hp 9153 floppy & disk

THe Nighthawk drive interface is strange. Very strange. it has the raw data signals of an ST412 drive (but on single-ended TTL lines, not differential pairs). It also hasa the strangest positioner interface you're likely to see. The positioner is a 2-winding stepper motor. You get (at the drive interface) to contrtoll the currents through the windings (there's a dual DAC in the drive).You also get soem kind of position feedback signal from the drive (there's an ADC in there too). There is no intellegence in the drive to control te DACs based on the output from the ADC, that is done in the controller (I assume in part by the 6809 firmware). Given there's an undocumented ASIC in the drive too, which has a register accessible over the interface, and for whaich I have no data, tryign to recreate the drive is going to be a big job.