This project provides an inexpensive means of remotely performing a STOP (ALT+S) to Mach3. This is accomplished with a Raspberry Pi (RPi) + Camera connected to a PC via a USB cable. Monitoring and activating the STOP on the CNC is done with a video streaming web application running on the RPi. Using this solution should be done at your own risk. I highly recommend contacting the manufacturer of your CNC, and purchase and install the recommended remote emergency stop specified.
Full instruction can be found at Remote CNC Stop and Monitor.
Note: This is not the same as an Emergency Stop (EStop) wired into your CNC control panel, but the execution of a hotkey to perform a STOP of Mach3.