Hp 7925H Disc Drive Simplified Block Diagram; Functional Diagram - HP 7925D Service Manual

Appendix A
7925
SPINDLE
~MOTDR
DISC PACK
ACTUATOR
ASSEMBL Y
. . - - - - D A T A - - - - . ,
LIGHT
FAULT
INDICATORS
READ/WRITE DATA LINES
SERVO
L L_-_--l~-y--II------t----
CODE - - - - '
I
I
I
I
I
I
I
I
I
L .
.......J
FROM
PR~~~~--+
-+----+f
SOURCE
~~~~~~RJ~----------------------------------------------------1
I
comRm.LER - - - - - - - - - - 1 I
I I
7311-93
Figure A-6. HP 7925H Disc Drive Simplified Block Diagram
The controller is a microprocessor-based design, interfac-
ing to the HP-IB through the HP-manufactured
Processor-to-HP-IB-Interface (PHI) integrated circuit. The
HP-IB protocol used in the controller is compatible with
the requirement of other HP-IB disc subsystems. The con-
troller command set (the set of legal commands passed as
data for the secondary get command) is a proper subset of
the HP 12745 (HP 13037 Disc Controller!HP-IB) subsys-
tem command set. A general description of the controller
command protocol is contained in Part VII of this appen-
dix. For a detailed description of the controller command
structure, refer to the HP 13365 Integrated Controller
Programming Guide, part no. 13365-90901.
The controller consists of two printed-circuit assemblies
(PCA's) which reside in the card cage of the disc drive. A
functional diagram of the controller is provided in figure
A-14 and table A-I describes the signal mnemonics used
in the diagram. Microprocessor PCA-A2 of the controller
replaces I/O sector PCA-A2 of the HP 7925 Disc Drive
while retaining the necessary sector circuitry. Data
PCA-AI of the controller occupies the previously unused
Al slot in the disc drive card cage. Self-test PCA-A13,
mounted inside the rear door of the disc drive and con-
nected by cable to data PCA-A1, contains the controller
self-test controls and indicators and the HP-IB input!
output connector. All of the electrical power required by
the controller is drawn from the disc drive power supplies.
In the controller, all data transferred during a read or
write operation flows through the PHI integrated circuit
and a 16-byte first-in, first-out (FIFO) buffer memory.
Therefore, the maximum number of bytes of data that the
controller can buffer is 24. In order to guarantee no loss of
data, an HP-IB channel with a transfer rate of greater
than 900 kilobytes is required.
A user can request normal or full-sector data transfer.
During a normal READ or WRITE, only the actual 256
data bytes within the sector are transferred, whereas in a
READ FULL SECTOR or WRITE FULL SECTOR the
preamble and postamble information is also transferred.
Thus, a full sector consists of 6 bytes of preamble, 256
bytes of data, 2 bytes of Cyclic Redundancy Check (CRC)
bits and 12 bytes reserved for Error Correcting Code
(ECC) information - a total of 276 bytes. The controller
does not support ECC and all error checking is handled by
CRC. However, all disc packs written by the controller and
the HP 13037 Disc Controller are interchangeable. There
is also a VERIFY command, in which the controller reads
from the disc as in a normal READ, checking the CRC
bytes, but not transferring data to the HP-IB. This com-
mand is used mainly to check for possible disc data errors.
During a READ or WRITE (normal or FULL SECTOR), a
hardware DMA configuration handshakes data between
the FIFO and the PHI chip. The microprocessor is used
only to switch the data path between the actual data and
the CRC bytes. Between each sector of a multi-sector
transfer, the microprocessor checks for errors and ad-
dresses the next sector to be transferred.
A-6