Disc Format - HP 7936 Support Manual

A5
=
Power Distribution PCA-A5
A6
=
Controller PCA-A6
A7 = LED PCA-A 7
AS
=
Primary Power PCA-A8
Certain of the signals listed in table 5-1 may be
monitored with test PC As that can be connected
to servo PCA-AI and read/write PCA-A2. Refer
to chapter 8 for details. In the SIGNAL column
of table 5-1, the servo test signals are identified
with an asterisk
(*),
and the read/write test signals
with a double asterisk (**).
In the DESCRIPTION column of table 5-1, the
circuits listed in upper case (capital) letters are
blocks in the functional diagrams.
5-5.
DISC FORMAT
Each data surface is divided into a number of
concentric circles called tracks. See figure 5-1.
From the outer diameter (OD) of the disc to the
inner diameter (ID), there are a number of 00
guard band tracks, one OD maintenance track,
three spare tracks, 1396 user da ta tracks, three
spare tracks, one 10 maintenance track, a number
of ID guard band tracks, and a landing zone.
The OD guard band is used for calibration. Both
the OD and ID guard band tracks allow for over-
shooting when seeking to the outermost and the
innermost data tracks. The maintenance tracks
contain service information including run time er-
ror logs, fault: log (data surface 0 only), and error
rate test logs. See figure 5-2. The maintenance
tracks are also used when reading and writing
during self test. The data tracks are used for read-
ing and writing host data. The spare tracks are
used for sparing out any data tracks containing
media defects. The head comes to rest on the
landing zone- when power is removed from the
drive.
Each data track
is
organized into smaller
sequentially-numbered blocks of data called sec-
Functional Description
7936 and 7937
tors. On each track, there are 124 of these physical
sectors, with each sector having 256 bytes of data.
See figure 5-3.
In addition to the 256 bytes of data, each sector
contains a 14-byte sync field, a 6-byte header, a
2-byte CRC field, a 12-byte ECC field, a data gap,
a servo field, and a servo code field.
The 14-byte formatter/separator field and the
6-byte header field comprise a 20-byte preamble.
This information is used for synchronization and
addressing purposes. The header field specifies
status, head, cylinder, and sector addresses, and
provides the spare sector information. The head
and cylinder are logical addresses and the spare
sector and sector bytes are physical addresses. The
status byte is used to determine whether a spare is
classified as primary (factory) or secondary (field).
If bit 6 of the status byte is zero, the sector is
spared secondary; if 1, the sector is spared primary.
If bit 7 of the status byte is zero, the entire track
has been field spared; if 1, the track has been
spared primary.
The data field is used to store 256 bytes of data.
Only the data field is transferred from the system
during most data operations. The preamble and
the postamble are checked by controller PCA-A6.
The 2-byte CRC field and 12-byte ECC field form
a 14-byte postamble. Controller PCA-A6 generates
the CRC and ECC 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 header field to the last bit in the data
field. During a read operation, this check informa-
tion is regenerated and compared in such a way
tha t the presence of errors is detected. The con-
troller ECC firmware corrects any data errors
detected by the hardware in the background while
the hardware is busy transferring subsequent sec-
tors in a multiple-sector transfer. In most cases,
this scheme will result in no loss of performance
for correcta ble data errors.
5-3