3.
Erroneous data bits removed
4.
Conversion
of
digital data
ElA
RS-422 to compatible differential datathere-
by providing immunity to common mode noise during transmission
HEAD
SELECT
1
READIWRITE
~
LSI
~
LOW PASS
~
PULSE SHAPER
~
ZERO CROSS
HEADS PREAMP
FILTER
DETECTOR
L
TIME
DOMAIN
UNE
~
MFM
FILTER
DRIVER
READ
DATA
FIGURE 24:
Read Operation
PC Board 20243-XXX
READ OPERATION:
PC
BOARD 20243-XXX
HEAD SELECT
6.5
6.5.1
The binary decoder
(le
5H) 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 Zener Diode CR2.
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.
LSI PREAMP
6.5.2
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.
LOW PASS FILTER
6.5.3
This filter network attenuates high frequency noise, which
is
outside the nor-
mal data signal range.
PHASE SHIFTER
6.5.4
Amplified data enters the circuit and
is
shifted 90
0
so peak data, which was
detectable over a fairly broad
range,
is
now moved to a highly sloped accurately
detectable position
at
the zero crossing.
ZERO CROSS DETECTOR
6.5.5
This element detects bit position as the slope
of
read data signaIs crosses the
zero threshold. At this point analog signais are changed todigital.
ST212
PRODUCT
MANUAL/Rev.
B
27
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