Honeywell BR3C9 Operation Manual page 123

separates records. These gaps are uaed
to
per.ait the read circuit to synchronize itself
so that it can reliably define logical ones
and zeros. Gap length is variable
in
accor-
cance with the nature of the following field
or record.
Track format, gap lengths,
and
record format
are a function of the operating system. Refer
to the applicable controller manual for fur-
ther information. Because of the drive
lcgi.c
operations, however, the following rules
~p~.
.
• Each gap (field-to-field) or record-to-
record) has variations in the number of
zeros. The last
two
bytes of each qap
contain hexadecimal 19. The first byte
is called the Sync
Byte~
the second,
the ID Byte.
Reacl data cannot
be
re-
turned
to
the controller until these
bytes are found.
• Par al~ records following Record Sera
(or ita equivalent), an adclre •• mark
is written between the zeros gap
aDd
the hex 19 bytes.
An
address mark
(All)
contains bytes without
any
flux
transi tiona
(-0 -
Or
-1-)
whatsoever.
Although the address
mark
is typically
three bytes of missing flux reversals,
the actual
AM
length is determined by
Data
Modifier Line. As long as Data
Mod~fier Line is ~p -- one byte or sixty
bytes -- all writing is inhibited.
Twelve bytes of zeros typically separ-
ated the
AM
bytes from the Sync Byte.
Principles of MFM Recording
This unit uses the MPM recording technique.
This system defines a
-1-
by a positive
transition occurring at the half-cell time
(Figure 3-37). A ·0· is defined
by
a posi-
tive transition occurrinq at the start of
the cell time, except when preceded by a
·I N
•
Ia
thia
case, no transition occurs durinq
the
cell time for a ·0·.
r.--
I
-II
0
-II
0 -I •
-I •
0
-I-
-,
.
1---1
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3-78
WRITE
OATA
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....
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WRITE
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AECOADED
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- ffl ·
- W
I
• ffll
FLUX
.-...ffl •
, . . READ
oa@~~~
____
~n~
______
~n
__________
~n~
__
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CONNIEC'TOR.
8. saGNAL. AS IT WOULD APPEAR AT HEAD COL
Figure 3-37. Modified Frequency Modulation Recording
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