Ah, got it. In the context of the thread, with all the RK05 discussion, I wrongly assumed that somehow there was an RK05 involved. I can imagine! You must be the first person in about 30 years to do a PUP implementation! :-) That doesn't surprise me one bit - the RK11-C was designed to drive Diablo drives, and the controller/drive interface was kept almost identical in the RK11-D (the only difference being the drive select stuff). But the drive (either RK05 or Diablo) only provides a bit stream and sector pulses over that interface, so turning that bit string into a sector is controller-specific, and it would have been somewhat astonishing if the PDP-11 and Alto interfaces had used a compatible low-level format. And, as Josh indicated, in fact, they did not. Noel