MPI 40 Series - Write Data Timing (FM Encoding); MFM Encoding

B. Selecting a read/write head (if necessary).
C. WRITE GATE being inactive.
Prior to a read operation, a time delay must be applied to sta-
bilize the position of the read/write heads. The timing required
for read operations is shown in Figure 3-6.
A read/write head senses flux changes on the diskette and gener-
ates a sine wave with the peaks occurring at the time of the flux
changes. Diskette Data from either head is routed through diodes
CR6, CR8, CR3 and CR4 {page 2, area DS) and on to amplifier U6 for
flux reversal detection. The filter network consisting of capaci-
tors Cll,12,13,14; resistors R14,15,16,17; and inductors L3,4,5,6
will filter out erroneous noise and shape the data read for zero
crossover detection. Pins 7 and 8 of differentiator US have a zero
crossing that corresponds to each peak of the read signal. These
outputs are AC coupled through C7 and CS to crossover detector U4
(area D2) which generates a low-to-high or a high-to-low digital
transition for each zero crossing detected. Ql and Q2 convert the
signal to a +SV and ground level signal. One-shot Ul7 and flip
flop U12 (area CJ) are used. to eliminate any false crossovers and
UlO, R48, and C28 will generate one pulse for each transition
detected; in essense, a frequency doubler since each transition
represents one data bit. The output of Ul0-8 (area C2) is sent to
one shot Ull-10 (area B4) where a 200ns READ DATA pulse is gen-
erated. This signal exits the drive on connector Pl-46 {page 3,
area Cl).
The Model 40 Series can be used as either a single density or
double density unit. If a double density recording (MFM encoding)
scheme is used, then the WRITE DATA should be precompensated to
minimize the peak shift effect. The controller should determine
the amount of compensation from the data pattern. Refer to para-
graph 3.7 for a discussion on data encoding and recovery.
C D C c _____ _
WRITE DATA (FMilJ u LJ u
-_.l
1
1_ 150nsMIN I I
I 1100ns MAXI I
2.00µs:!:.10µs~ :-- l-4.00µs±20ns~
Figure 3-12: Write Data Timing (FM Encoding)
The MPI Model 40 Series read/write heads have a tunnel erase
element for erasing between two adjacent tracks during writing.
Figure 3-8 shows Erase "Turn On" and "Turn Off" time with respect
to the WRITE GATE input line. Stepping is inhibited while erase is
on.
3-22