Table of Contents
HP 9895A Disc Memory Command Set
A-5
HP and IBM Formats
The 9895A can work with discs which use either the HP single- or double-sided formats or
the single-sided IBM format. After a disc is inserted in a unit, the first status request for that
unit will cause the controller to determine which format is present. This information is
available as part of the returned status.
Many details of operation vary slightly for the two formats. These include the allowable
range of target address, the updating of target addresses and the effect of the D bit. These
differences will be noted in the appropriate command descriptions.
Loading of the Recording Heads
Control of the position of the heads is done by the controller. Any command received
correctly that needs to read or write the disc will toad the heads for its operation. In addition,
the controller will keep the heads loaded for approximately 2 seconds after the command
completes the operation in anticipation of a subsequent command. In multiple drive config-
urations, the time the heads remain loaded on inactive drives can increase from 2 seconds to
approximately 10 seconds, as a function of the operation in progress on some active drive,
the interleave of the disc in the active drive and the frequency of the command from the host
system. If the host system should malfunction, and leave the controller in a state expecting
to receive or send data, the controller will abort the operation in approximately 60 seconds
and proceed to its idle loop where the heads could be unloaded. The host system has the
ability to lock the access door on any
I
all of the drives to prevent unwarranted access to the
discs; in this case, the doors will not be unlocked if the above error condition should occur.
Table of Contents
