HP 7925D Service Manual page 254

PART VII - RECORDING FORMAT AND
COMMUNICATION PROTOCOL
A-93. INTRODUCTION
This part describes the recording format used in the
HP 7925H Disc Drive and provides details of the HP
7925H command set. The bit numbering notation
shown in figure A-24 is used throughout the section. A
16-bit word is made up of two bytes, namely the left-
hand (upper, most significant) byte and the right-
hand (lower, least significant) byte. Note that the HP
7925/1000 numbering scheme is the reverse of the HP
300/3000 scheme in that the least significant bit is
numbered 0 and the most significant bit is numbered
15.
A-94. RECORDING FORMAT
The HP 7925H track and sector recording format is
illustrated in figure A-25. There are 48 sectors on each
track and each sector is separated from the next by an
intersector gap (ISG) having a nominal duration of 27
microseconds. Each sector contains a number of fields,
described as follows.
• SYNC. The sync field consists of 24 bytes of all zeros.
The field is generated at the beginning of each sector
written, and is used to synchronize the read elec-
tronics to the data stream during a read operation.
• PREAMBLE. The preamble is a 6-byte field that
precedes the data field of each sector. The six bytes
are defined as follows:
SYNC - The sync bytes form one word (two bytes)
of value 100377 (octal). The leading 1,
since it follows the sync field, signifies
TRANSFERRED FIRST
the start of a valid data stream to the
data separator. The least significant 1
(making the bytes 100377 instead of
100376) indicates that there is no valid
error-correcting information in the ECC
field of the postamble. The controller
will not support error correction at pres-
ent, but will write an arbitrary pattern
into the ECC field so that the READ
FULL SECTOR and WRITE FULL
SECTOR of all compatible disc drive
subsystems will function properly.
CYLAD - Two bytes containing the 16-bit cylinder
address of the sector. The address may
not be the same as the physical address
of the sector if the sector is part of a track
which has been flagged with the S
(spare) or D (defective) bit.
S, P, D - Three bits of track status information.
The combination used must be the same
for all sectors of a particular track (cyl-
inder and head address). The three bits
are defined as follows:
S bit - When set to 1 indicates that the
track being accessed is a spare
track in active use.
P bit - When set to 1 indicates that the
track being accessed is write
protected. Such a track may not
be written on unless the READ
ONLY switch on the disc drive
TRANSFERRED SECOND
rr---------.JA'----------__y,...----------'A'------ )
HP-IB
HP 300/3000
HP 7925/1000
8
a
15
7
14
6
2
13
5
3
12
4
4
11
3
5
10
2
6
9
7
8
8
8
7
7
9
6
10
5
11
4
4
12
3
3
.13
2
2
14
15
o
7300·110
Figure A-24. Bit Numbering Notation
A-81