Table of Contents
Advertisement
SCSI Interface Overview
Page 7
remains higher than normal, use a cleaning cartridge to clean the read/write heads. If
this procedure does not clear the problem, change to a new tape cartridge.
If a hard error (unrecoverable error) occurs during operation, the drive terminates
operation immediately and returns a Check Condition. The Initiator should cease any
further read or write functions and issue a Request Sense Command to determine
the type of error.
When the drive detects a write error, it attempts to rewrite the data up to 128 times.
After the 128th attempt, the error is considered unrecoverable and the operation
terminates. When a hard error is encountered, replace the tape with a new cartridge
and repeat the function or clean the heads with a cleaning cartridge.
Variable and Fixed Mode Recording
The Viper drive can write either fixed or variable block sizes. The recording mode is
determined by the Fixed bit in the SCSI Write and Read commands.
If the Fixed bit is set, the Mode Select command sets the size of the next block or
multiple blocks to be written with the next Write command. When a Write command
is issued with the Fixed bit set, the current block size is implemented. The transfer
length specifies the number of blocks to be written with this size. If a Write command
is issued with the Fixed bit set and the current block size set to 0, the Viper drive
returns a Check Condition with Illegal Request Sense Key. When writing with the
Fixed bit set, each Write command specifies the number of contiguous blocks to be
written of a fixed size, resulting in fixed-mode blocks. If the Fixed bit is reset, then
only one block can be written on the tape per SCSI Write command CDB. In that
case, the Write command CDB transfer length specifies the size of the block to be
written in bytes. With the Fixed bit reset, the current block size specified with the last
Mode Select command is ignored.
Setting the block size to 0 in the Mode Select page descriptor is not required.
Therefore, with the Fixed bit reset, each SCSI Write command may specify a
different byte count, resulting in variable-mode blocks.
The host may switch between fixed and variable mode recording. By issuing the
Mode Select command to specify different block sizes, blocks can be written to the
tape with different block sizes in the fixed mode. Also, the host may change the block
size after BOM, allowing on-the-fly block-size changes.
The Read command Fixed bit also specifies fixed or variable mode. When reading in
variable mode, the host must know the size of the block to be read from the tape in
advance in order to avoid causing the Viper drive to return a Check Condition with
Incorrect Length indicated in the Sense data (ILI). Also, the data transfer may be
truncated (cut off) when the recorded block does not match the transfer length in
variable mode or the current block size in fixed mode.
The Read command includes a SILI bit to Suppress ILI Check conditions.
When the SILI bit is set, the host usually specifies the maximum block size before
reading so that the data blocks are not truncated, and no Check Conditions are
generated.
Advertisement
Table of Contents
