	O
can	O
benefit	O
from	O
third-party	O
enhancements	O
.	O
</s>
<s>
Plug	B-Device
compatible	I-Device
does	O
not	O
mean	O
identical	O
replacement	O
.	O
</s>
<s>
However	O
,	O
nothing	O
prevents	O
a	O
company	O
from	O
developing	O
follow-on	O
products	O
that	O
are	O
backward-compatible	B-General_Concept
with	O
its	O
own	O
early	O
products	O
.	O
</s>
