HP 7925D Service Manual page 183

PART I - THEORY OF OPERATION
This Part contains a brief introduction to the HP 7925H
Disc Drive addressing structure, the HP 7925H environ-
ment, the functional makeup of the HP 7925H, the format
of the disc data surfaces, and a list of mnemonics and
abbreviations. Also provided is a detailed discussion of the
integrated controller and each of the eight systems which
compose the drive function.
The principal difference between the HP 7925H and the
HP 7925 Disc Drive described in the main manual is that
the HP 13037 Disc Controller used by the HP 7925 is
replaced by disc controller circuitry contained within the
HP 7925H. This integrated controller (hereafter referred
to as the controller) provides a simple interface between
the Hewlett-Packard Interface Bus (HP-IB) and the HP
7925H. (See figure A-I.) The characteristics of the con-
troller permit up to four HP 7925H's to be interfaced to a
single HP-IB channel. Upon receipt of command se-
quences via the HP-IB, the controller decodes and gener-
ates all of the necessary timing sequences for the disc
drive. In addition, the controller handles all of the input/
output communications with the HP-IB controller-in-
charge. A self-test function incorporated in the controller
assists in isolating certain malfunctions to the printed-
circuit assemblies (PCA's) and other components of the
disc drive.
A-1.
A-2.
INTRODUCTION
ADDRESSING STRUCTURE
(2.42 MHz at 2700 rpm). A unique index pattern is en-
coded on the servo track between physical sectors 0 and
63. It is used to sense the start of physical sector O. The
sector counting electronics counts these clock transitions
to keep track of the physical sectors as they pass beneath
the heads, and when the index pattern is detected at the
end of each revolution, it resets its counter to zero and
begins counting for the next revolution.
The disc drive keeps track of physical sectors as they pass
beneath the heads. The controller, on the other hand,
deals only with logical sectors in order to minimize system
intervention during automatic head and/or track switch-
ing. This feature of the controller enables multiple sector
operations to continue beyond the end of a track without
waiting for another revolution of the disc to take place.
Logical sectors are staggered as the tracks progress
downward through the cylinder, so that sector 63 on the
next track will follow sector 63 on the current track (see
figure A-3). This logical structuring of sectors permits the
controller to verify the address fields and track status of
sector 63 on the new track and then immediately continue
with the data transfer to sector 0 of the new track. The.
mapping from logical sector to physical sector is performed
by the controller microcode before the sector address is
transferred to the disc drive. An inverse mapping opera-
tion occurs in the case where the disc drive returns its
present sector address in response to a controller
command.
A-4. SURFACE MODE
The controller operates in two modes, the surface mode
and the cylinder mode, to access the data storage areas of
the disc drive. (See figure A-4.) The following paragraphs
discuss controller/disc drive operations in the two modes.
In the surface mode of operation, only one head is selected.
The head is positioned over a particular track and then
data is written or read starting with the lowest numbered
track and continuing to the highest numbered track. A
surface of information therefore consists of all sectors on
all tracks at a given head address. Data transfers will
continue with sector 0 of the next track after the address
fields and track status indicators of sector 63 of that track
have been verified by the controller. This process con-
tinues until there is no more data or no more storage space
left on this surface of the disc.
The disc pack used with this disc drive is comprised of
seven discs. The top and bottom discs provide physical
protection for the five center discs. These five center discs
provide nine data surfaces and one servo surface. As
shown in figure A-I, the disc drive accesses data on the
nine data surfaces with nine read/write or data heads.
Head positioning information and sector clocking are de-
rived from the fifth (servo) surface through a read only or
servo head. There are 815 ensured cylinder positions
available for data storage. Cylinder addresses range from
zero to 822. Each data cylinder consists of nine data
tracks, one on each data surface. Tracks are addressed
when both cylinder and head addresses are specified. Each
data track is divided into 64 physical data sectors. Sectors
are addressed when both head and sector addresses are
specified for a given cylinder. Head addresses range from
zero to 8 and sector addresses range from zero to 63.
The physical location of each data sector is determined by
counting clock transitions which are derived from the
servo code written on the servo surface (see figure A-2L
There are 53,760 clock pulses produced per revolution
A-3.
ADDRESSING MODES
A-I