6300
Another minor function control performed is the processing
of
the Write Enable (IWEXR)
and Erase Enable (IEEXR) signals from the
I/O
interface. These signals are gated by
Selected And Ready, Position Mode, and File Protect Mode, then transmitted to the
Read/Write PCBA for controlling those functions
on
the Read/Write PCBA.
In
addition,
the Not Write or Erase signal is generated for application to the
Start/Stop
Control Logic.
Also performed within the Read/Write control logic is the gating of
Read
Enable with
Selected and Ready. This signal is then transmitted to the Read/Write PCBA.
The Write Protect logic combines the state
of
the specific WRITE PROTECT switch
on
the
operator panel with the state of the Platter
Select (IPSXR) line such that the respective
protect switch and the Platter Select line are decoded to set or reset the
File Protect Mode
latch. The state
of
the File Protect Mode latch determines whether a particular platter is
permitted write and erase operations, depending upon whether
it
is deSignated as a
protected platter by the respective
WRITE PROTECT switch on the operator panel.
The state of the File Protect Mode latch is gated with
Selected and Ready and is
transmitted
to
the
I/O
interface by the File Protected Status Driver which transmits the file
protect condition of the disk drive to the controller
on
the File Protected (IFPXD)
I/O
line.
The state of the
WRITE PROTECT switches is indicated by the operator panel
PROT
(Protect) lamps regardless
of
which platter is selected. The lamp indication depends
entirely
on
the state
of
the respective switch. The lamp drive signals
are
generated by the
protect lamp drivers according to the switch states.
4.7.3
SPINDLE SPEED CONTROL
The rotational speed
of
the spindle is controlled
to
within ± 1 percent
of
the nominal
value. This
tight
control is maintained so that the spindle speed is not affected by line
frequency variations and to avoid disk speed variations due to tolerances
of
the drive train
components. The time reference for the spindle speed control is derived from the Crystal
Oscillator and Clock Countdown logic that is shown in Figure 4-9 as part of the
Start/Stop Control Logic.
The actual speed
of
the spindle is derived by sensing notches in the phase lock ring with a
magnetic transducer.
In
addition to the speed sensing function, Phase Lock Ring and the
Magnetic Transducer
are
used in conjunction with the Sector Electronics (Paragraph
4.7.5). The Phase Lock Ring is a
flat circular plate with notches in the periphery. It is
mounted integral with the spindle and therefore the rotational speed
of
the Phase Lock
Ring is the same
as
that of the spindle. The Spindle Speed Control is a true servo loop in
that power is controlled to the drive motor which, in turn, rotates the spindle; the actual
speed
of
the spindle is sensed and compared with a time reference and the result of the
comparison is used to correct the amount
of
power applied to the drive motor.
In
this
manner, the loop corrects the existing speed
of
the spindle to the correct speed within the
ability of its resolution and response capability.
It is important to note that although the
loop is a true closed
loop servo, it is not a linear servo system. The reason is that the result
of the
time/
speed comparison is a single binary
digit
and therefore can have only two
possible states, a zero or a one, i.e., the speed is either too fast or too slow. For this
reason the actual speed regulation takes place in a
limit
cycle type
of
operation where the
actual spindle speed varies between
an
upper and a lower boundry
as
determined
by
the
response
time
of the loop and the resolution of the error detector. However, certain of the
loop characteristics are like a linear servo, in that power to the drive motor is varied for
control by changing the time of occurrence of the application
of
power within a given
power
line cycle in a manner which is usually referred to
as
phase angle control.
In
other
4-22
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