Building the HobbyCNC 4 Axis Stepper Driver

Package Contents

Three days after I had placed my order with HobbyCNC the box containing the parts arrived on my doorstep. I purchased a package. That is: It contains the driver board, power supply components and 4 127 oz-inch stepper motors. It is expected the builder will supply their own transformer (117/24VAC 10A) and case for the power supply. Everything was well packaged. Nothing was broken or missing.

PCB Layout

The PCB is double sided, solder masked and silk screened. Since this board controls 4 steppers, 4 sections of the board are roughly the same. Each stepper is controlled by step and direction pins from a PC parallel printer port. The board also provides 5 inputs that can be used for limit switches. HobbyCNC does not provide a schematic with the kit, but the silkscreen on the PCB and the step by step directions make it easy to put the board together.

After Soldering...

All components on the board are through hole. I used both a 12W and 25W soldering iron to build this board. The wide, high current traces around the stepper driver chips suck up a lot of heat, so a higher power soldering iron is needed. The stepper driver chips are the last things to go on. Before that is done the assembly instructions run through some tests to ensure the preceeding assembly has been correct. By doing this we avoid damaging the expensive driver chips.

Board Testing

The board is based around the SLA7062M Stepper Driver Chip from Allegro Microsystems. The board allows the user to set the step mode in which the chip is used. That is: 1/2, 1/4, 1/8 and 1/16 stepping. The motor current can also be set with the potentiometers on the board. Both the step mode and motor current can be set on a per-axis basis.

After the step mode and current have been set the stepper motors can be connected. Since I was using HobbyCNC motors this was a simple matter of following the wire by color instructions, other unipolar stepper motors will take some investigation with an ohm-meter to establish the correct wiring. Bipolar stepper motors cannot be used.

It turns out that CNC gives new life to old laptops. Common multitasking operating systems (Linux, Windows) are non-realtime and have unpredictable task switching performance. Since DOS is single tasking, a program writer is assured of predictable performance, so DOS is still a good choice for CNC control. I used TurboCNC running on an otherwise useless 486 laptop to test out the stepper motors. Once TurboCNC is configured with printer port step and direction pins for each axis, the jog command can be used to verify the operation of the stepper motors.

Now that the driver board is working we can wean ouselves from the benchtop power supply by building a dedicated power supply. We will also package everything neatly and safely into a custom case.