SCSI tape question
Jon Elson
elson at pico-systems.com
Sat Dec 13 12:57:01 CST 2014
On 12/13/2014 12:38 AM, Chuck Guzis wrote:
> On 12/12/2014 09:04 PM, Jon Elson wrote:
>
>> OK, that narrows things down QUITE a bit. I worked with
>> some Exabyte
>> drives and others that had a lot of constraints on what
>> you could do,
>> and you could do some interesting things. I think you
>> could write
>> zero-length blocks, and they would show up just like that
>> when
>> read. Clearly on 9-track, there SHOULD BE no such thing
>> as a
>> zero-length block, but I suppose some drives could
>> actually write
>> such a thing at 1600. A preamble, CRC, LRCC and
>> postamble, with
>> no data. But, that OUGHT to be an illegal command.
>
> I seem to remember on certain tapes, that very short
> records are discarded or reading; something like a
> "noise" threshold of 7 bytes. But that could be for
> 7-track and 800 9-track. I don't recall--but regardless,
> mistaking a 0-length record, should one occur, for a
> filemark is not catastrophic.
Yes, quite correct. Turning on or off the erase head would
leave a "noise burst" that
could be picked up by the read amps. So, in NRZI mode, you
had to ignore them.
Good drives had something like a current ramp-up control to
eliminate these while
writing.
In PE mode, there would be a preamble and postamble of 40
transitions each,
so you have no way of mistaking a little noise for the
beginning of a valid block.
Jon
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