HP 7942 Service Manual page 36

Theory of Operation
7942 and 7946
the signal originates is listed, followed by a boxed
numeral identifying the sheet of the functional
block diagram where the assembly is shown.
Most of the mnemonics listed in table 3 - 1 have
"- L" or "- H" suffixes. These suffixes identify active
low or active high logic signals, respectively. Sig-
nals without such suffixes are usually bus or analog
signals.
In addition, Section IV, Service Information, con-
tains system cabling and signal distribution
diagrams for the disc drive.
3-3. DISC FORMAT
The two plated metal discs in disc drive assembly
A 1 provide three data surfaces, each with one
read/write head. The fourth surface is used with a
servo head for prerecorded servo data. The four
discs in the 55 megabyte version provide seven
data surfaces and one servo surface. See figure
III
Each data surface is divided into 987 concentric
circles called tracks. See figure 3-1. From the out-
side diameter of the disc there are: 1 self -
test/maintenance track, 483 data tracks, 1 7 spare
tracks, 485 da ta tracks, and 1 self -
test/maintenance track.
The two self test/maintenance tracks are used for
testing reading and writing. These tracks also con-
tain service information, including run -time logs,
fault logs, and a spare table directory. See figure
3-1. The self -test/maintenance tracks are located
3-4
at the inner diameter (ID) and outer diameter (OD)
of the disc to permit read/write testing at these
locations. Also, duplicating the service information
at two locations reduces the possibility of loss of
data due to recording medium failure. The 968
data tracks are used for reading and writing data.
This provides the user with 968 addressable cylind-
ers. The 1 7 spare tracks are used for sparing out
tracks containing hard errors.
Each data track
IS
organized in to smaller
sequentially-numbered blocks of data called sec-
tors. Figure 3 - 2 shows the track format, based on
32 data sectors, each having 256 bytes of data in-
formation. The beginning of each sector is iden-
tified by a prewritten identification (I D) field
which contains the physical sector address plus
cylinder and head information. This ID field is fol-
lowed by the data field. The beginning of both the
ID field and the data field is flagged by unique
two-byte characters called address marks. The
first byte in both address marks is a hexadecimal
A 1 pattern. This is followed by an FE pattern for
the ID address mark and an F 8 pattern for the
da ta address mark.
A summary of the recording capacity provided by
user available data tracks in the HP 7941 and HP
7945 is provided below.
Oat a Bytes
Per
Sector 256
Track
8,129
Head
7,929,856
7941 23,789,568
7945 55,508,992
Sectors
Pe r
32
30,976
92,928
216,832
Tracks Heads
Pe r Pe r
968
2,904
6,776
3
7