Digital Equipment RW504-ZA, Руководство по обслуживанию

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• Various bus-level communication errors
In all of these cases, the command is rejected immediately and the autochanger is
not moved.
6.6.4 Move Errors
If an error is detected during an autochanger motion, the state of the machine
is recorded in internal memory and a retry procedure is called. Errors of this
type may be either physical or logical, and may be recoverable or unrec overable.
Results of the error recovery are returned to the host when the command
completes. If possible, the cartridge is returned to its original location before
command completion, putting the autochanger back into its original state.
Logical errors refer to conditions in which source locations were found
unexpectedly empty or destination locations were found unexpectedly full.
These conditions indicate that a cartridge was moved without the knowledge of
the autochanger, possibly during service. At this point, the host must become
involved in locating the source of the error.
The host’s actions can include issuing a Read Element Status command to
find the difference between the host’s location (element) list and that of the
autochanger, followed by an Initialize Element Status command to find the actual
locations of all disks. When the differences are determined, a final check of data
on the disk should be done, and the disk must be returned to the appropriate
location (element).
The picker ‘‘element status’’ always reflects the physical state of the picker. The
autochanger does not give a status when the picker is found unexpectedly full or
empty because this logical error is not allowed. At power-on, the picker is checked
to see if it contains a cartridge. The mechanical design of the picker prevents a
cartridge from being fully inserted into the picker without first going through an
initial power-on cycle in the autochanger.
Physical errors refer to conditions in which something physically changes in
the system that prevents normal operation of the motion. These can be either
temporary or permanent. Error recovery attempts to recover from every physical
error without host intervention. Any error that is detected through overforce,
sense of touch, or by a sensor calls a procedure to attempt the recovery.
One exception to the no-host-intervention rule is in the drive/autochanger
interaction. If the autochanger indicates that a cartridge has been inserted
into the drive, but the drive does not read it, the host must become involved in
identifying the source of the problem. Likewise, if the drive is commanded to
eject a cartridge and does not do so, the drive is considered to be empty and the
host must identify either the final position of the cartridge or determine if the
drive has failed.
Theory of Operation 6–25