Read Operation Pc Board 20292-Xxx; Mfm Read Data; Head Select; Lsi Preamp - Seagate ST212 Product Manual

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