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ACCESS MODE
Read/
Write
and
Head
Control
Latches
New Media
I - -
MFM
~
Inner Track Select
~.
To Drivers
Bits 0,1,2,
(filter control)
and Sense
Command
3,4, and 5
Inner Tracks
~
Information
Decode
Head Engage
-
....
Head Select
-
."
Bits 6 and 7
Parity
~
r--+
Selector
us Out
Access
Generator
r--r
B
Parity
Control
(bit 6 on)
Busln
~
0
Check
1
(bit 7 on)
}
To Stepper
Motor
Machine Check
2
(bit 6 off)
and Sense
3
(bit 7 off)
Information
In access mode, the system can check the status of the
drive and diskette, move the heads, select the correct
side and current, and load or unload the heads. The
adapter is in access mode when no read, write, or
diagnostic operation is in progress, and a diskette drive
has been selected.
After an initialization sequence, the adapter is set to
access mode.
The commands for the adapter in access mode are sent
by the system over the 'bus out' line, access control
byte. As the access control byte enters the adapter, it is
checked for odd parity. This parity is generated by the
system. If an even parity is detected, the machine check
line is activated and the commands are ignored.
Bit 0 of the 'bus out' line is turned on to reset the 'new
media' latch in case it has been set. The 'new media'
latch is set anytime an index pulse is not sensed for 200
ms or the index pulse duration exceeds 16 ms. If a
diskette is not inserted in the drive, the diskette drive
cover is open, or the diskette has stopped turning or
slowed down with the index sensing mechanism
covered, the index pulse sensing time will exceed 200
ms. If the diskette has stopped turning or slowed down
with the index hole over the index sensing mechanism,
the index pulse duration will exceed 16 ms.
3-30
The 'M FM control' latch is set or reset depending on
the format to be used in the operation following the
access operation. If the following operation is in the
MFM format, bit 1 of the 'bus out' line is set on and the
'MFM control' latch set. If the following operation is to
be in the FM format, bit 1 of the 'but out' is turned off
and the 'M FM control' latch is reset.
As the read/write heads move toward track 76 of the
diskette, the density of the recorded information
increases. This happens because the physical length of
the track is smaller on the inside tracks. Since the same
amount of information is recorded on each track, the
information on the inner tracks is more compact than
the information on the outer tracks. When the heads
reach track 60 accessing toward track 76, bit 2 of the
'bus out' line is turned on. This sets the inner track select
(filter control) latch which filters out the clock pulses.
When the heads are accessing toward the track 00 and the
head location is between track 00 and track 59, this bit is
turned off and the inner track select (filter control) latch
is reset.
A lower current is required at the inner tracks for write
operations. The inner tracks latch is used to provide a
lower current at the inner tracks. Any time the heads are
located at track 42 or greater, bit 3 of the 'bus out' line
is turned on. This sets the inner tracks latch. Bit 3 of the
'bus out' line is turned off, and the inner tracks latch is
reset any time the head location is track 41 or less.
/ '
,
d--- '"
,~-,.)
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