Are old SCSI tape drives not all created equal?

jim stephens jwsmail at
Fri Aug 19 12:36:51 CDT 2016

On 8/19/2016 9:24 AM, js at wrote:
> On 8/17/2016 6:17 PM, Chuck Guzis wrote:
>> On 08/17/2016 02:59 PM, js at wrote:
>>> Hi, Chuck.  Excellent question -- and they do respond per your
>>> minimum, but beyond that, I'm not sure.  When a drive wouldn't work,
>>> I only thought to check for unit ready, unit identify, and to see
>>> what would happen with a START or STOP unit command.
>>> Even the Teac MT-2ST would respond to those 3 (for the START or STOP
>>> command, it retensions the entire tape).   Interestingly, the Teac
>>> also doesn't provide a unit name like all the others do eg. "ARCHIVE
>>> PYTHON etc..."   It just shows up as a blank during bootup on a PC
>>> with an Adaptec SCSI card.  This lack of name seems to make it
>>> invisible to Windows (XP) ASPI.
>>> I have MSDOS software than allows one to issue direct SCSI commands,
>>> but doing that is beyond my present know-how.
>> Well, that's all good.  SCSI tape covers a lot of ground--from 9 track
>> 1/2" open-reel drives and includes various technologies, from simple
>> DCxxx QIC carts, to DDS, SLT, DLT...  All have their peculiarities.
>> For example, some permit rewriting of blocks; others put this strictly
>> off-limits.  Lots of features are vendor-optional, which include things
>> such as partitioned data sets and robot auto-loaders. Read-after-write
>> verification is optional (but is a good thing, particularly if the drive
>> firmware includes recovery by erase-and-rewrite.
>> Linux can be pretty decent about a one-size fits all and has several
>> optional packages that people have submitted, including the st toolkit.
>> If you can program C, I might have some DOS I/O library functions that
>> may interest you.
>> Generally speaking, the "safe, always there" commands are INQUIRY
>> (0x12), TEST UNIT READY (0x00), REWIND (0x01), REQUEST SENSE (0x03),
>> READ(6) (0x08), WRITE(6) (0x0a)  WRITE FILEMARKS (0x10), MODE SENSE
>> (0x1a), MODE SELECT (0x15), UNLOAD (0x1b) and perhaps SPACE (0x11).
> Chuck,
>   Where might I find information on how to form SCSI command data 
> blocks so as to try the above commands?   I sent just an "01" to the 
> TEAC MT-2ST, and it did rewind.. but did not react to any of the other 
> above commands just by sending single bytes.
>   Oddly, the OnStream drive did *not* accept an 01 command.
Try sending 0x01 followed by 5 or 7 0x00's before dropping select. i 
don't think the Teac should have responded to a single byte after select 
then drop select.  Commands are packets of either 6 or 8 bytes, and 
other should be avoided, unless the vendor documents an exception.

Select with attention which is a reconnect could send shorter packets 
for status and the like, but not a command.
> Thank you-
> -John
>> Of course, commands such as MODE SENSE, MODE SELECT and REQUEST SENSE
>> have variable implementations.  Status for a given condition isn't
>> guaranteed to be the same across devices; for instance on the Qualstar
>> SCSI half-inch drives like to return a record of zero length instead of
>> setting the "filemark hit"  status on a read operation.
>> Generally speaking, however, as long as you stick to the above list and
>> the simplest options, you'll be good with anything.
>> --Chuck

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