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rt-stepper products
USB to DB25 dongle
dongle breakout board
4-axis motor drive
sensor kit
  • Converts parallel port CNC controller to USB.
  • Both manual and auto CNC operations supported.
  • PC software runs on Windows, Linux and Mac OS X.
  • EMC/LinuxCNC compatible.
  • CAM compatible with Fusion 360.
  • Measure spindle speed in RPM.
  • Synchronized threading.
  • Integrated 4-axis motor drive CNC controller
Please note, rt-stepper dongle hardware does NOT use Mach3 software.

April 2020, announcing PyMini software release 2.9. Corrected some PyMini backplot panel simulation issues. Some gcode commands were not being displayed properly in the backplot. This change only effects the PyMini backplot panel display, the gcode commands executed correctly on the CNC machine.

December 2019, announcing PyMini software release 2.8. Updated the homing script, added support for two new parameters HOME_LIMIT and HOME_VELOCITY. The new parameters are set in the rtstepper.ini file and are only used by the homing script. Fixed an Auto/Cancel issue when using the gantry support. This fixed a problem where a two motor driven gantry would get out of sync after a Cancel.

For previous announcements see the News Archive.

Eckler Software offers a unique USB solution for CNC controllers with the rt-stepper USB-to-Parallel dongle. Users can use the rt-stepper dongle to convert a parallel port CNC controller into a USB CNC controller. Designers can integrate the rt-stepper dongle hardware into there own CNC controller. The rt-stepper 4-axis motor drive CNC controller is a good example of integrating the dongle hardware with the Pololu/StepStep A4988 carriers.

Most CNC controllers are designed to be driven with a PC parallel port. Unfortunately parallel ports are thing of the past and many new PCs have no parallel port. Commercial USB-to-Parallel dongles do exist, but these dongles are designed for printers not CNC controllers. CNC controllers require XYZ step/direction signals. With the rt-stepper dongle you can now drive XYZ step/direction signals over the USB bus.

Driving a CNC controller with a PC parallel port requires a real time OS otherwise the stepper motors can loose synchronization. With the rt-stepper dongle this restriction is removed. The rt-stepper dongle provides the real time stepper motor control which means you can now use your favorite OS such as Linux, Mac or Windows. Each dongle provides a step resolution of 46,875 Hz.

Most commercial gcode interpreters do NOT support laptops with parallel port chipsets. This is not a problem with the rt-stepper dongle. You can use any Linux, Mac or Windows laptop or netbook with USB support. Laptops can provide dedicated CNC setups with minimal investment and work space.

PyMini is a powerful minimalist graphical user interface written in python which has the same look and feel in Linux, Mac and Windows. A user can write their own gcode program using their favorite text editor or run a CAM generated gcode program on PyMini. You can verify the gcode program with or without the dongle connected and view the xyz paths executed on the backplot panel.

There is only one process that runs on the PC - PyMini. PyMini is a GUI front-end program that communicates to the dongle. Both manual and auto CNC operations are supported. With a index pulse connection the dongle will measure spindle speed in RPM. PyMini main screen is showed below, for information see the PyMini documentation.

The PyMini software uses the same gcode interpreter and trajectory planner as the EMC/LinuxCNC project. In fact PyMini GUI functionality is based on the EMC/LinuxCNC Mini GUI which was written in TCL/TK instead of python. See the "Gcode Overview" section at for gcode syntax. Note, mcodes are handled differently in PyMini. Most mcodes are implemented via python plugin scripts. This means spindle, coolant and homing can be executed via custom python scripts.

I started using EMC back in 2002. I developed this product after using EMC for several years with my Sherline CNC mills. EMC worked great, but requires a PC with a real-time kernel and a parallel port to control the mill. Now I have the best of both worlds EMC/LinuxCNC based software with no real-time kernel requirements.

I have two mills and two CNC controllers - Camtronics and Xylotex. I now use the PyMini software exclusively. I can use the latest notebooks, which have no parallel port, with my old CNC controllers.

