Quantum PowerStor L200, User Manual

Page: 72 / 112

Chapter 5 SCSI Interface
SCSI Message System
56 Quantum|ATL PowerStor L200 Series Library User’s Guide
Bus Free
5 There are several situations when the device will go to
BUS FREE
unexpectedly, as defined in the SCSI-2 standard (see sections 4.1.1
and 4.3):
• An internal hardware or firmware fault that makes it unsafe for
the device to continue operation without a full reset (similar to
a power-up reset).
• ATN asserted or bus parity error detected during non-tape
data transfers.
Bus Parity Errors
5
Bus parity errors (single bit errors) are very serious. They imply
the possibility of undetected double-bit errors on the bus. Most
likely these result in undetected data corruption. On properly
configured SCSI buses, parity errors are extremely rare. If any are
detected, they should be addressed quickly by improving the
configuration of the SCSI bus. A well-configured SCSI system in a
normal environment should be virtually free of bus parity errors.
Bus parity errors cause the library to either retry the operation, go
to Status phase, or go to
BUS FREE
and prepare sense data.
Retrying of parity errors during the data out phase is normally not
done, but can be enabled by changing the
EnaParErrRetry
parameter in the
Vendor Unique (VU) EEROM
mode page. This
feature is disabled by default because of negative impacts on
device performance. Additionally, the data stream on writes
cannot be pipelined.