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RI!AD OPBAnONS
General
The read mode of operation is initiated when
the drive receives a read command and the
desired head is selected.
The head then
detects any flux transitions on the pack
and converts them into an analoq voltaqe.
!his analoq siqnal (representinq written
data on the pack) is amplified and applied
to the AGe amplifier.
This output is ap-
plied to the Data Latch where it is con-
verted to MPM data.
It is then sent
to
the
Phase Lock Oscillator which syncronizes
itself
to
the data frequency and provides
sync information to the Data Seperator.
The Data seperator converts the data pulses
from the Data latch into clock and data
siqnals.
The Serdes assembles the serial
(bit) information into. parallel (byte) in-
formation for transfer to the controller.
A maximum of 30
DIS
ia required from the
application of read data for the read cir-
cuits (Fiqure 3-46) to stabilize to a steady
state condition.
Bosic Read Sequence
The basic read operation for a
~ad
command
is as follows:
10 Controller places command on the Com-
mand Code Lines and raises Device
Command Strobe.
2. The drive decodes the command and ver-
ifies that it is safe to read (not Pack
Unsafe and heads are On Cylinder).
If
it is safe, the read logic is enabled.
3. Analog data read by the head is ampli-
fied and shaped.
The signal is then
analyzed by the read detection data
latch to find flux transitions.
Each
transition generates a data pulse.
In
this context, however, data pulse means
only that there was a transition:
whether it is a data ("1") or clock
pulse is not yet defined.
4. A phase lock oscillator synchronizes
itself with the data pulses frequency
and phase.
During the first 5 micro-
seconds following the rise of Read Gate,
a signal called Fast Start acts as a
booster pulse to help the oscillator to
quickly attain synchronization.
5. The data separator receives inputs from
the phase lock oscillator and the data
pulses.
It separates the clock "data"
pulses from the true data "data" pulses.
The clock pulses are used as the inter-
nal read clock.
The data pulses are
applied to SERDES.
3-90
6. Data is loaded into SERDES and shifted
throuqh SERDES by the read clock. ·The
bit counter is disabled and no data is
transferred to the controller.
When
SERDES contains hex 19 (0001 1001) of
the Sync Byte, Data Good is generated.
This means that a valid field is ap-
proaching.
The bit counter starts count-
ing.
Data
Good
cannot come up durinq
Fast Start.
7. With Data
Good
up, the bit counter
starts
countingc
8. When the bit counter equals 8, SEROES
contains the hex 19 of
the
ID Byte.
This byte is loaded into the Byte Re-
gister
and the
drive
raises Serial
Read
In.
The
Byte
Reqister:
acts
as a buffer
to
hold
the assembled byte
~or
the controller
since the first bits of the next byte
are already entering SERDES.
The output
of
the
Byte Register is applied to the
multiplexer for transmission to the con-
troller via the bidirectional data lines.
9. Serial Read In drops when the controller
raises Serial Write Out.
The Serial Read In/Serial W:ite Out sequenc-
ing continues for each byte read.
Device
Command Strobe and the command must remain
up until the read operation is. terminated.
The Read Header functions identically to Read
except that hex 19 cannot set Data Good prior
to the sensing of an address mark.
Moststeps
of the basic Read function apply: however,
Fast Start does not rise with Read Gate (step
4).
In addition, replace step 6 with the
following:
1. Data is loaded into SERDES and shifted
through SEROES by the read clock.
The
bit counter is disabled and no data is
transferred to the controller.
2. When at least 2.5 microseconds of miss-
ing flux transitions have passed (this
will occur only during the AM bytes of
missing clock/data), and when the first
pulse of the "0" bit is sensed (this is
the first bit of the 12 bytes of zeros
separating the AM bytes from the Sync
Byte), a S-usec Fast Start timeout is
initiated.
Data Good cannot come up
during Fast Start.
l. After Fast Start times out, the read
logic begins looking for hex 19 in
SERDES.
When the Sync Byte is detected,
Data Good comes up.
4. The remaining Read steps apply without
change.
83318200
A
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