600005_CM6000_Drive_Debug_Procedure_Aug84.pdf

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COMPUTER
MEMORIES,INC.
DRIVE DEBUG PROCEDURE
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C:on
row~r-ep,
the
driv~
enters an initili:ation procedere, where various
system Farazreters such as disc speed, servo velocity, power supply
voltage and track zero are checked. After verifying these
para~eters,
the
~icroprocessor
issues a Drive Ready signal to the controller,
indicating the drive is ready to perform read/write operations. The
controller in this case being the AMT tester.
Positioning of the read/write heads to various tracks is achieved by
setting the direction
line,
(logic
a
for seek in, logic 1 for seek
out) and clocking the -step line. After locating the desired track,
the drive issues a seek complete signal to the controller.
There are many conditions which must be met in order for the drive to
properly read and write data on the disc surface.
.
The serial: Data' Bit Rate of the drive is a direct function of the disc
speed, hence proper disc speed must be maintained in order for the
controller to properly decode the data it receives from the drive.
The CM6000 operates on a rate of 5 megabits per second. This corres-
ponds to a spindle motor speed of 3600 RPM.
Each time the spindle motor rotates 360 degrees, an index pulse from
the spindle motor encoder is issued to the microprocessor. The
~icroprocessor
continuously monitors these index pulses and issues a
correction signal labeled "motor"" when the elapsed time between index
pulses is greater than or less than l6.8ms. This variable duty cycle
signal is filtered via R5 on the main board and C4 on the motor control
board and is used to control the spindle motor speed.
wnen wrltlng data on the disc surface, proper
~rite
current should be main-
tained. Excessive write current may result adjacent track data being dis-
turbed dee to increased flux lines generated by the readwrite heads.
txcessive write current also increases "Bit Shift" and
~ay
result in reduce,
..dncow r.argins.
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Insufficent write current is troublesome fer a
nu~ter
of reasons.
~~~e6~cr
that in order to write new data on the eisc scrface, the
old data
~ust
first be erased. Insufficent write
c~rrent ~ay
result
in new data being written over .partially erased old data. A cOJrunon
symptom of this condition is
~andom
soft errOrs during a read operation.
Figure 1 is an example of data that was written without a sufficent
amount of current.
Note the peak to peak
a~plitude
variations and the
al::sense of a l::l.ank horizontal line at the zero crossing point.
Ficure 2 should clarify the reaSOn for this. Notice that the
center of each data cit cell drops down and actually touches
the zero crossing point. This may result in the drive sending
a logic 0 instead of a loglc 1 and vice versa. The
eMGOOO
in-
corporates the use of a
·Desnake·.~ircuit~
consisting of Z41. Z23
and Z32, to help overcome this
p~oclem.·
However sufficient write
current should be maintained for reliable operation.
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Figures 3 and 4 are
exa~ples
of cata written with the correct
amount of write current. Note in Figure 4 t.he center of each
tit cell never comes in contact with the :ero crossing point.
ef course different data patterns will result in different
waveforms and may vary slightly from head to head and from
drive to drive.
C€<OSSING.
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l.n order for the drive heacs to serve any useful function,
tb~y ~ust
first be atle to locate a track, and stay on that
~rack
for the encire
360
degrees of rotation
d~rins
a read or
write opeation. Not an easy tasK when you consider thac three
tracks are about as wide as the thickness
'of
this piece of
paper!
.
.
.
The
eM6DOD
incorporates the· use of a closed loop, servo wedge
positioning system, as outlined in Fi9ure
5.
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·Ol.:;o~t sic~las,
channel A and channel-B.
Ficure 6t is the
difference si;nal of channel
A
and B. This signal may be observed
Fl:l.:re
ba
1~.L\,;:'_"''''''-''
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-------_
at T.P. 10. Each half cycle represents one tracK On the disc
surface, with the zero crossing of each half cycle denoting the
center of the track. Fi9ure
~
may help the reader visualize this
;-elationship.
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