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The loading (lowering) or unloading fraising) of the
read / write heads is controlled by bit 4 of the ' bus out'
line. When this bit turns on, the heads are loaded. In
the loaded position, the heads can read or write
information on the diskette surface. If two index pulses
are detected and a command was not issued to the
adapter, this bit is turned off and the heads are
unloaded.
There are two read/write heads on the diskette drive. In
order to read or write the correct side of the diskette,
one and only one head must be selected. Bit 5 of the
'bus out' line determines which head is used in the read
or write operation. If this bit is off, head 0 is selected.
If this bit is on, head 1 is selected.
Four access lines are used to move the heads from
track to track. When the adapter first entered access
mode, the heads were located at track 00. This was
accomplished in the initialization phase of the operation.
In track 00, bits 6 and 7 of the 'bus out' line are both
on. With these bits on, access lines 0 and 1 are active.
These access lines remain active as long as the heads
stay at track 00 and no access command is issued by
the system to change the status of bits 6 and 7. Access
commands move the read/write heads sequentially from
track to track in both forward and reverse directions.
Track
00 01 02 03 04 05 06 07 08 09
Bus Out Bit 6
1
0
0
1
1
0
0
1
1
0
Bus Out Bit 7
1
1
0
0
1
1
0
0
1
1
Access Li ne 0
1
0
0
1
1
0
0
1
1
0
Access Line 1
1
1
0
0
1
1
0
0
1
1
Access Line 2
0
1
1
0
0
1
1
0
0
1
Access Li ne 3
0
0
1
1
0
0
1
1
0
0
To move the heads from track 00 to track 01, bit 6 of
the 'bus out' line turns off and bit 7 remains on. This
pulses the stepper motor and turns it just enough to
move the heads to track 01. Bit 6 turning off with bit 7
remaining on deactivates the 'access 0' line and
activates the 'access 2' line. The 'access l' line remains
active. These two access lines (1 and 2) remain active
as long as the heads are located at track 01 and no
access commands have changed the status of bits 6
and 7. To move the heads from track 01 to track 00, bit
6 is turned on and bit 7 remains on. This turns off the
'access 2' line and turns on the 'access 0' line. The
'access l' line remains active. The stepper motor now
turns in a reverse direction to move the heads back to
track 00.
ACCESS MODE SENSING
In access mode, sense information is sent to the system
by the adapter. This information is in the access sense
byte of the 'bus in' line. The bits in the access sense
byte represent the condition of the different control
latches in the adapter. Bits 6 and 7 are set according to
the condition of the access lines.
This byte of information tells the system the operational
state of the diskette drive. (See Access Sense, under
Adapter Commands in this section.)
The system has sent commands to the adapter over the
'bus out' line. After these commands were sent, the
system sensed the condition of the adapter by analyzing
the access sense byte of the 'bus in' line to ensure that
all of the commands were operated on and the diskette
drive is in the exact operational status as demanded by
the commands.
Operations
3-31
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