Addressing Modes; Surface Mode; Cylinder Mode; Sector Clock And Index Generation - HP 7925D Service Manual

Theory of Operation
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 1-2).
There are 53,760 clock transitions produced per revolution
(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 1-3). 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.
7925
1-3. ADDRESSING MODES
The controller operates in two modes, the surface mode
and the cylinder mode, to access the data storage areas of
the disc drive. The following paragraphs discuss
controller/disc drive operations in the two modes.
1-4. SURFACE MODE
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 transfer 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.
1-5. CYLINDER MODE
In the cylinder mode of operation, the heads are positioned
over a particular cylinder and then data is written or read
starting with the lowest numbered head and continuing to
the highest numbered head. A cylinder of information
therefore consists of all sectors on all tracks at a given
cylinder address. Head switching occurs after the data in
sector 63 of the current track has been transferred. Head
switching is sequential, that is, head 1 will be selected
after head 0, and so forth. 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. An end-of-cylinder will
OCCllI
after the data in sector 63 of the last track has been
transferred. Cylinder switching (a seek operation) may
take place at this time and the process repeated.
DIRECTION
/ O F HEAD
/ MOTION
NOTES: 1. SERVO CODE = 6720 (MINUS 3) DI-BITS PER REVOLUTION.
2. SECTOR CLOCK
=
53,760 CYCLES PER REVOLUTION (2.42 MHz AT 2700 RPM).
3. ONE INDEX PULSE IS GENERATED PER REVOLUTION.
7311-33A
Figure 1-2. Sector Clock and Index Generation
1-2