BASF 6106 - Track Zero Oetector

2.2.5 TRACK ZERO DETECTOR
This logic generates the TRACK OO-signal when
the read/write head is positioned at track zero.
The host system uses this signal to recalibrate
th~ positioning system. When the position of the
read/write-head
is unknown the host system sends
step out pulses until TRACK 00 goes low.
The track zero detector is comprised of the track
zero switch, a comparator
circuit and a phase
detector circuit, which detects the correct
phase of the stepper motor. The track zero
switch
mounted on the deck assembly is activated
by the head carriage. The logic circuit supports
mechanical
and optical track zero switches
(see Fig. 2-21).
When the head carriage moves out,the track zero
switch
must be open before the read/write head
re ach es track four. When the head carriage moves
• sv
towards the track zero position the track zero switch
must close after track four and be fore track zero.
The TRACK ZERO-signal will be active when the track
zero switch
is closed and phase A and phase C of
the stepper
motor are activated. OUTENABLE (0 [NA)
must be high.
Fig. 2 -
22 shows the timing diagram when the host
system attempts
to step the head carriage beyond
track zero. The mechanical stop on the spiral wheel
prevents the read/write head from moving out further
and holds it near track zero. But the TRACK OO-signal
will be deactivated, because the stepper motor is
in a wrong phase (0 BI, 0 CI). If the host system
sends three
more step out pulses, the phasing of
the stepper
motor is correct again, the TRACK 00-
signal is activated and the read/write head is posi-
tioned
at track zero.
Ta
THE
HaST
SYSTEM
f~T 0 ENA
INTER FACE
FIGURE 2 - 21 TRACK ZERO DETECTOR
OE:\A
DI RECTI ON
IN
STEP
I I I I I
I
I
I
I
TRACK
5 4
3 2
I
0
9
NEAR TRACK 00
0
ß
A
•
~.
II
I
7
ß
·C
I
4Ii
fJ
D
/
•
4
•
•
TRACK 00 S!\'ITCH
,...--. - -,
'-- .. -
&
TRACK 00
•
•
FIGURE 2 - 22 TRACK ZERO· TIMING DIAGRAM
2 - 16
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