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c. Data bit is entered into the Oiff
256 FF and Oiff 512 FF*when con-
troller drops Serial Write out.
(Sample time).
d. FF K1200 is set.
It functions in
this command as it did during steps
3 and 4 to ensure correct command
sequencing.
e. Drive drops Serial Read In two micro-
seconds after fall of Serial
W~ite
Out.
11. Controller places difference byte on
bidirectional data lines.
Since the
difference is 150, the byte is 1001
0110.
Controller also raises Seek
Forward Lower (Tag 03) and Serial
Write Out.
a. Drive decodes command.
b. Drive raises Serial Read In.
c. Data from bidirectional data lines,
Oiff 256 FF and Oiff 512 FF*is
applied to inputs of Difference
Counter.
It is not gated in until
step d.
These inputs are comple-
mented by inverting the inputs to
t~e
counter.
Including bit 2 8 and
2 , decimal 150 (1001 0110) becomes
873 (11 0110 1001).
The Difference
Counter increments with each cylin-
der pulse durinq the seek:
seek
length is zero (T=O) when counter
is at its maximum count of 1023 or
511 for BR3E4/3E5.
d. When controller drops Serial Write
Out Sample time
(DC~l)
is up for one
microsecond.
FF K1200 checks command
sequencing and, at the same time,
the Difference Counter common enab.le
line gates 873 into the counter.
e. When
DC~l
times out,
DC~2
is up for
one microsecond.
This is Seek En-
able time.
The remaining events
occur in sequence.
f. Seek Initiate is generated.
As long
as there is not a Pack Unsafe condi-
tion and Unit Ready is up, the For-
ward FF sets.
g. If the seek length is not zero, the
Seek FF sets.
On the other hand,
with a zero-length seek, Seek FF
does not set; positioner Busy is up
for
DC~2
time, followed by DEN.
h. When
DC~2
times out, drive drops
Serial Read In.
No further control-
ler/drive communications are needed
to complete the seek.
*Not applicable to BR3E4/3E5.
83318200
0
12. The following drive events occur with
the Seek FF set:
a. The output of the position conver-
ter is gated to the desired velocity
function generator to generate the
Position Error signal.
This initi-
ates voice coil amplifier current.
b. A 1.2 microsecond Seek Start pulse
is generated.
13. Seek Start initiates the following
events:
a.
A~OO
millisecond delay is trigger-
ed.
If On Cylinder is not obtained
before the delay times out, Seek
Incomplete (bit 27 of OS3) , Device
Fault, and FLT are generated.
b. Bit 2 0 of Cylinder Address Register
is gated to Slope FF. If this bit
is up, seek is to an odd-numbered
cylinder.
The FF will then be set.
The track servo signal is, there-
fore, inverted before becoming the
fine servo signal (position error
signal when T
<
1) during even
~.
c. Fine FF is cleared.
The position
error signal is obtained via the
coarse gate.
14. The positioner starts its seek. Refer
to Positioner Motion.
15. As the positioner begins to move, the
track servo signal increases.
This
signal functions as the fine position
signal in the servo circuit.
When it
exceedS about 0.7 volt, On Cylinder
Enable drops.
The following events
occur:
a. If On Cylinder Enable is down for
more than 800 microseconds, the On
Cylinder FF clears.
This causes
steps band c.
b. Positioner Busy status is raised.
c. The heads are deselected.
"d. The Schmitt triggers driving On
Cylinder Enable have hysteresis.
Fine position must exceed 0.7 volt
to initiate Not On Cylinder while
it must be less than 0.3 volt to
enable,the On Cylinder delay.
16. When the seek is completed and On
Cylinder is generated, Positioner Busy
status drops.
This raises DEN to the
controller.
3-57
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