HP 7925D Service Manual page 255

Appendix A
7925
DATA.
The data field consisting of 256 bytes of data.
The Sand D bits are mutually exclusive.
The P bit may be set in any combination.
POSTAMBLE. The postamble is a 14-byte field that
follows the data field of each sector. The 14 bytes are:
SECTOR - One byte containing the sector (rota-
tional position) address of the sector.
• Read data passed from the disc to the I/O channel.
• Write data passed from the I/O channel to the disc.
• Control commands (Op codes) and associated
parameters.
• Controller status on completed commands.
An HP-IB primary or secondary is distinguished from data
by the assertion of the Attention (ATN) line on the HP-IB.
Only the HP-IB controller-in-charge (CIC) can assert
ATN. Primary and secondary, as used in this section, are
subsets of the total class of primaries and secondaries
available under HP-IB protocol. In particular, the
primaries recognized by the controller are primary ad-
dress to talk/listen (Primary 1) and primary untalk/
unlisten (Primary 2). Refer to IEEE Standard Digital In-
terface for Programmable Instrumentation for additional
details.
In general, four different types of information flow be-
tween the controller and the HP-IB channel. These types
are:
The controller presents a bus-controlled device inter-
face on the HP-IB. In this mode of operation, the
controller-in-charge of the bus (for example, an HP
300 System I/O channel) explicitly addresses devices
(for example, the HP 7925H) to communicate over the
HP-IB. This is accomplished by using the primary
and secondary commands defined in IEEE Standard
488-1975, IEEE Standard Digital Interface for Pro-
grammable Instrumentation. Communications be-
tween the controller and HP-IB (except for the
SECONDARY (HARD) CLEAR and IDENTIFY se-
quences) observe the general sequence protocol shown
in figure A-26.
A-95. CHANNEL COMMUNICATIONS
A 12-byte field reserved for error-
correcting information. The present con-
troller will not support error correction.
The controller writes an arbitrary pat-
tern on the ECC field. A READ FULL
SECTOR command will return the pat-
tern after the other fields have been
transmitted.
Two bytes of cyclic redundancy check
(CRC) information for detection of errors
during readout.
ECC-
CRC-
is not set and the FORMAT
switch on the disc drive is set to
override the protect feature.
D bit - When set to 1 indicates that the
track being accessed has been
flagged defective.
HEAD - Five bits containing the head address of
the sector. The address may not be the
same as the physical head address of the
sector being flagged with the S or D bit.
The S, P, D bits and the head address
together form one byte of the preamble.
•
•
ISG
J
--
--
SECTOR n
+
1
--
ISG SECTOR n
/ '
I
/
/
I
I
/
r
{ ISG
J
SYNC
I I -. . . . ,
FIELD» PREAMBLE DATA POSTAMBLE
24 , , - - - - - - -.........-:-_- _ _
------------------J/-------~
BYTES'" 6 BYTES _ 256 BYTES I 14 BYTES \
,," -- I \
./ I \
",./ - I \
./
\
• ./ _ _ . - - - - - - . 8 7 6 5 1 8 - - 1
I \
l_c_S_Y_N_C_.......
_C_Y_L_A_D---I~
HEAD
I
SECTOR -
I
(-C-R-C-I E_C_C_*
l
2 BYTES 2 BYTES 1 BYTE 1 BYTE 2 BYTES 12 BYTES
*
NOT WRITTEN OR READ BY HP 7925H.
7311-103
Figure A-25. Track and Sector Format
A-82