Thanks for the links. I assume that if I use the analog dimming mode, that my distances to the LED can be much longer than with PWM dimming.

Are there limits to how far away the LEDs can be from the Power LED Shield? (I seem to recall that a BuckPuck wants the LED to be 6" away max.)

I've gotten this code to work at 2000 steps/sec using 12v.
http://arduino.cc/forum/index.php/topic,89159.0.html

This is a hardware issue, not a software issue. Irrelevant post.

Ah yes, that diode is rather suspicious now...  Note that motors aren't pure inductance, each half of each winding is coupled to the other half, which may in practice limit the inductive spikes from a unipolar motor in certain modes.

And there is the issue of step speed - diodes across the windings slow the motor's response down (a diode and zener diode pair per winding can be used to tame the spike and speed up switch-off)

Interesting...how exactly would the diode and zener diode pair be wired? And would it be compatible with the diodes built into the ULN2003? My stepper motors are maxing out at about 400 steps/sec. It would be nice to increase that.

Isn't that's why you put a snubber diode, and perhaps a parallel ceramic capacitor, across the motor coil?

The ULN2003 has snubber diodes built in to each darlington, all connected to the COM pin. That's why the COM pin should be connected directly to the motor power supply. I've attached the logic diagram of the ULN2003.

"The diode probably protects against reverse polarity supply connection."

Perhaps so, but the back EMF from the stepper coils will hammer the darlingtons without that escape path!

A followup question:

When the stepper is not in use, I wish to cut the high voltage motor supply so the motor can cool down. If I cut the motor supply without cutting the input control voltage, will it hurt the UNL2003?

In other words, is it hard on a darlington to have voltage applied to the base when the collector is floating?

After further reflection, I may have a better understanding of why the two 1k resistors are on the inputs. Those two darlingtons are serving as inverters, and sink only 5mA, compared to the other four, which are sinking upwards of 300mA each (at 50% duty cycle). So perhaps the resistors are lowering the base current to accommodate the lower sunk current? Taking the resistors out should still be fine though, right?

It would just mean a little more current draw on the control line (I'm measuring 1.9 mA vs 2.2 mA base current per control line for with and without the resistor).

There is a circuit diagram for two-wire control of unipolar stepper motors at:
http://www.tigoe.net/pcomp/code/circuits/motors/stepper-motors/
attributed to Sebastian Gassner.

I don't see the need for either the diode connected to COM, nor the two input resistors, all marked in yellow in the diagram attached.

All three should be removed and replaced with simple connections.

The diode on COM is particularly worrisome as it blocks the back EMF escape route that can damage the Darlingtons in the ULN2003 when driving an inductive load, such as a stepper motor.

Am I correct in making these changes, or am I missing something?

OK, thanks for clarification. It took me forever to find this solution, so to help others find it, I'll post:

initialize array of Stepper objects
initialize array of Stepper instances
make array of Stepper objects
make array of Stepper instances

as search phrases that will hopefully help others find this!

Stepper motors[] =
{
   Stepper(80,5,6),
   Stepper(75,9,10)
};

Stepper motors[] =
{
   stepper(80,5,6),
   stepper(75,9,10),
};
will create an array of Stepper objects.

Shouldn't those two "stepper"s be capitalized? i.e., Stepper(80,5,6), Stepper(75,9,10)