I am struggling configuring an emergency stop that is instantaneous AND offers a resume possibility, I am thinking of going for an external relay cutting power.
Regarding the Resume capability, you already have a recent thread running. Have you updated RRF as @gloomyandy suggested?
Generally spoken, a true emergency stop means to physically cut mains power. Any software-driven approach can go wrong if controller and/or firmware malfunctions for some reason. Given that you have to switch power off, follow the path of M911: Configure auto save on loss of power.
If you don’t want to shut down the system completely, go with Pause/Resume - that’s exactly what it’s for. As this mechanism is software-driven, you can easily add a relay to switch off the spindle.
So you can have both an Emergency stop and a soft stop (aka Pause). But don’t mix them. By piling up additional hard- and software components to catch potential malfunctions, you introduce more potential points of failure - not exactly what you want in an emergency situation.