Originally I built the dongle PCB on my mill. Now the dongle PCB is made at a local USA circut board manufacturer, but I still make the custom case with PyMini. You don't get a feel for a product unless you use it.

I used Eagle CAD to develop and build the PCB. The engraving software I used for the case is called engrave-11-inline. Engrave-11-inline is a good example of using a real programming language, in this case python, to develop gcode output.

Buy one rt-stepper dongle with case $79 + 5.35 shipping (US address only).
Buy one rt-stepper dongle with case $79 + 16.95 shipping (non-US address only).
Buy one rt-stepper dongle with sensor kit and case $96 + 5.35 shipping (US address only).
Buy one rt-stepper dongle with sensor kit and case $96 + 16.95 shipping (non-US address only).

Buy one rt-stepper dongle, no case $59 + 5.35 shipping (US address only).
Buy one rt-stepper dongle, no case $59 + 16.95 shipping (non-US address only).
Buy one rt-stepper dongle with sensor kit, no case $76 + 5.35 shipping (US address only).
Buy one rt-stepper dongle with sensor kit, no case $76 + 16.95 shipping (non-US address only).

Buy one rt-stepper dongle breakout board $29 + 5.35 shipping (US address only).
Buy one rt-stepper dongle breakout board $29 + 16.95 shipping (non-US address only).

The rt-stepper dongle breakout board is available for breadboarding or hard wiring your own connectors. This is a REV-2 dongle that supports all the same functions as the regular dongle. It includes the same USB connector, but no DB25 or RJ-11 connector. The above picture on the left shows what is included with the breakout board. Each board comes assembled and tested, but the two 10-pin headers are NOT soldered to the PCB board. You will have to solder in the 10-pin headers yourself. Reference the above picture on the right for pin identification. By convention each pin number corresponds to a DB25 connector pin signal. See the PyMini documentation on how these signals are used.

2             DB2  (DB2-9 are step/direction signals)
3             DB3
4             DB4
5             DB5
6             DB6
7             DB7
8             DB8
9             DB9
10            DB10-INPUT0  (index pulse)
11            DB11-INPUT1  (limit switches)
12            DB12-INPUT2
14            DB14-OUTPUT0
16            DB16-OUTPUT1
The board can be configured for USB bus power or external 5V DC power. By default the board is configured for USB bus power. Remove jumper JP3 for external 5V power. When using USB bus power care must be taken to NOT exceed USB 1.1 bus power specification (~100ma).

Buy one rt-stepper 4-axis motor drive $89 + 5.35 shipping (US address only).
Buy one rt-stepper 4-axis motor drive $89 + 16.95 shipping (non-US address only).
Buy one rt-stepper 4-axis motor drive with sensor kit $106 + 5.35 shipping (US address only).
Buy one rt-stepper 4-axis motor drive with sensor kit $106 + 16.95 shipping (non-US address only).

The rt-stepper 4-axis motor drive is a complete CNC controller with the rt-stepper dongle hardware fully integrated. Each axis is independently controlled using standard StepStick A4988 carriers. This 4-axis motor drive lets you control bipolar (4 wire) stepper motors. Unipolar (6 or 8 wire) stepper motors can also be controlled as bipolar stepper motors when properly connected.

The StepSticks that come with the 4-axis motor drive are adjusted to 1A current limit. These StepSticks use 0.1 ohm sense resister with a max current of 1.5 amps. You can set the current limit using this formula.

0.8 = 1 * 8 * 0.1 (vref = current * 8 * sense-resister)
For a 1A current limit the StepStick pot is adjusted 0.8VDC (vref). You can do this with no motor plugged in. Measure vref at the pot wiper, clockwise increases vref, counterclockwise decreases vref.

All StepSticks in the 4-axis motor drive are socketed so they can be replaced. Stepsticks need heat sinks for 1A or higher operation. The heat sinks must be installed using thermal glue otherwise the heat sink will fall off once the sticky tape heats up.

