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SC1 Variable Speed Control Data Sheet

Overview Features  

This card lets you control your spindle with step and direction signals, as if it was an axis motor. It converts the step signal into and an analogue (0-10VDC), which is what most Variable Frequency Drives (VFD) use for external control. This card also has a relay that lets you use the direction signal to direct the 24VDC signal from your VFD to the appropriate input signal of your VFD to drive CW or CCW.


A Variable Frequency Drive or Inverter works by modifying the frequency for AC motors. You can control most of these devices with an external analogue signal (0-10VDC). That is, if there is 5VDC coming into through the control signal, the motor will run at 50% of full speed, if there was 10VDC, the motor will run at 100% of full speed. If there is no signal coming out, then the motor will stop.

This unit can also be used on many DC motor controllers by replacing the potentiometer that controls the speed.


          Inputs a frequency (step) and outputs an analog 0-10VDC.

          Uses the direction signal to tel you VFD to go CW or CCW.

The board comes with an on-board electromechanical relay that can be used for spindle direction control. This relay can also be used to turn on or off you spindle.

          Optoisolated output signals.

The analog and CW and CWW signals are optically isolated, so this board can be used with drives that make grounds common with the mains that drive the VFD or motor.

          Al TTL +5VDC or +3.3VDC Signals.

Interface directly with parallel port interface products and other cards. 5VDC (TTL) signals are very common among automation devices.

          Screw-On connections for all terminals.

You only have to screw-on the wires to make all your connections

Flexible design.

It works with other Rhonmac-cnc products, directly through your parallel port, or through many other parallel port control products.

Installation Requirements:

It requires a +12VDC power supply to operate.

WARNING: To keep the output signals optoisolated, these must not have common ground or current with other circuits you are using.

You will require a voltmeter to fine tune your system.


Before connecting anything, please be sure to read your VFDs manual and make sure you understand all the safety issues.

Please note the wiring diagram bellow.
















RJ45 Connector:

The RJ45 connector is for connecting the boards directly using a standard RJ45 cable. A standard RJ45 cable has straight through wiring. That means that Pin1 = Pin1, etc

This Connector is been provided in many of our boards and soon we will have breakout boards that will come with this connectors. This is to ease the installation process by eliminating the screw on terminals. The RJ45 connectors are used not just to carry the I/O signals, but also to power the external board. That way you will have it all in one connection.

Pin 4 carries the step signal and pin 5 carries the direction signal.

Pin Description

1 +12vdc, 2 +5Vdc, 3 NC1, 4 NC2, 5 NC3, 6 NC4, 7 NC5 and 8 Ground

Configuring your Control Software: It is strongly recommend you read your control softwares manual. You need to configure your control software to control the spindle as if it was an angular axis. This card requires a 25 KHz input signal to deliver 10VDC. So you have to set the speed of the motor (spindle) at maximum. For acceleration values adjust them to where you feel comfortable. Keep in mind the acceleration of the motor must also be set in your VFD. For configuring Mach follow these steps:  

1. Go to Config / Ports & Pins / Motor Outputs. Enable the spindle and select the port and pins you wired for step and direction.

2. Go to Config / Ports&Pins / Spindle Setup. In the motor control box, check Use Spindle Motor Output and Step /Dir Motor. Under Pulley Ratios set the pulley ratios of the machine.



  3.        Go to Config / Motor Tuning / Spindle. On Steps per unit put 1,000, set velocity to maximum. For Acceleration, choose the acceleration that you feel comfortable with. Start slow, increase acceleration as you test your system. Under Step Pulse length, use a number from 1 to 5, but start with 1. This number is directly proportional to the final voltage you will get in the analog output. Use this number and the fine tuning pot to adjust the voltage you want to get at max speed.


Text Box:  Text Box:  Fine Tuning:

Make sure that when you reach the max speed in the control software you get 10VDC out. This voltage can vary depending on many things, including the electrical properties of parallel port or breakout board you are using, the length of the step pulse your software is delivering, and the normal hi or low status of your step pin. Play with the fine tuning pot in the card, the normally hi or low status of your pin, and the pulse width.

Replacing a Potentiometer:

This circuit can be used to replace a potentiometer of DC motor speed control circuits. This speed controller circuits are very commonly used by SIEG, KB Electronics, and many other oriental machines. Before explaining how to do it, please first keep in mind that it can be done if the voltage that goes though the pot is +12vdc or less. This circuit cannot be used for AC currents.

In most cases the terminals that go to the potentiometer will carry these signals:

P1 = GND



These are the steps for replacing a potentiometer:

1.        Measure the voltage difference between P1 and P3. Make sure it measures under +12vdc.

2.        Fine tune the analogue output to the output voltage you got from step 1

3.        Connect the ground from the analogue output to the ground of the potentiometer (P1).

4.        Connect the analogue output to the wiper connection of the potentiometer (P2).

  If the reference voltage from pot is between +10 and 12vdc, you can use it to power the annalog circuit. In this case, connect P1 to the ground of the power terminal, and P3 to the +12vdc power connector.

                                                                                       SC1 Board Dimesion





For the SEIG X-3 Mill follow connections as below 

SC1Speed Control Board


Direction Control is not implemented in the Seig X-3 speed control





The final voltage must be adjusted to 7Vdc.

The P1 and P2 must be disconnected from the potentiometer and connected to Ground and analogue output on the SC1 board as indicated opposite by the red lines





X-3 control board as viewed from inside the electrical cover at the back of the machine. Instruction sheet supplied with SC1 board for X-3 conversion.


Our policy is one of continual improvement therefore specifications may change without notice. Pictorial representations are intended as a guide only and may vary depending on machine model or product offered.