<s>
The	O
Cromemco	B-Device
4FDC	I-Device
Floppy	B-Device
Disk	I-Device
Controller	I-Device
is	O
designed	O
to	O
interface	O
both	O
5.25	O
-	O
and	O
8.0-inch	O
floppy	B-Device
disk	I-Device
drives	I-Device
to	O
the	O
S-100	B-Architecture
computer	I-Architecture
bus	I-Architecture
used	O
in	O
Cromemco	O
and	O
other	O
IEEE	B-Architecture
696	I-Architecture
computers	O
.	O
</s>
<s>
It	O
also	O
contains	O
an	O
RS-232	O
serial	B-Protocol
I/O	I-Protocol
channel	O
with	O
software-selectable	O
baud	O
rates	O
from	O
110	O
to	O
76,800	O
.	O
</s>
<s>
In	O
addition	O
,	O
it	O
has	O
a	O
1KB	O
resident	O
2708	O
ROM	B-Device
containing	O
Cromemco	O
's	O
RDOS	O
,	O
the	O
resident	O
disk	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
The	O
4FDC	O
was	O
designed	O
to	O
drive	O
Persci	O
277	O
8-inch	O
single-density	O
floppy	B-Device
drives	I-Device
.	O
</s>
<s>
They	O
used	O
a	O
fast	O
voice	B-Device
coil	I-Device
actuator	O
and	O
not	O
a	O
stepper	B-Algorithm
motor	I-Algorithm
to	O
position	O
the	O
drive	O
read	O
write	O
head	O
.	O
</s>
<s>
The	O
FDCX4	O
,	O
in	O
addition	O
to	O
using	O
an	O
analog	O
phase-locked-loop	O
data	O
separator	O
in	O
all	O
modes	O
,	O
also	O
used	O
write-precompensation	B-Device
.	O
</s>
<s>
When	O
this	O
switch	O
is	O
ON	O
the	O
1KB	O
ROM	B-Device
containing	O
RDOS	O
cannot	O
be	O
accessed	O
by	O
the	O
computer	O
.	O
</s>
<s>
When	O
this	O
switch	O
is	O
OFF	O
the	O
RDOS	O
program	O
resides	O
in	O
the	O
computer	B-General_Concept
memory	I-General_Concept
space	O
from	O
address	O
0xC000	O
to	O
0xC3FF	O
.	O
</s>
<s>
Switch	O
2	O
is	O
the	O
RDOS	O
disable	O
after	O
boot	B-Operating_System
switch	O
.	O
</s>
<s>
If	O
this	O
switch	O
is	O
ON	O
the	O
1KB	O
ROM	B-Device
containing	O
RDOS	O
will	O
automatically	O
be	O
disabled	O
after	O
CDOS	B-Operating_System
is	O
bootstrapped	O
in	O
from	O
a	O
disk	O
thus	O
clearing	O
memory	O
space	O
from	O
0xC000	O
to	O
0xC3FF	O
for	O
system	O
use	O
.	O
</s>
<s>
(	O
In	O
this	O
mode	O
the	O
ROM	B-Device
is	O
actually	O
disabled	O
by	O
an	O
output	O
to	O
port	O
40H	O
which	O
is	O
done	O
automatically	O
by	O
CDOS	B-Operating_System
)	O
.	O
</s>
<s>
If	O
switch	O
2	O
is	O
OFF	O
,	O
RDOS	O
remains	O
in	O
memory	O
space	O
even	O
after	O
CDOS	B-Operating_System
is	O
loaded	O
.	O
</s>
<s>
RDOS	O
contains	O
two	O
programs	O
;	O
1	O
)	O
the	O
CDOS	B-Operating_System
bootstrap	B-Operating_System
program	I-Operating_System
and	O
2	O
)	O
the	O
console	O
monitor	B-Operating_System
program	I-Operating_System
.	O
</s>
<s>
Switch	O
3	O
is	O
the	O
boot	B-Operating_System
enable	O
switch	O
.	O
</s>
<s>
When	O
this	O
switch	O
is	O
ON	O
the	O
bootstrap	B-Operating_System
program	I-Operating_System
will	O
execute	O
(	O
thus	O
loading	O
CDOS	B-Operating_System
)	O
without	O
first	O
entering	O
the	O
monitor	B-Operating_System
program	I-Operating_System
.	O
</s>
<s>
When	O
this	O
switch	O
is	O
ON	O
,	O
diskettes	B-Device
cannot	O
be	O
initialized	O
under	O
software	O
control	O
thus	O
preventing	O
a	O
"	O
runaway	O
"	O
program	O
from	O
unintentionally	O
altering	O
the	O
diskette	B-Device
initialization	O
.	O
</s>
<s>
This	O
switch	O
must	O
be	O
OFF	O
when	O
initializing	O
diskettes	B-Device
.	O
</s>
<s>
All	O
signals	O
from	O
the	O
drives	O
are	O
TTL-buffered	O
and	O
have	O
150	O
ohm	O
pullups	O
.	O
</s>
<s>
Signals	O
to	O
the	O
drives	O
from	O
the	O
1771	O
are	O
TTL-buffered	O
with	O
separate	O
buffers	O
for	O
mini	O
and	O
maxi	O
connectors	O
.	O
</s>
