Digital Equipment RW504-ZA, Service Manual

Page: 172 / 220

which side of the picker faces the mailslot-end of the chassis and allow
calibration of the picker’s position in relation to the drive and storage slots.
6.6.2 Error Recovery Processes
The autochanger uses the following processes to recover from errors:
• Inline recovery
• Find home sequence
• Calibrate
For certain well-defined error conditions, recovery operations that have little
effect on position or performance of the autochanger are executed inline. These
are used only if the error condition can be determined exactly, and in cases where
further motion may make recovery difficult.
The purpose of FIND HOME is to initialize the machine to a known state. For
power-on, this means finding a ‘‘home’’ (zero) position for the carriage/picker
assembly.
The calibrate procedure is then called to further locate reference points other
than the zero locations found during FIND HOME. Using sensors, the picker
is characterized as to its relationship with the mechanism. The positions of
the drive, mailslot, and storage slots are calculated based on the location of the
sensors.
For recovery after power-on, many subsets of FIND HOME may be called. One
mode, for example, only determines which side of the picker faces the mailslot-end
of the chassis. If any of the subsets of FIND HOME fail, the full FIND HOME
sequence is run. A successful running of FIND HOME gives the autochanger
code the exact positions of each end of the carriage rail and the rear plane of the
picker assembly. Also, the autochanger then knows which side of the picker is
facing the mailslot-end of the chassis and whether the picker holds a disk or not.
6.6.3 SCSI Detected Errors
For the vast majority of potential error conditions that may exist, the SCSI
interface retrieves immediate information about the error with no motion
required. Potential error conditions include:
• Machine not being ready for a new command due to another previously-issued
command or a previously-detected hardware fault that prevents motion
• Illegal request to move a cartridge from an empty location or to a full location
• Illegal request to do an unsupported command or operation
• Invalid syntax or parameters in a command
6–24 Theory of Operation