The problem with this type
01
system is when the data is
recovered, il Îs not self-clocking. Self-c1ocking codes include
Frequency Modulation (FM)
and Mooilied Modulation (MFM).
The actual flux reversai raIe
01
the two codes
is
lhe same;
Table
6 shows the diflerences.
Frequency
Modulation
(FM): Information is always recorded
by inserting a clock between each data bit. A
"'"
bit is delined
as a
flux transition between clocks whi\e a "0"' is delined as the
absence
ollhis
flux transition. Clocl<s are always flux transition.
Modlfled
Frequency
Modulation
(MFM):
!nformatÎon is
encoded using data and clocks. The longest tÎme between
lIux
Iransitions is the same as the FM code bul clocks are not
recorde<! between data bifs.
Deflnltions:
t.
",
..
is delined as a lIux transilion occurring al the hall-cell
lime.
2.
·'0" is delined as a lIux transition occurring al the start ot
th)
cell time. A pulse
al
the beginning
01
the cell is a
cloc~;
however. a clock is not always written. Clock is suppressed
il there
is a ..,.. in this cell or il Ihere was a
..
,
..
in Ih·,
preceding
cell.
Table 6.
5elf·Clocklng
Codes
Bil
OenSÎty
Data
T.ansler
RaIe
8ils'lrack
BirSIOisk
Ce"
lime
DOUBLE FREOUENCV
,,,,.
"'"
249.984
Hl
42,664
3,208.128
'"
6536
MODIFIED FREQUENCV MODULATION
3672 (outer track)
6536 (Inner lrilCll)
499.968
Hl
83.328
6.416.2S6
2"
6536
Adlustmenta
and
Jumper
Options
The dala separator must be adjusted wilh the t
791
in an idle
condition (no
command
currently ln operation). Adjust A2
potentiometer tor a l.4V level on test point 25. Then adjust
A1
potenliometer
to
yield a 4MHz square wave al pin t6
01
U28.
The write precompensation must
he
adjusted while executing a
continuous·write
command
(Example: Format). Adjusl A3
polentiometer
ta
yield 250·ns·wide pulses at test point
27.
This
results in
a writa precompensation value of 250 ns.
There are a number of jumper options availabte on a conlroller
board.
(Table
7)
describes
the standard configuration
as
normally used by the Modal Il system.
10
Standard Configuration
Inltalled
Jumpers
Functlon
8
to
C
8-inch drive ready signal
J to K E0H-EFH port addressing
LtoM
active high XFEAAO
P la a
2·MHz FOC
dock
T
10
U
drive
0 INT"JEXT select
Optional Configuration
Inll.lled
Jumper.
Functlon
010
E prime signal
to
prinler
Oto
A
t
MHz
FOC
clock
M
ta
N active low XFEARQ
Ito
J
A0H·AFH port addressing
Ato
8
mini-drive ready signal
H
10
F,
S
to
T
Two internai mini-drives
INT'/EXT select
Table 7. AdJuBtment Table
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