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Chapter 4 SCSI Interface

General SCSI Bus Operation

Quantum|ATL PowerStor L500 Series Tape Library User’s Guide 65

The Test Unit Ready command is used to determine whether the

library would accept, for example, a Move Medium or a Position to

Element command without returning Check Condition status.

Bus Free

There are several situations when the device may 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

Bus parity errors (i.e. single bit errors) are very serious because

they imply the possibility of undetected double-bit errors on the

bus, which most likely would result in undetected data corruption.

On properly configured SCSI buses, parity errors should be

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 (see

section 6.1.1 in the SCSI-2 specification). Retrying of parity errors

during Data Out Phase when writing is normally not done, but can

be enabled by changing the EnaParErrRetry parameter in the VU

EEROM Mode Page. This feature is disabled by default due to

negative impacts on device performance (also, the data stream on

writes cannot be pipelined).

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Quantum L500 Guia do Utilizador Página 79