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keeps the head following the track exactly, by adjusting the head
position so that the amplitudes of the two servo bursts written on
each side of the data track are equal.
For example, in Figure 2-3,
bursts A and D on logical track
a
are kept equal.
There is one head for each surface: four on the Q250, six on the Q280.
Each head can either read or write.
When reading, the output is
amplified first by the preamplifier chip in the HDA, then by an AGC
(automatic-gain-control) amplifier.
That output is filtered, then
differentiated to find the zero crossings, and converted to pulses by
a pulse detector.
A phase-locked loop circuit derives synchronized
data and clock signals for DICEY.
Careful design makes drop-ins or
drop-outs rare.
RLL 1,7 encoding allows 1.33 data bits to be stored
for every magnetic transition on the disk.
write
The preamplifier/write driver chip inside the HDA selects the head
(under command of the microprocessor) and sends current reversals to
the head, which writes magnetic transitions on the disk.
2.2.3
SCSI Bus Physical Interface
A 50-pin connector is provided at position J1 on the PCB for con-
necting to the standard SCSI bus.
See Figures 2-5 and 2-9.
The
standard SCSI single-ended non-shielded connector is used.
J1 is a
Universal Header connector and has a key slot to prevent installing
the mating connector improperly.
The recommended mating connector is
AMP receptacle PIN 1-499508-2 (with strain relief) or PIN 1-746195-2
(without strain relief), or equivalent.
Unkeyed mating connectors
should not be used due to the danger of plugging the connector in
backwards.
A 50-conductor flat cable or a 25-signal twisted-pair ribbon cable
with a minimum conductor size of 28 AWG and a characteristic impedance
of 100 ohms +/- 10% is recommended. SCSI devices are daisy-chained
together using a common cable.
To minimize discontinuities and signal
reflections, cables of different impedances should not be used in the
same bus.
Table 2-1 gives J1 pin assignments.
Note that to achieve
satisfactory transmission quality, all odd pins except pin 25 are
connected to ground; pin 25 is left open.
Maximum total cable length
is 20 feet (6 meters), a length suitable for use in a cabinet.
2-8
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