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The rules for MFM recording may be summarized
as follows:
1. There is a flux transition for each "1"
bit at the time of the "1".
2. There is a flux transition between each
pair of "0" bits.
3. There is no flux transition between the
bits of a "10" or "01" combination.
Sorting data is accomplished by synchroniz-
ing an oscillator with the read data during
the first continuous gap of zeros after read
is enabled.
The analog data can be decoded
to binary "l's" and "a's" once the oscillator
is synchronized.
A write function is similar to a read, except
the incoming binary "l's" and DO'S" are
changed to the
prop~r
write current levels
and then written on the disks.
The advantages and disadvantages of MFM re-
cording are listed as· follOWS:
1. Fewer flux reversals are needed to re-
present a given binary number because
there are no flux reversals at the cell
boundaries.
This achieves higher re-
cording densities of data without in-
creasing the number of flux reversals
per inch.
2. Signal-to-noise ratiO, amplitude re-
solution, read chain operation, and
operation of the heads are improved by
the lower recording frequency achieved
because of fewer flux reversals requir-
ed for a given binary number.
3. Pulse polarity has no relation to the
value of a bit without defining the
cell time along with cell polarity.
This requires additional read/write
logic and high quality recording media
to be accomplished.
Erase before writing
using MFM recording.
is recorded over any
be
on the disk (this
to as a hard write).
Disk Surface Recording
is not required when
The data being written
data which may already
technique is referred
Surface recording involves the magnetization
of minute areas on the surface of a highly
retentive magnet material.
In order to re-
produce the recorded information, the mag-
netic state of the material is read back by
using the retained or residual flux to induce
voltages in the Read circuits.
83318200
A
The relationship of a fixed recording head
with a magnetic recording surface is shown
in Figure 3-38.
Also shown is one cycle of
alternating current and the resulting current
flow through the coil and the flux changes
on the disk surface.
The current
flow~
the coil reverses with a change from a posi-
tive to a negative direction, causing the
polarity of the recorded flux to also
reverse~
Data is written by this flux reversal that·
is, it is the change in
fl~
that will be
re~d
back as data.
This reversal generates
a readhack pulse.
Constant current (no flux
reversals) does not generate a pulse; there-
fore, it does not represent meaningful data.
This constant current state occurs while Data
Modifier Line is up.
Figure 3-39 shows the phase difference be-
tween the induced magnetic force (flux) and
the resulting current while reading data off
the disk.
WRITE OPERAnONS
General
The controller sends the Write data to the
drive via the bidirectional data lines.
The
data passes through the receivers and is
input to SERDES.
SERDES converts the paral-
lel data into the necessary serial format.
Next the data goes to the Write Compensation
circuits which adjusts for problems caused
by variations in read data frequency.
The
Write Driver circuits then process the data
and send it to the head for writing on the
disk.
Figure 3-40 shows the Write circuits
Satic Write Sequence
. The basic write operation sequencing is as
follows:
1. Controller places command on the Command
Code Lines and raises Device Command
Strobe and Serial Write Out.
2. The drive decodes the command and veri-
fies that it is safe to write (not pack
Unsafe and heads are On Cylinder).
If
it is safe, the write logic is enabled.
3. The drive raises Serial Read In.
The
controller drops Serial
W~ite
Out, but
data must remain stable on the bidirec-
tional data lines until the drive drops
Serial Read In.
4. Data is loaded from the bidirectional
data lines into SERDES.
3-79
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