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Radio Shack EC-293 Technical Information Page 25

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TECHNICAL INFORMATION
Floppy Disk Controller IC
The
Floppy
Disk Controller (FDC) in
the
Expansion Interface
directly controls
the
drive
motors;
track stepping and direction; write
gating and data.
It
also
inputs
information
on
the
diskette index
position, track zero occurrence, write
protection
and data/clocking -
all
at
the
Mini Disk Card-Edge
on
the
Expansion Interface.
The
FDC
registers (CMD/STATUS, TRACK, SECTOR, DATA) are located at
TRS-80 memory addresses 37ECH, 37EDH, 37EEH and
37EFH,
respectively.
Drive selection
is
through
Z47. Only
one
drive is selected at a time. A
time-Qut timer circuit (Z33)
is
activated/reactivated each time a
drive is selected/reselected,
thereby
protecting
the
disk drives in
the
instance
of
program "crashes", as
the
drives are
not
designed for
continuous motor-on use.
After
two
or
three seconds,
the
MOTOR
ON line (15, pin 16) will deactivate (go high) unless Z33
is
retrig-
gered
by
a drive selection/reselection. Pin 6
of
gate
Z46
provides a
signal
to
the
FDC (pins 23 and 32) when a head load has been
commanded (FDC status
=READY).
TRSDOS programming takes into account
that
the
Disk drive
motor
requires
one
second
to
come up
to
operating speed, and
that
head
loading takes 80 milliseconds
to
stabilize. At
the
end
of
an FDC
operation, an
interrupt
is
generated (pin 39
of
FDC goes high)
which,
through
gate
Z34
(pins
11
and 10), sets Z28, pin 9. This
interrupt request
is
terminated
by
reading
the
FDC status register
(address 37ECH)
- which makes pin 39
of
the
FDC go low - then
reading from
37E0H,
which resets pin 9
of
Z28.
Because
of
the
complexity
of
floppy disk operation, Radio Shack
does
not
encourage customers
to
bypass
the
TRS-80 Disk Operating
System for disk I/O,
nor
can we answer customer questions in this
area. Customers who are
intent
on
such applications should start
by
reading these publications:
• Shugart SA400 OEM and Service Manuals
• Western Digital
FD
177lB-O1
Data
Sheet
24
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