Singer 40 - Model 42 Disc Pack Configuration

General Description
The Model 42 uses a pair of disc packs, one mounted on top of the other. Both
packs must be mounted for the disc drive to be operable. The bottom pack
(Model 41A Disc Pack) is referred to as the resident pack and the top pack
(Model 41B Disc Pack) is referred to as the removable pack. Regardless of the
terminology, both packs can be removed from the disc drive by the operator.
When both packs are mounted, they rotate together as though they were one pack.
It is anticipated that the resident pack will remain mounted for extended periods
of time with the removable pack being changed from run to run, day to day, etc.,
as dictated by the requirements of each installation. The resident pack and the
removable pack each consist of three magnetic oxide coated discs joined to one
another on a common vertical axis. The discs are approximately 14 inches in
diameter and there is approximately 0.4 inch between successive discs. Each
pack has four recording surfaces. The top surface of the top disc of each pack
and the bottom surface of the bottom disc of each pack are not used for record-
ing data.
On all three disc packs (Models 41, 41A, and 41B) each recording surface con-
tains 200 concentric tracks and each track is divided into 50 equal parts called
sectors. Each sector contains 100 characters. A write or read operation
always writes on or reads from a single sector.
The Model 41 Disc Pack has a maximum data storage capacity of 10 million
characters. The Models 41A and 41B Disc Packs each have a maximum data
storage capacity of 4 million characters.
The read/write head mechanism of the Model 40 Disc Drive has five arms, one
for each interdisc gap. Each arm has 'two heads: one for the surface above the
arm and the other for the surface below the arm. All five arms move together.
The read/write head mechanism of the Model 42 Disc Drive has four arms. The
mechanism is identical in appearance to the read/write head mechanism of the
Model 40 Disc Drive except that the middle of the five arms is missing.
Like all peripheral devices for the System Ten computer, the disc drives are
independently powered and are turned on and off from their own operator control
panels. Once a pack (or a pair of packs in the case of the Model 42) is mounted
and the power is on, the operator starts the pack rotating by moving the START /
STOP switch on the operator control panel to the START position. When the pack
is rotating at an acceptable operational speed (2400 rpm ± 2%), the READY indi-
cator light on the operator control panel turns on. With the power on, the disc
dri ve is in either of two states, called local and on-line.
When in local, the disc drive is disconnected from the central processor.
When on-line, the disc drive can perform read and write operations in response
to Read and Write instructions executed in the central processor.
The ENAB LE/DISAB LE switch on the operator control panel serves as a com-
bined ON LINE and LOCAL switch. When the' ENABLE/DISABLE switch is in
the DISABLE position, the disc drive is in local. When the READY light is on
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1-2 Models 40 and 42 Reference Manual