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'Multiplexing stepper motors'
1996\09\01@212119 by Mark K Sullivan

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>Now, I would like expand the concept to be able to drive five (or
>even six) stepper motors off of a single processor.  I however, don't
>have enough available pins to control each motor directly.

That's easy!
Use the ULN2003 open-colletcor driver to control the phases.  Used diodes
between each stepper (anode) and the ULN2003 outputs (cathode) to isolate the
motors from each other.  Now use one high-side driver per motor to select it for
stepping.  Note that you can only move one motor at a time and you won't have
holding torque when another motor is selected.

- Mark Sullivan -

1996\09\02@122939 by Mark K Sullivan

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>I did try hooking up two stepper motors
>together.  However, even when I didn't have the two common wires for
>one motor connected to anything (normally they would have been
>connected to +24vdc), the motor still stepped in sequence with the
>other motor.

That's because you still have the coils in parallel.  You can think of the
unipolar motor winding pair as a center-tapped winding which you are driving
push-pull.  You had the center-tap open but you still had the ends of the
winding connected together.

>I didn't try using any diodes however.

That'll fix it.

>As far as the holding torque, I don't believe that is a problem.
>Also can you recommend a high side driver?

For the high side, use a PNP bipolar, darlington, or P-channel FET.  Connect the
Emitter (or source) to V+, the collector (or drain) to the motor common.  Use
one of your ULN2003 channels (or another open collector low-side switch) to pull
the base (or gate) toward GND to turn on the switch.  For the bipolar case, use
a current-limiting resistor between the PNP base and the open collector driver.
For the FET, connect a turn-off resistor from gate to source and connect the
source directly to the open collector driver.  I don't know what the current
required by your motor is, but an IRF9520 would be a typical choice for several
amps.

- Mark Sullivan -

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