Heatkit 5 1/4 floppies

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Apr 7 19:10:09 1999

"Richard Erlacher" <edick_at_idcomm.com> wrote:
> Apple drives
> used a form of GCR on their drives, the heads of which were positioned with
> a software timed DC motor, while nearly everyone else used FM or MFM on
> drives which used steppers driving band actuators.

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
Received on Wed Apr 07 1999 - 19:10:09 BST