I recently purchased 3 axis electronics with the USB controller board. I'm trying to install an E-stop switch. Where do I connect that on the controller board?

There are many ways to wire an emergency stop switch. The simplest, and arguably the most certain, way is to wire the switch into the incoming power so that throwing the switch kills all power to every part of the machine. Here is a diagram showing how a customer wired such a switch on their blackTooth laser which you could adapt to your needs. http://www.buildyourtools.com/phpBB3/viewtopic.php?f=12&t=4872 Alternatively, you could use an input pin on your USB board. The wiring would depend on whether and how you might be using limit switches (single or multiple switches; normally open or normally closed). Our website datasheet for the USB board is at https://buildyourcnc.com/item/electronicsAndMotors-electronic-component-USB-Controller-Breakout#prettyPhoto/2/ and shows how to configure the switches for each set up. If you use this method, you'd essentially wire the e-stop as a limit switch.