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INDEX DETECTION CIRCUIT
The Index detection circuit (Figure
3-33)
generates a 2-microsecond pulse at the start
of each new loqical track.
This signal is
returned to the controller as Index
Mark
and
also resets the sector counter to zero.
Prior to reaching the Index area,
both
even
and odd dibits are available.
Dihits Pre-
sent
FP
(K10S) is held
in
the preset state.
Gate I701 is held at a continuous low state:
this causes the counter to be continually
loaded with zero with each clock pulse from
A708.
The Index pattern is the special pattern of
missing dihits illustrated in Figure
3-33.
Flip-flop K10S clears, allowing the counter
to
increment with each 806 kHz pulse via
A708.
The counter can continue
to
increment
only if
the
precise pattern continues to be
sensed.
Any other combination of missing
dibits (such as when tracks are crossed
during seeks) will cause the counter to
be
reset to zero.
When the counter reaches a decoded value of
5,
two
of the three input gates to Index Set
FF K706 are available.
The next even dibit
triggers A708 to set the FF.
In turn, A707
provides a 2-microsecond Index pulse.
Note that Index is inhibited while the heads
are over either a forward or reverse EOT
area.
SECTOR CIRCUIT
The sector circuit (Figure 3-34) permits the
controller to determine the current angular
position (sector) of the read/write heads .
wi th respect to Index.
The number may
be
obtained by an Angular Position In (Taq 24)
command.
This command is one of the few
commands that may be issued during either
diagnostic or regular operations.
Note that
this command cannot be used to generate an
interrupt when a select sector is reached;
it can only determine the sector available
at the time that Angular Position In is
issued.
The count transmitted on the DLI
remains frozen until Angular Position In
drops.
Each track may be considered as subdivided
into 128 segments of equal size.
They are
numbered from 000 to 127.
Sector 000 is the
first sector following Index.
The 806 kHz clock Signal is used to generate
the sector count.
Each positive-going clock
pulse increments a divide by 104 counter.
When the counter reaches 104, the sector
counter is incremented by one.
The divide
by 104 counter restarts from zero: the pro-
cess continues so that every 104 odd/even
3-72
dibits increases the sector counter by one.
When the sector counter equals 127, the di-
vide by 104 counter can no longer increment
the sector counter.
Index should occur at
this
time
to reset both counters.
If Index occurs before the sector c.ounter
reaches 127, the counter is malfunctioning.
This sets Rotational Position SenSing CRPS)
Error, which is bit
23
of
DS3.
The sector register is unaffected during
sector counter operations.
The contents of
the sector counter are gated into the sector
register when API is issued.
Angular Posi-
tion
In
(Tag
24).
The register remains at
the same value until the next Angular PCsitica
In.
The
sector counter continues
to
operate
normally.
Durinq dia9DQstic operations (Set Diagnostic
Escape Latch set), the sector counter con-
tinues to operate
but it cannot
be
gated
into the sector register.
The reqister is
reset by Set Diaqnostic EScape (Tag
IF)
and
incremented by an Incr Sector Counter com-
mand (Tag
33).
Its value may then be deter-
mined by Angular Position In.
READ/WRITE OPERAnONS
GENERAL
The drive processes the read/write data
~
ferred between the disk pack and controller.
The controller and disk pack each handle
differen~
types of data.
The controller
transmits and receives only
8
bit parallel
bytes of
NRZ
data.
The disk pack requires
serial
MFM
data.
Therefore the drive must
convert the data to the proper format before
transferring it between the two.
During Write operations the drive receives
parallel data from the controller, converts
it to serial data (serialization) and writes
it on the disk pack.
During read operations
it reads serial data from the disk pack,
converts it to parallel (deserialization)
and transmits it to the controller.
The read/write circuits (Figure
3-35)
per-
form this conversion and the heart of these
circuits is the serializer/deserializer
(SERDES).
It is used during both read and
write operations to make the proper data
conversions.
The remainder of this section
describes SERDES along with the other
circuits
involved in processinq read/write data.
HEAD SELECTJON
The head select circuit must select the de-
sired head before a read or write operation
83318200
A
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