On Wed, 7 Apr 1999, Richard Erlacher wrote: Interesting. All of the SA400s that I've seen used the spiral groove on a rotating disc. The only 5.25" drives of that vintage that I have with a helical lead screw were the Micropolis 35 track 48TPI and 77 track 100TPI (NOT 96TPI!!). Was there any sub model designation for the Shugarts to differentiate different positioners? -- Fred Cisin cisin(a)xenosoft.com XenoSoft http://www.xenosoft.com 2210 Sixth St. (510) 644-9366 Berkeley, CA 94710-2219

"Richard Erlacher" <edick(a)idcomm.com> wrote: Correct about GCR, but not about head positioning. If you look at the schematics of the Disk ][ controller card and that of the drive, both of which were in the DOS reference manuals up through the early 3.3 manuals (before the Apple //e shipped), you will see that the controller uses four outputs of a 74LS259 (or 9334) 8-bit addressable latch to control the four phases of the head positioning stepper motor. IIRC, in the drive the four TTL-level phase signals from the controller are inputs to a ULN2003 which actually drives the motor phases. The stepper motor has an additional stable states halfway between any pair of adjacent tracks. However, due to the head width, it was not possible to get twice the number of usable data tracks. But you could use non-standard track positions as long as they were at least two steps apart. One of the earliest copy protection schemes was to use track 0, 1.5, 2.5, 3.5, etc. Of course, the copy programs quickly started supporting half-track positioning. The screwy part of Apple's head positioning system is the absence of a track 0 sensor. The technique to home the positioner was to step away from track 0 a few steps (call it N), then step toward track 0 by more than 68+N steps (the number 96 comes to mind but I could be mistaken). This results in the famous "Disk ][ mating call" sound. In fact, you can actually get quarter-track positioning, but you have to keep one (two?) of the stepper coils driven to hold the positioner in place. This can be viewed as a very crude form of microstepping. Of course, you can potentially also write data *during* the postitioner motion. However, this was seldom done even for copy protection, because the repeatability was poor even on the same drive, and very poor between drives. Because the stepper motor was under software control, the RWTS subroutines (Read/Write Track & Sector, the low-level disk driver) actually used a ramped acceleration/deacceleration profile, for faster seek times than in typical systems which used a fixed track-to-track step time. Eric