The 4-axis motor drive is designed to handle 12-24VDC 5A. This means the design allows for 4 motors running at 1.25A or 3 motors running at 1.66A. This is plenty of power for running NEMA 17 or NEMA 23 stepper motors. Even with StepSticks set to 1A current limit I had no problem running my Sherline machines.

Please note, the 4-axis motor drive does NOT come with a DC power supply. You can purchase an appropriate power supply here at amazon.

There are four jumper areas JP1-4 for selecting motor step size. Use this table for selecting the step size for each motor.

1, 2
3, 4
5, 6
JPn pins
full step
1/2 step
1/4 step
1/8 step
1/16 step
The X specifies what pins to jumper. In the above picture each motor was set to 1/4 step size, JP1-4 pins 3 and 4 are jumpered together.

PyMini uses a configuration file to set runtime options. Here are two configuration file examples - stepstick_5000.ini and stepstick_shapeoko.ini. The stepstick_5000.ini is for my Sherline 5000 mill and stepstick_shapeoko.ini is for my Shapeoko 2 CNC router. In the configuration file you can define how each axis maps to a motor connector. For example stepstick_5000.ini axis X,Y,Z,A are mapped to connector X1,X2,X3,X4 respectfully and stepstick_shapeoko.ini axis X,Y,Z,Y are mapped to connector X1,X2,X3,X4 respectfully.

The RJ-11 connector is fully compatable with rt-stepper sensor kit. This connector is also useful for wiring limit switches and digital outputs see the PyMini documentation. The 4-axis motor drive schematic can be referenced here.

Overall Dimensions: 4.04 in.(L) x 3.11 in.(W) x 0.85 in.(H)

Buy one rt-stepper sensor kit $22 + 5.35 shipping (US address only).
Buy one rt-stepper sensor kit $22 + 16.95 shipping (non-US address only).

The rt-stepper sensor kit creates the index pulse for measuring the spindle speed. The kit as shown in the above picture provides all the necessary hardware for mounting to a Sherline spindle motor. Each kit bolts on to the DC speed controller case with no modification made to the Sherline equipment. Use the above picture for reference when installing.

The plastic sensor mount is made out of ABS. The black disk with reflector tape is made out of light absorbing black construction paper. Since the disk center hole is a tight fit it can be placed on the spindle pulley with or without using a glue stick. The 3/4 inch socket head screw replaces the original 1/2 inch socket head screw.

Use the provided RJ-11 cable to connect the sensor board to the rt-stepper dongle. No additional wiring is necessary for the sensor when using the rt-stepper dongle with RJ-11 connector. The generated index pulse is connected to the dongle on INPUT0 (DB10). A straight through RJ-11 cable is required not a reversed wired cable.

For optimal sensor sensitivity the distance between the sensor and the black reflective disk should be 1/16 inch (0.0625). After tightening down the sensor mount make sure the distance is correct. The mount is slotted so you can adjust it up or down with the socket head screw.

Save on shipping with multiple orders to the same address. Here are two options for multiple orders.

  • Send me your order reqest via my email address below and I can email you a PayPal money request. Be sure to include your shipping address with your email request.
  • Click the PayPal button for each order and I will refund you the extra shipping cost. Actual shipping will be adjusted to a single shipping charge (at shipping time).

The rt-stepper dongle hardware is based on the Microchip PIC18F2455/PIC16F1459 processor. If you are board designer and would like to integrate the dongle hardware into your own application. I am willing to sell you preprogrammed pic chips for your application. Based on my sublicense agreement with Microchip this arrangement works best because each pic chip must be programmed with unique USB descriptors (ie: pid/vid, serial number, product ID).

Be sure to use the rt-stepper dongle as your reference design with your application. Because any hardware or software support would be based on the dongle only. Please email me for more information.

© 1996-2020 Eckler Software

Last updated January 10, 2020