Viper 60s Theory Of Operation And Maintenance Manual page 140

THEORY OF OPERATION
6.3.2 Write Data (Backup) Operation
Each formatted block of data is written immediately after the preceding block. Assuming the
proper control signal protocol is in progress, write can be commanded by the host under
certain conditions. In any case, Viper circuits verify that the cartridge is in place and not write
protected.
If
the host has not issued a tape position command to begin recording, the drive defaults to the
beginning of tape (BOT) before writing begins.
Writing to tape can begin following a read operation. In this case, the last file written on tape
is
found, and the drive waits in the vicinity where the last data was recorded. When a write
command is received, data blocks supplied are written after the previously recorded block of
data.
Writing data to a streaming tape means that the tape should be in constant motion. For the
tape to remain in constant motion, the flow of data from the host must be sufficient to keep the
tape drive buffers full of data.
If data transfers from the host are interrupted, an underrun occurs. If data transfers from the
host are under 90 kilobytes per second, the tape will not stop but the drive may, at intervals,
write a duplicate of the preceding data block. The duplicate block is transparent to the host.
If the data falls below 45 kilobytes per second, the drive responds by writing a second copy of
the last block and then writes a large postamble, stops tape motion, changes direction, and
positions head over already written data. When the data transfers resume, the drive searches
for the end of the last block and begins writing.
Underruns should be avoided since they consume tape, and the repositioning increases write
time and reduces data throughput.
During a write operation, the early warning hole for the last track is sensed, the drive stops
accepting data from the host at the next block boundary. The drive completes writing all data
blocks contained in the buffers and raises the -EXCPT signal to the host.
In response, the host can command one block of data or a filemark written. The drive reports
that the tape is at the end of media again and then writes another data block. This block, or
blocks of data can be used to describe the file as incomplete.
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