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ArtistA logic schematic consisting of a pair of cross-coupled NAND gates forming a latch, whose output feeds into the D input of a D-type flipflop. The clock input of the D-type flipflop is labelled "Phase 2", and the two inputs of the latch are labelled "Phase 1" and "Phase 3".
When the Apple ][ disk drive recalibrates to track zero, it makes a disturbing buzz as the disk head repeatedly hits the track zero limit. This is because (unlike the Atari drives) there is no track zero sensor. This circuit, connected to Phases 1,2, and 3 of the actuator stepper motor, sets the D-type flip flop to indicate which direction (in or out) the head is moving. So we can stop the stepper motor when the track zero sensor activates, until the head moves inwards again.
The circuit is also useful when connecting PC drives to the Apple, since they use step/direction actuators instead of 4 phase actuators.
How it works: When phase 2 occurs, the D-type flipflop samples the latch, to see whether the previous phase was phase 1 or phase 3. Thus it indicates whether the head is moving outwards or inwards, If outwards and the track zero sensor is active, then disable the actuator driver chip (thus stopping head banging). If inwards, then re-enable it, so the head can move again.
