Breakout Boards

Breakout boards are the interface between a computer and a CNC machines various different electronic components (actuator drives, spindle speed controller, limit switches, etc). Breakout boards will break out every pin or wire from an interfaces connector so that we can easily interface with it. CNC breakout boards often have many features that allow us to more easily and safely add more functionality to our CNC machines.

Breakout board is a general term for a PCB (Printed Circuit Board) that breaks out the various pins of a particular connector to several terminal connectors. The purpose of these boards is just to make it easier to interface with the various pins of the particular connector it is ‘breaking’ out. However CNC breakout boards do a lot more, they add additional electronic components and give the boards special features tailored for CNC operation.

Some of the features you may come across are:


Opto-isolation is a feature you will see on most CNC breakout boards. Opto-isolation isolates the electrical signals sent between the computer’s interface (usually the parallel port) and the breakout board itself. Opto-isolation works by replacing electrical contact with light. This is done by using electrical components called opto-isolators.

Opto-isolators have a tiny internal LED (Light Emitting Diode) and something called a phototransistor. When a signal from the computer arrives at an opto-isolator it powers the tiny internal LED, this then causes light to fall onto a phototransistor which is sensitive to light. This causes the phototransistor to close, resulting in a closed circuit. In turn this new closed circuit condition will allow a completely different voltage source to flow on the other side of the opto-isolator.

A breakout board featuring opto-isolation will then be electrically isolated from your computer, keeping your computer safe from the voltage operating within your CNC electronics and will also prevent any potential short circuits from damaging your computer.

Variable Speed Control

Variable Speed Control is a feature you might find on some breakout boards. This feature will allow the control of a VFD (Variable Frequency Drive) or other speed controller. This means that if you have a spindle or router controlled via some type of speed controller that you will be able to control it directly from your computer. Turn the spindle on/off and change the speed.

This is extremely useful when combined with CAM software that will allow you to program different spindle speeds for different operations. As you run a G-Code program it will be able to automatically change the speed of the spindle by telling your CNC controller software what spindle speed it wants. The CNC controller software will then send an electrical signal out to the breakout board which will then through the Variable Speed Control electronics be turned into a voltage, usually within a 0-10 volts range. This voltage will be proportionate to the speed requested so:

If zero volts is sent then it will mean zero speed, so the spindle will be turned off. If 5 volts is sent then it is half way between off and full power, so the spindle will run at half of its capable speed. These ranges are usually editable within the programming of the VFD so that you can be sure a certain voltage will equal a certain speed.

Charge Pump / Watchdog Timer

This feature usually known as a charge pump (also called a watchdog timer) is a special circuit that is featured on some breakout boards. It is a safety feature that ensures the breakout board is only open and responsive while a CNC controller software is running and operating in good condition. If the software should crash or freeze the breakout board will instantly disable itself and automatically shutdown the actuators and a spindle if connected.

This is an exceptional safety feature that I highly recommend having. If you want to learn more about the charge pump and how it works, I have a dedicated page to it here.

This product page shows a breakout board which I’m referring to.