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Reverse seeks function in an identical man-
nner, except that all phases and polarities
are reversed.
Total seek times for forward
and reverse seeks are identical for seeks of
equivalent
leng~~s.
Return to Z.ro Seek (RTZS)
The R'l'ZS function is a Seek where the heads
are repositioned to cylinder 000.
This
function is commanded when the controller
issues Recalibrate (Tag 12), Reset Diagnos-
tic Mode (Tag 15), or a DIN.
One of these
commands is required to clear a Seek In-
complete condition (DS3, bit 2 7 ). This bit
is set if On Cylinder is not generated with-
in 500 milliseconds from the start of the
seek, or
if
On Cylinder is lost.
It is also
required to clear Positioner Overtravel
(Oetailed Status Byte 3, bit 2 6 ).
However,
the error is cleared only by OIN if a Pack
Unsafe (CHECK indicator lighted) condition
exists.
See Figures 3-25 and 3-26 for Re-
calibrate (ReB) timing.
The Recalibrate pulse sets the R'l'Z latch and
clears the EOT Seek Error Latch.
This en-
ables the RTZ gate: this bias voltage forces
an average 7 ips reverse motion of the posi-
tioner.
When the positioner passes cylinder 000, no
more even dibits are detected.
This is the
Reverse EOT area.
The lack of even dibits
inhibits cylinder pulses, allowing the
~
ity integrator in the track servo circuit to
reach a negative output in excess of 1.28
volts.
This sets the Reverse EOT FF.
The
integrator is reset, but reverse motion con-
tinues unimpeded.
After an additional reverse motion of about
two to four tracks, the velocity integrator
output again exceeds 1.28 volts.
The RTZ
latch is cleared while the Load latch sets.
The logic now functions in a manner equiva-
lent to the First Seek sequence.
With the Load latch set, the Load gate sup-
plies a voltage to command a 7 ips forward
motion.
The velocity integrator, this time
indicating forward distance, clears the Load
latch to permit continued motion under con-
trol of the fine servo signal.
The posi-
tioner then servoes into cylinder 000.
On Cylinder is available 1.75 milliseconds
after the RTZS is completed.
The sequence
must be completed within 500 milliseconds
after RTZS initiation, or else Seek Error is
set.
On Cylinder drops Positioner Busy. This
raises DEN.
The RTZS function is also used during normal
power off sequencing.
If the controller
issues a Set Standby State (Tag 13) command,
or if the START switch is pressed
by
the
83318200
C
operator, the control interlock opens.
This
raises the Unload Heads signal in the drive
loqic.
The RTZ latch sets to initiate a 7
ips reverse seek.
This time, however, the EOT Enable circuit is
disabled so that the velocity integrator sig-
nal has no effect.
In turn, the Load latch
is disabled.
Reverse motion continues until
the heads unload.
The RTZS function occurs automatically to the
heads unloaded condition if dibits are lost
for more than 200 milliseconds.
The Pack
Unsafe FF is set to prevent another First
Seek until the CHECK indicator hasbeen
~.
END OF TRAVEL DETECTION
The End of Travel (EOT) circuit determines
when the heads are positioned outside of the
normal data cylinders.
This function is
used during Load and RTZ sequences and to
indicate an error condition during a seek.
Forward EOT indicates that the heads are
within the inner guard band.
Assume that
the controller has commanded a forward seek
to an illegal cylinder past cylinder 822 or
410 for BR3E4/3E5.
Sequencing is a follows
(See Figure 3-27).
1. As the heads move forward, the velocity
integrator output produces a signal
proportional to velocity (the input to
the integrator) and time (provided by
the integrator capacitor).
The output,
which is a positive-going ramp during
forward seeks, represents distance
travelled.
It is pulled back to
groun~
by cylinder pulses.
As long as cylindar
pulses are generated, the output cannot
reach an effective value.
2. After track 822 or 410 for BR3E4/3ES i3
passed, no more odd dibit tracks are
detected, resulting in no more cylinder
pulses to reset the velocity integrator.
When the output exceeds approximately
1.25 volts (4-8 tracks) Forward EOT
Enable comes up.
This signal, in con-
junction with the even dibits picked
off of the inner guard band, sets the
Forward EOT FF.
3.
With the Forward £OT
FF
set:
a. Seek FF (set the start of the seek)
is cleared.
This stops the seek
function.
The output of the posi-
tion converter in the servo circuit
is blocked to indicate a zero posi-
tion error.
b. The difference counter is set to
1023 (T=O), or 511 for BR3E4/3E5.
3-59
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