The next question is, what's the "SS/80
Command Set" ?
Sorry, I misunderstood. It looks like SS/80 is the HPIB command set used
by the 9153, and not the actual interface to the drive itself.
The only document which seems to talk about the actual drive is this one
http://www.hpmuseum.net/document.php?hwfile=2723
And sadly it's a bit light on any details.
ob