6.4 READ OPERATION: PC BOARD 20292-XXX
HEAD
SELECT
(interface)
t
READIWRITE
HEADS
+
READ
DATA
MFM
READ
LSI
r-----
LOW PASS
CONVERSION
DATA
PREAMP
FILTER
READIWRITE LSI
(interface)
FIGURE
23:
Read Operation
PC
Board 20292-XXX
6.4.1
MFM READ
DATA
The data recovered by reading a prerecorded track
is
transmitted to the host sys-
tem by a differential pair
of
MFM READ
DATA
lines. The transition
of
the
+MFM
READ
DATA
line going more positive than the -MFM READ
DATA
line represents a flux reversaI
on
the track
of
the selected head.
6.4.2
HEAD SELECT
The binary decoder (IC
60)
selects the desired head based
on
the status
of
the
head select lines.
If
the DC·voltages are too low (possibly causing
an
inaccurate
write operation), pin
12
will force the decoder to choose a nonexistent head.
During read operations the head center tap
is
set
at
approximately +5 Volts
by the Read/Write LSI (IC 6H, pin 27).
By
referencing 0 to
+5
Volts, actual
ground appears as - 5 Volts, which eliminates the necessity
of
a negative power
source normally required by the LSI preamp, IC 3H.
6.4.3 LSI PREAMP
With the head center tap active, data from the selected head will flow into the
preamp. This preamp amplifies the read signal and also acts as a high-pass filter.
6.4.4
LOW PASS FILTER
This filter network attenuates high frequency noise, which
is
outside the nor-
mal data signal range.
6.4.5 READ
DATA
CONVERSION
Amplified analog data enters the Read/Write LSI· (IC 6H)
at
pins 1 and 2 and
exits as differential MFM Read data
at
pins
17
and
18.
The Read/Write LSI
functions are:
1.
Read data and shift 90°
2.
Analog to digital conversion
26
ST212
PRODUCT
MANUAL/Rev.
B
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