I am spoiled. I am used to having the motor control start and stop with a gcode command. I looked through the FluidNC documentation and came up with a workaround that has been great for me.
I set up a small ESP8266 circuit inside of the recommended E-Stop button. When it is powered on it sends a UDP multicast out to let me know it is online, then it monitors the Maslow telnet 23 signal. With a response of “Run” (polled twice), the router starts. it monitors the status during operation run time. When it gets an “Idle” (polled twice) it drops the relay and router stops. If I get an “Alarm”, it drops the relay immediately. Hitting the estop also drops the relay.
My motor start time ( basically my M3 ) is controlled by a G91 G1 Z.01 F10 G90 command. This gives it enough time to hit speed.