Need Help!- Newbie's homebrew controller problem

[mossy173]

Well, I'm on the cusp of having a working homemade desktop CNC engraver/plotter, save for one last problem (hopefully), and it's a bizarre one.

Basic architecture: parallel pins lead to three PIC12F675's that are just programmed to advance stepper motor states when they get hit with step pulses (depending on direction value). Very simple. Those winding power states go through a bunch of transistor arrays to the motors (unipolar).

Problem: only one axis will work at a time. The other two are unresponsive, and sometimes the PIC controlling one of them gets very hot. Thing is, all three work in separate trials (while calibrating each axis individually with the stepconf wizard that comes with emc2). But when I launch emc2 itself and try to jog around with more than one axis, only the first axis I touch will work, and the other two will not until I cycle the power on the machine. When I do that, once again, only the first axis I jog will work, and the other two will not. This is true for all three axes.

I have rewritten the firmware controlling each axis. I have found that cycling power to each individual microcontroller (by unplugging it and plugging it back in) has the same effect as cycling machine power. I have checked for shorts and the like. I cannot imagine why this is happening.

Does anybody have any ideas as to where to look?? I will provide any information requested.

[H500]

Do you have a schematic of your setup?

[mossy173]

Here's a sketch of the electronic setup. The parallel port is connected through 2kohm resistors to pins 3 and 4 of one pic12f675 per axis. Pins 0, 1, 2 and 5 connect through darlington arrays (ULN2003) to the inputs of a six-pin unipolar stepper motor. Each line going into the steppers uses three pins of the darlington arrays in parallel because of current consumption (each pin can handle 500mA).

So, once again, the problem is using all three axes simultaneously. Controlling each individually works fine, but when I try to send signals to multiple axes, only the first I try to move will respond until I cycle the power to the board.

Thoughts??

[H500]

Try putting a big (2200uF) cap across the power supply, close to the PICs. Each PIC must also have a .1uf decoupling cap across the power pins. I assume your power supply is big enough for all three motors.

That should fix your problem

[mossy173]

Thanks so much, H500! I'll do that as soon as I can! Out of frustration and curiosity, though, would you mind explaining what is being solved, and how that will solve it? I'd like to know what's going on.

[H500]

Do a search for power supply bypassing or decoupling on digital circuits and they will explain it in detail. Without it, the power supply will dip for a few microseconds when the pic is switching it's outputs. The 2200uf does the same at lower frequencies. Additionally, it will absorb the back emf from your motors to prevent the voltage from rising too much. Both can cause the pics to malfunction either temporarily or permanently.

[mossy173]

That did it, it works now, and has (sorta) written EMC2!! thank you so much!