Serial Wombat
a general-purpose digital interface device for hobbyists, engineers and students

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Serial Wombat Unipolar Stepper Mode

Note: The protocol for stepper motor mode has changed slightly for version 2.0 and above. Users of previous versions should download documentation specific to their version on the download page.

The Serial Wombat can control up to 8 unipolar stepper motors using 8 sets of four pins.

Using your Serial Wombat as a stepper motor controller is easy! Just follow these simple steps:

• Build your interface hardware. The Serial Wombat is a logic level device, and can't power stepper motors directly. For high current motors I typically tie the common wire to the power supply, and use logic level low-side MOSFETs to switch the coils. For lower current motors I often save a couple of bucks and use a darlington pair array.
• Tell the Wombat which pins are connected to the four motor wire drivers.
• Tell the Wombat what to do: Spin one direction or the other, or move the motor to a certain position. You can select half steps for greater precision, full steps with one coil to reduce current, or full steps with two coils to increase torque.

Repeat the above steps for up to 7 more unipolar stepper motors

Each stepper motor requires at least two messages to configure: message 200 and 201. Message 202 can be used after initialization to move the motor to a new absolute position without changing any other parameters.

Message 203 is used to allow the stepper motor to move in one direction or the other until a digital input changes. This allows the Wombat to automatically stop the motor if a limt switch is used.

Message 204 is used to move the motor an arbitrary number of steps clockwise or counterclockwise. This can be used without any knowledge of absolute motor position.

Message 205 is used to specify the current motor position. This command can be used to initalize the internal step count within the Wombat, so that future absoulte position commands are referenced from that number and the current position.

Message format:

The pin assigned to Unipolar Stepper Mode provides the current step position as its public data.

Examples:

Configure pins 33,34,35, and 36 to drive a unipolar Stepper Motor Clockwise in half steps with a 4 frame delay between steps. A darlington pair is being used to switch the motor, so drive a pin high to energize a coil, and low to deenergize it.

200 33 23 34 35 36 1 0 ; Set pin 33 to Unipolar Stepper Mode
                           ; Pins 34,35,36 are other pins
                           ; Pin high to energize a coil
                           ; Pin low to deenergize it.

201 33 23 1 4 0 0x55 0x55  ; Second configuration message, pin 33 to Unipolar stepper
                           ; Mode 1 (Spin CW)
                           ; 4 frames between steps
                           ; Half Step (0)
                           ; Last two bytes aren't used in this mode

Configure pins 33,34,35, and 36 to drive a unipolar Stepper Motor Clockwise in full high torque steps with a delay determined by pin 2's public data between steps. A darlington pair is being used to switch the motor, so drive a pin high to energize a coil, and low to deenergize it.

200 33 23 34 35 36 1 0 ; Set pin 33 to Unipolar Stepper Mode
; Pins 34,35,36 are other pins
; Pin high to energize a coil
; Pin low to deenergize it.

201 33 23 1 4 0 0x55 0x55 ; Second configuration message, pin 33 to Unipolar stepper
; Mode 1 (Spin CW)
; 4 frames between steps
; Half Step (0)
; Last two bytes aren't used in this mode

Background:

Check out this great tutorial by Microchip on stepper motors.