Read Operation: Pc Board 20243-Xxx; Head Select; Lsi Preamp; Low Pass Filter - Seagate ST212 Product Manual

3. Erroneous data bits removed
4. Conversion of digital data ElA RS-422 to compatible differential data there-
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