Format Differences - HP 9895A Service Manual

2-12 Theory of Operation
Track Numbering. Each track has a physical address as previously described. There is also
a logical track address associated with each good track. The logical track address is written in
the ID field of each sector on the track.
If
a disc has no bad tracks, the logical address of a
track is the same as the physical address.
A disc with N bad tracks can be made to look like a 77 - N track disc with no bad tracks. To
do this, the logical track address stored in the ID field of each sector of every bad track is set
to !FF. Tracks of this type are known as invisible tracks.
All
visible tracks are then sequen-
tially assigned logical track numbers. Logical Track 0 is the outermost good track, not
necessarily physical Track 0.
Format Differences
Table 2-3 summarizes the principal differences between the IBM Format and the HP For-
mat. Details of these differences are provided in the following paragraphs.
Coding. The IBM Format uses a single-density encoding scheme known as frequency
modulation (FM). The rules for FM coding are as follows:
•A 0 bit cell has no data transition.
•A 1 bit cell has a data transition.
•Every bit cell has a clock transition.
The minimum distance between transitions is one-half the bit cell, that Is, the distance from
a clock transition to a data transition.
Table 2-3. Format Differences
Feature HP Format IBM Format
Usable Physical Tracks
0 - 76 0 - 76
Sectors Per Track
30
26
Sector Numbering 0 - 29
I - 26
Bytes Per Sector 256
128
Data Order LS Byte First
MS Byte First
LS Bit First
MS Bit First
Coding MMFM
FM
Precompensatlon Required Yes
No
Sync-Up Bytes Four Bytes of !00 Six Bytes of !00
And
Four Bytes of !FF
Address Marks Extra Clock
Missing Clock
Transitions Transitions
CRC Includes Address Mark No
Yes