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HP 9895A Disc Memory Command Set A-3
Parallel Poll Response
Parallel poll is used as an additional means of communication between the 9895A and the
bus controller. If the 9895A is ready to accept the next part of a command sequence, it will
respond to the parallel poll conducted by the bus controller.
After accepting most secondary command bytes, the 9895A disables the parallel poll re-
sponse. This indicates that the device is busy processing the current part of the command
sequence. The actual disabling of parallel poll response may occur up to 100 microseconds
after the secondary is accepted by the 9895A. Thus, if the 9895A has parallel poll enabled,
and the bus controller is fast enough to send a command sequence and then conduct a
parallel poll before the 9895A has disabled the poll, the bus controller would see the wrong
parallel poll response. To solve this problem, an intentional delay can be introduced, or a
DSJ command (this disables parallel poll) can be issued before other commands.
The exception to the parallel poll response interlock concerns the Clear commands. The
DSJ command, unlike the Clear commands, may not be a valid or recognized command in
all states of the controller. Thus, the controller may reject or not even see the DSJ command
and not disable the parallel poll response. If the bus controller had expected the DSJ to
lower the parallel poll in the case where the controller can not accept the DSJ, the bus
controller would see the wrong parallel poll after the Clear command.
Cylinder and Track Numbering
Starting from the outer cylinder, cylinders are numbered sequentially from
a
to 76. These
numbers are also the physical track addresses. A track is the intersection of a cylinder and a
head. There is also a logical track address associated with each good track.
If
a disc has no
bad tracks, the logical address of a track is the same as the physical address.
A disc with bad tracks can be made to look like a slightly smaller disc with no bad tracks. To
do this, the bad tracks are specially marked to indicate that they have no logical address. A
track marked in this way is referred to as an invisible track. The remaining good tracks are
sequentially assigned logical track numbers. Logical track
a
is the outer-most good track (it
mayor may not be physical track 0).
During normal operations, the user need be concerned only with logical addresses. The
9895A controller will take care of finding the proper physical address.
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