FLOPPY DISK INTERFACE
DESCRIPTION
General
The
Floppy
Disk
Interface consists of a Western Digital
WD2797-02 Floppy
Disk
Controller
(FDC), a series of
buffers, a decoding
circuit
and
the
interface connector.
Three
control
lines
for
the
disk
drives are generated
directly
under
software control, via
the
Parallel Interface
on
the
System
Board.
Two
of
these
control
lines
are routed via
the
decoding
circuit
to
provide individual select lines
(DSO,
DS
1),
for
the
disk
drives.
The
third
control
line
is
the
head
load control signal (HLD) for loading
the
read/write
head of
the
selected
disk
drive.
The
motor
control
line
(Motor On) is hard-wired to
OV,
thus
providing a
permanently
active control signal to
the
disk drives.
The
configuration of
the
drives is set so
that
the
motor
operates,
only
when
a
disk
is
within
the
drive.
The
remaining
functions
for, controlling
the
movement
of
the
read/write
head, transferring
data
to
and
from
the
disks,
and
monitoring
status
signals from
the
drives are
implemented
by
the
FDC. A detailed description of all
these
connections
and
all
other
connections
to
the
interface is
provided
in
tabular
form
at
the
end
of
this
chapter.
All
disk
transfer operations performed by
the
FDC
(formatting, reading disk data,
writing
data
onto
disks), are
controlled
by
the
system
software,
and
involve
both
the
8086
CPU
and
the
8089 lOP.
The
lOP
is used to perform
DMA
data
transfers
between
system
memory
and
the
FDC
on
a byte-by-byte basis.
The
actual
sequence
of events performed
by
the
processors
and
the
FDC
during disk transfer operations
is
quite
complex,
involving
an
interchange
of
information
between
the
two
processors and
the
generation of various
disk
drive
control
signals.
Due
to
this
complexity, a
simplified
view
of
disk
write,
disk
read
and
disk
formatting
operations
is
presented
in
the
paragraphs below to
illustrate
some
of
the
basic principles. A
more
detailed approach is
taken
in
subsequent
paragraphs.
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