Mesa 7i96 and E-Stop

There are so many ways to set up an estop.
But at the end of the day, you need to trigger an input in linuxcnc on an estop so the system knows whats going on. 
It can then do whatever is required by enabling outputs etc.
On my first machine I used one side of the estop switch to turn off the AC power to the stepper power supply and the other side to enable an input to linuxcnc
It is best practice to use the estop-latch component to manage external estops.

My latest machine uses a safety relay and does what Andy suggests, disables the  3 phase spindle using a breaker and sends 24v to an input.
Linuxcnc then uses 3 seperate outputs to disable the ethercat drives which have an appropriate input. Power to the motors is retained.
It does not use estop-latch to enable the safety relay (it could) but relies on an external reset button on the safety relay circuit.
The safety relay requires a different estop switch with two NC  contacts so there is redundancy for safety.