For the servo system, add another slave card and servo motor.
For the stepper system, several methods of controlling focusing are available:
a. Connecting parallel port pins 16 and 17 to a pair of relays to control a small DC motor for focusing. Pin 16 focuses 'out' and pin 17 focuses 'in'. This can be operated with the field rotator concurrently.
b. As above in option 3, with the addition of parallel port pins 1 and 14 for slow speed control. Field rotation is disabled with this option.
c. Using the field rotation motor control circuit with a small stepper motor for focusing. This uses parallel port pins 1 and 14 for pulse and direction.
d. Adding a second MC3479 bipolar stepper control circuit with a small stepper motor for focusing. This uses parallel port pins 16 and 17 for pulse and direction. Both the field rotation and focus motors can be operated simultaneously. Along with the altitude/declination and azimuth/hour angle motors, this means that four motors are controlled at the same time.
If a stepper motor is used, the focus position is kept track of by virtue of counting the stepper pulses. Particular focus positions can be returned to.