SCSI tape question

[Jon Elson]

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