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

FM FORMAT PRINCIPLES
FM (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 O-bits), only clock pulses are
present. These pulses occur at a frequency of every
4 microseconds.
When data is present, the pulse frequency changes. Each
1-bit is inserted halfway between adjacent clock pulses
so that there is double the frequency. If a O-bit is recorded,
nothing is inserted between the clock pulses, so the pulse
frequency is not changed.
The raw data in FM format is 250 000 bits (31 250 bytes)
per second.
Read Data
Each clock pulse or 1-bit is recorded on the diskette
surfaces 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 1-bit. If there is no
change in the direction of the current between two adjacent
clock pulses, the diskette drive control circuitry recognizes
that a O-bit is present.
Voltage Change
for Clock
Pulses Only
Voltage Change
for Clock and
Data Pulses
Resultant Bit
Pattern Recorded
on the Diskette
J
I
I
C
I
I I
--.....r4JJ.sl~
I I
I
I
I
I
I
I
I
I
I
I I
I I
I
D CDC D C D C
I
I I I I I I
0
0
I
The C and D above the line show the clock and data bit times.
I
I
I
I
I I
I
I
I
D C D C
I I I
0
The numbers under the line show whether a 1-bit or a O-bit is recorded.
62 Theory
I
I
i
I
I
I
I I I
I
I
I
o
CDC 0
D C 0
I I I I I I
0
0 0