Sector Clock And Index Generation; Logical Vs. Physical Sectors - HP 7925D Service Manual

7925 Appendix A
track status indicators of sector 63 of that track have been
verified by the controller. An end-of-cylinder will occur
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.
The data field is used to store 256 bytes of data. Each byte
is defined as being 8-bits. Only the data field is transferred
to and from the system during most data operations. The
preamble and postamble are normally generated and
checked by the controller.
The 30-byte preamble is used for synchronization and ad-
dressing purposes. It is comprised of a 24-byte sync field; a
2-byte sync field; a 2-byte cylinder address field; and a
2-byte field which specifies the head and sector addresses
and provides the spare, protected, and defective track
status indicators.
SECTOR FORMAT A-5.
The smallest addressable data storage area on a data
surface is a data sector (see figure A-5). Accessing a data
sector is accomplished when the controller specifies the
address of the cylinder, head, and sector. Each data sector
contains a 30-byte preamble, a 256-byte data field, and a
14-byte postamble.
The 14-byte postamble consists of a cyclic redundancy
check (eRC) word and 12 bytes of error correction code.
The controller generates the CRC information during a
write operation and appends it to the other information
written in the sector. The check information itself depends
on the value of every bit from the first bit in the sync word
to the last bit in the data field. During a read operation,
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 A-2. Sector Clock and Index Generation
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
A-5. CYLINDER MODE
.... 4 3 2
SECTOR
o
63 62 ....
o
H
~
2
D
3
4
63
62
~INDEXZONE
62
61
61
60
T
7311-34
Figure A-3. Logical vs. Physical Sectors
A-3