Mfm Format Principles; Read Data - IBM 5280 Maintenance Information Manual

MFM FORMAT PRINCIPLES
MFM (modified frequency modulation) format is a method
of recording data on a diskette surface. The frequency of
pulses is changed (modulated) to represent data.
When no data is present (all a-bits) only clock pulses are
present. These pulses occur at a frequency of every
2 microseconds.
When data is present, the pulse frequency changes. Each
l-bit is inserted halfway between adjacent clock pulses.
The clock pulses are suppressed however, so the frequency
remains the same. All clock pulses will be suppressed
unless two O-bits (no pulse) occur next to each other.
When this happens, the clock pulse that would normally
occur at the clock time between the a-bits is not
suppressed. Therefore, the pulse frequency changes for
that specific time.
The raw data rate in MFM format is 500 000 bits (62 5000
bytes) per second.
Read Data
Each clock pulse or l-bit is recorded on the diskette surface
as a change in magnetic direction (flux)' from the bit
immediately preceding it. As this change in flux passes
the read/write heads, it causes the current in the read/write
heads to change direction. This change in the direction of
the current is recognized by the diskette drive control
circuitry as either a clock pulse or a l-bit. If there is no
change in the direction of the current at data time, the diskettl
drive control circuitry recognizes that a a-bit is present.
MFM format, when compared to FM format, can record
twice the amount of data on a diskette surface.
Voltage Change
--12:+-
I
I
for Clock
I
Pulses Only
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I I
Voltage Change
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L
for Clock and
--f!
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Data Pulses
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t
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t
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I I
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Resultant Bit
D CDC D C D C D C D C D C D C D C D C D C D C
I
,
I
,
I I
I I I I I
Pattern Recorded
I I
I
I I
I
I
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on the Diskette
0 a 0 a 0 0
The C and D above the line show the clock and data bit times.
The numbers under the line show whether a l-bit or a a-bit is recorded.
64 Theory