6300
Cylinder Address Strobe then the Demand Address lines are examined to determine if they
contain a valid address.
If a valid address is present, this address is accepted by the
Position Control logic.
If the address is
an
illegal address, i.e., it lies outside the range
of
the valid addresses, then this is signalled by the Illegal Address (NLlAXG) line.
During a strobe, and any time during a Seek operation, the Position Control logic and the
positioner status are indicated
on
the Busy Signal (NLBSXG) line. Illegal Address and
Busy
Signal are outputs of the Load Address and Illegal Address Control and the Busy
logic.
Information describing the current position
of
the positioner is stored in the Current
Address Counter which is
an
up/down
type
of
counter. The direction
of
the count and the
amount
of
the count
are
determined by the Count Control logic on the basis
of
the Position
Reference Clock
(SPRCG) and Position Quadrature Clock (SPQCG) signals from the Servo
PCBA. These are the digital position transducer signals derived from the outputs
of
the
position transducer.
The positioning system in the
03000 Disk Drive functions
on
the basis that the physical
position of the positioner is known to the Position
Control Logic at all times. This is
because the logic has kept track
of
all moves made by the positioner since initialization,
i.e., current position information is initialized at the
time
the heads are loaded.
In
other
words, the system knows where
it
is because it was told where
it
started from and it kept
track
of
every move thereafter. Furthermore, it knows how far
it
has
to
go
to
achieve the
demand address because
it
knows where its current location is. The particular mode
of
operation
of
the Position Control logic is determined by the Mode Control portion of the
logic in conjunction with the Operation
Control logic. The various inputs and outputs
of
this portion
of
the Position Control logic can be seen in the block diagram (Figure 4-16).
There
are
four modes
of
operation
of
the Position Control logic. One mode is the Position
Mode which causes the positioner servo to operate as a position type servo and hold a
particular
cylinder position.
During a
Seek operation it is necessary to operate the positioner servo
as
a velocity type
of
servo. This,
of
course, is a negation of the Position Mode line.
In
addition
to
the velocity
reference
enabled by the Velocity Reference Enable (NLVREG) line, the direction
of
the
velocity is specified by the Forward Direction (LFDX1) line, and the particular velocity
reference level is determined
on
the basis
of
the amount of difference between the current
address and the demand address. This difference is specified to the Velocity Function
Generator on the
Servo PCBA by the Address Difference lines. It is important to note that
the difference between the current address and the demand address is obtained by
performing a ones-complement arithmetic subtraction
on
the binary values contained in
Current Address Counter and the Demand Address Register. This subtraction process is
performed by the
Subtractor and Complementor logic. Since the arithmetic is ones-
complement arithmetic,
an
end-around carry is used. This carry is under control
of
the
Carry Control logic and the algebraic sign
of
the velocity desired is determined
on
the
basis of the binary
value
of
the carry. The actual subtraction is mechanized using
an
integrated circuit binary full-adder. When the heads are being loaded, it is necessary to
force the carry to a particular state; this is accomplisched by the Carry Control logic
on
the
basis
of
the states of certain bits in the Current Address Counter.
The other two modes of operation
of
the Position Control logic are the Forward Slow Mode
and the Reverse
Slow Mode. The two slow modes are a slow velocity type
of
operation.
The Forward
Slow Mode is used during loading of the heads and the latter portion
of
a
4-26
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