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head reads information from the servo track
surface af the disk pack.
This information
is known as dibits: dibit is a shortened
term for dipole bit.
Dibits are prerecorded
on the servo surface durinq manufacture of
the disk pack.
Do not confuse
the
servo
surface with the other 19 disk pack record-
inq surfaces.
Dibits are the result of the manner in which
flux reversals are recorded on the servo
tracks.
One type of track, known as the
EVen track, contains negative dibits.
The
other track, the Odd track, contains posi-
tive dibits.
At the outer edge of the servo surface is a
band of' positive dibit tracks.
This area
is the Reverse End of Travel (EOT) or outer
guard band
0
Then, there are servo tracks
alternately recorded with neqative and posi-
tive dibitso
Finally, toward the inner edqe
of the pack, there are tracks containinq only
negative dibits.
This is the Forward
EOT
or
inner guard band.
When the read/write heads are located at the
centerline of a data track, the track servo
head is actually centered between two of the
prerecorded servo tracks and is readinq an
edge of each.
The detected Signal is a mix-
ture of the two adjacent dibit siqnals.
The
amplitude of each dibit component is pro-
portional to the read coil overlap of the
recorded servo tracks.
With the head center-
ed, the amplitudes of the
two
types of dibits
are equal.
As the head moves away from its
centered position, the amplitude of one dibit
component increases while the other
~.
This error voltaqe is the track servo signal.
The basic elements of the track servo cir-
cuit are illustrated in Figure 3-27.
Table
3-5 explains the track servo circuit func-
tions.
01811 GATING
After being differentially amplified, the
servo signal is applied to gates that sepa-
rate the dibit signals by sensing the posi-
tive and negative flux reversals (Figure
3-29).
A positive dibit consists of a posi-
tive-going waveform immediately followed by
a negative-going waveform.
On the other
hand, a negative dibit consists of a
~tive
going waveform followed immediately by a
positive-going waveform.
The dibits are analyzed by the positive and
negative gates.
Each gate output switches
to the low state when it senses its respec-
tive dibit.
The negative-going pulses con-
trol single-shots and
JK
FF's to generate
the odd/even dibits.
3-66
The even/odd dibits are used to enable the
EOT detection circuit and to generate the
basic machine clock signal.
TRACK SERVO SIGNAL GENERAnON
The servo Signal is generated by peak de-
tectors that monitor their respective dibits.
The positive peak detector (Figure 3-29)
provides an output proportional to the amp-
litude of the positive dibits.
It senses
only the positive waveform of positive di-
bits:
Gate
1
low inhibits it from reacting
to either the negative waveform of positive
dibits or the entire negative dibito
An RC
network integrates the peak detector output
to provide a smooth output.
The resulting
signal output is greatest, therefore, when
the servo head is
c:entered
over an odd dibi t
track.
The neqative gate works in a similar manner.
The.servo signal is provided by a Summing
amplifier.
It receives inputs from the peak
detectors.
(The positive peak detector out-
put is first inverted.'
Therefore, the out-
put represents the difference between the
two peak
de~ector
outputs.
The track servo signal is at its maximum
neqative value when the servo head is posi-
tioned over the outer guard band or over one
of the odd dibit tracks.
It is at its maxi-
mum positive value when the servo head is
positioned over the inner guard band or over
one of the even dibit tracks.
The track servo signal is applied to the ser-'
vo·circuit and to the cylinder detect circuit.
In the servo circuit, it is used to generate
the fine servo signal that controls movement
during the last one-nalf track of a seek or
durinq a Load sequence.
The cylinder detect
circuit generates cylinder pulses as the
track servo Signal approaches a null.
Circuit gain control is achieved by applying
the outputs from the peak detector buffers
to the AGe summing amplifier.
Its output is
negative in proportion to signal strength:
the stronger the signal, the less negative
the AGC voltage.
This signal is fed back to
the AGC amplifier to control the resistance
of a FET within the amplifier.
The FET is
connected across the differential inputs to
the amplifier.
The less negative the AGC,
the less tne resistance: therefore, more of
the signal from the track servo head is
shunted by the FET to reduce circuit gain.
The Dibits Detect One Shots (A336 and A337)
prevent the circuit from being turned on by
random noise spikes while the heads are un-
loaded or being loaded.
When the preamp
83318200
A
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