A note from the field as to what can happen with a 3494...
I came in yesterday morning to find an Int Req on the robot:
both 3590 drives in the last frame had failed to unload, at very
different times of the night.  The two drives in the first frame
had no problems.  After some 12 hours of analysis and erroneous
guesses by various IBM levels, it was finally determined that the
ARTIC (A Real-Time Interface Coprocessor) had partially failed.
This card in the industrial computer within the 3494 manages
RS-232 and RS-422 communication, as serial connections to a host
and command/feedback info to the tape drives.  The last two drives
had ejected their tapes and had tried to tell the Library Manager
about this, but could not.  When the robot needed to re-use the
drive but knew there was a tape in it, it posted the Int Req.
Attempting to clear the Int Req at the control screen resulted in
the LM being hung with a clock icon.  A Ctrl-Alt-Del reboot would
then always result in a memory dump and reversion to a Shutdown
selection box.  With all the 3494's internal logging one would
think that diagnosis of such a problem would be straightforward;
or at least the manifested symptoms would be well-known.  Not.

So the next time you get an Int Req and the tape is sticking out
of a drive but not removed by the robot, suspect a problem with
the drive's communication with the library manager.
      Richard Sims, BU