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can
be
performed.
See Figure 3-36 for the
head 02 circuit.
The Bead Address Register is set by a Bead
Address Out tower (Tag
09) c:oDllllaDd
from the
controller.
It is automatically reset
to
~
zero by any
RrZS
functionG
Bit
20
of
the
register is examined to determine if the
head will have an odd or even number.
Since
(in
this example) bit 20 is off, the even
head decoder is enabled.
The head is enabled
by applying a qround to the read/write head
center t_p.
NOte that all heads are automatically de-
selected while not On Cylinder or for 6
microseconds following a Head Address Out
Lower command.
Two head select errors may occur at any time:
1. NO Head Select is generated if the re-
gister is set
to 19
or greater.
Since
these head numbers are nonexistent,
there has been
~
equipment malfunction.
2. Multiple Bead Select is sensed by the
SCE/SCD circuit.
If only one qround
(or no ground) is sensed, the SCD vol-
tage comparator output is +5 volts (no
error).
Two
or more grounds puts more
resistors in parallel, causing the vol-
tage comparator to indicate a Multiple
Head Select error.
.
As Fiqure 3-36 illustrates, anyone of four
head select or write errors can generate a
current faultG
With the Current Fault FF
set:
1. Pack Unsafe FF sets.
a. CHECK indicator lights.
b. Read and Write Gates are disabled.
c. Seek Forward Lower, Seek Reverse
Lower or Recalibrate commands are not
executed.
d. This error status can be cleared only
by pressing the CHECK switch or by
a DIN from the controller.
2. Bits 24 through 27 of DS2 are enabled.
Refer to Sense Operations.
The Current Fault FF cannot set to indicate
an error while Data Modifier Line is up.
Since there are no flux transitions while
writing the address mark, AC Wkite Fault will
occur.
This is not an error condition at
this time.
3-76
BASIC READ/WRITE PRINCJPt.ES
G. .....
caI
Read/Wltite
data
is transferred between the
controller and drive
in
bytes.
Each byte
consists of eight bits.
The parity bit is
Dot written on the
pacJc.
Information is recorded on,
and
read from,
the disk by means of
19
read/write heads.
Data is written by injecting
a
large current
through a coil within the selected head.
This generates a flux field that magnetizes
the
iron oxide particles
bound to
the disk
pack surface.
Each particle is then the
equivalent of a miniature bar maqnet with a
North and South pole.
The writing process
orients the poles to permanently store the
direction of the flux field as the oxide
passes under the head.
In turn, the direc-
tion of the flux field is a function of write
current polarity while its amplitude depends
on
the:
amount of current: the greater the
current,
the
more oxide particles that are
affected.
Readinq is an equivalent process.
As
the
oxide particles pass beneath the head, the
stored flux field intersects the coil within
the head.
This induces a current Which,
when amplified and analyzed, defines the
written data.
In order to define the binary digits stored
on the pack, the frequency of the flux re-
versale must be carefully controlled.
Sever-
al recording methods are available; each has
its advantages and disadvantaqes.
The drive
uses the Modified Frequency Modulation (MFM)
recording technique.
Refer to Principles of
MPH Recording.
The main advantage of MFM
is that it permits higher recording densities
without increasing the number of recorded
flux reversals per inch.
The length of time required to define one bit
of information is the cell.
Each cell is
nominally 155 nsec in width.
Since-there are
eiqht bits per ,byte, each byte requires 1.24
~sec
to write or read.
Although data is
transferred in parallel (by byte), it is
written or read serially (by bit).
Track Format
Each track has Index as its starting point.
Information is then recorded in groups of
related data in the form of records.
In
turn, records may be subdivided into fields.
A gap of zeros separates each field within
each record and, in addition, another gap
83318200
A
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